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The MySQL (TM)
software delivers a very fast, multi-threaded,
multi-user, and robust SQL
(Structured Query Language
)
database server.
MySQL Server
is intended for mission-critical, heavy-load
production systems as well as for embedding into mass-deployed software.
MySQL
is a trademark of MySQL AB
.
The MySQL
software is Dual Licensed
. Users can choose to
use the MySQL
software as an Open Source
/Free Software
product under the terms of the GNU General Public License
(http://www.gnu.org/licenses/) or can purchase a standard
commercial license from MySQL AB
.
See section 1.4 MySQL Support and Licensing.
The MySQL
web site (http://www.mysql.com/) provides the
latest information about the MySQL
software.
The following list describes some sections of particular interest in this manual:
MySQL Database Server
,
see section 1.3 What Is MySQL AB?.
MySQL Database Server
,
see section 1.2.2 The Main Features of MySQL.
MySQL Database Software
to new architectures
or operating systems, see section E Porting to Other Systems.
MySQL Database Server
,
see section 3 Tutorial Introduction.
SQL
and benchmarking information, see the
benchmarking directory (`sql-bench' in the distribution).
Important:
Reports of errors (often called bugs), as well as questions and comments, should be sent to the mailing list at [email protected]. See section 1.6.2.3 How to Report Bugs or Problems.
The mysqlbug
script should be used to generate bug reports.
For source distributions, the mysqlbug
script can be found in the
`scripts' directory. For binary distributions, mysqlbug
can
be found in the `bin' directory. If you have found a sensitive
security bug in MySQL Server
, you should send an e-mail to
[email protected].
This is the MySQL
reference manual; it documents MySQL
Version 4.0.5. Being a reference manual, it does not
provide general instruction on SQL
or relational database
concepts.
As the MySQL Database Software
is under constant development,
the manual is also updated frequently.
The most recent version of this manual is available at
http://www.mysql.com/documentation/ in many different formats,
including Texinfo, plain text, Info, HTML, PostScript, PDF,
and Windows HLP versions.
The primary document is the Texinfo file.
The HTML version is produced automatically using a modified version of
texi2html
.
The plain text and Info versions are produced with makeinfo
.
The PostScript version is produced using texi2dvi
and dvips
.
The PDF version is produced with pdftex
.
If you have a hard time finding information in the manual, you can try our searchable version at http://www.mysql.com/doc/.
If you have any suggestions concerning additions or corrections to this manual, please send them to the documentation team at [email protected].
This manual was initially written by David Axmark and Michael (Monty) Widenius. It is currently maintained by Michael (Monty) Widenius, Arjen Lentz, and Paul DuBois. For other contributors, see section C Credits.
The copyright (2002) to this manual is owned by the Swedish company
MySQL AB
. See section 1.4.2 Copyrights and Licenses Used by MySQL.
This manual uses certain typographical conventions:
constant
mysqladmin
works, invoke it with the
--help
option.''
When commands are shown that are meant to be executed by a particular
program, the program is indicated by a prompt shown before the command. For
example, shell>
indicates a command that you execute from your login
shell, and mysql>
indicates a command that you execute from the
mysql
client program:
shell> type a shell command here mysql> type a mysql command here
Shell commands are shown using Bourne shell syntax. If you are using a
csh
-style shell, you may need to issue commands slightly differently.
For example, the sequence to set an environment variable and run a command
looks like this in Bourne shell syntax:
shell> VARNAME=value some_command
For csh
, you would execute the sequence like this:
shell> setenv VARNAME value shell> some_command
Often database, table, and column names must be substituted into commands. To
indicate that such substitution is necessary, this manual uses
db_name
, tbl_name
and col_name
. For example, you might
see a statement like this:
mysql> SELECT col_name FROM db_name.tbl_name;
This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this:
mysql> SELECT author_name FROM biblio_db.author_list;
SQL keywords are not case-sensitive and may be written in uppercase or lowercase. This manual uses uppercase.
In syntax descriptions, square brackets (`[' and `]') are used
to indicate optional words or clauses. For example, in the following
statement, IF EXISTS
is optional:
DROP TABLE [IF EXISTS] tbl_name
When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (`[' and `]'):
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):
{DESCRIBE | DESC} tbl_name {col_name | wild}
MySQL
, the most popular Open Source
SQL database, is
developed, distributed and supported by MySQL AB
. MySQL AB
is a
commercial company founded by the MySQL developers that builds its business
providing services around the MySQL
database.
See section 1.3 What Is MySQL AB?.
The MySQL
web site (http://www.mysql.com/)
provides the latest information about MySQL
software and
MySQL AB
.
MySQL
is a database management system.
MySQL
Server. Since computers are very good at handling large
amounts of data, database management plays a central role in computing,
as stand-alone utilities, or as parts of other applications.
SQL
part of
``MySQL
'' stands for ``Structured Query Language
''—the
most common standardised language used to access databases.
Open Source
.
Open Source
means that it is possible for anyone to use and modify.
Anybody can download the MySQL
software from the Internet and use it
without paying anything. Anybody so inclined can study the source code
and change it to fit their needs. The MySQL
software uses the
GPL
(GNU General Public License
),
http://www.gnu.org/licenses/, to define what you
may and may not do with the software in different situations.
If you feel uncomfortable with the GPL
or need to embed
MySQL
code into a commercial application you can buy a
commercially licensed version from us.
See section 1.4.3 MySQL Licenses.
MySQL Database Server
is very fast, reliable, and easy to use.
If that is what you are looking for, you should give it a try.
MySQL Server
also has a practical set of features developed in
close cooperation with our users. You can find a performance comparison
of MySQL Server
to some other database managers on our benchmark page.
See section 5.1.4 The MySQL Benchmark Suite.
MySQL Server
was originally developed to handle large databases
much faster than existing solutions and has been successfully used in
highly demanding production environments for several years. Though
under constant development, MySQL Server
today offers a rich and
useful set of functions. Its connectivity, speed, and security make
MySQL Server
highly suited for accessing databases on the Internet.
MySQL Database Software
is a client/server system that consists
of a multi-threaded SQL
server that supports different backends,
several different client programs and libraries, administrative tools,
and a wide range of programming interfaces (API
s).
We also provide MySQL Server
as a multi-threaded library which you
can link into your application to get a smaller, faster, easier-to-manage
product.
MySQL Database Server
.
The official way to pronounce MySQL
is ``My Ess Que Ell'' (not
``my sequel''), but we don't mind if you pronounce it as ``my sequel''
or in some other localised way.
We once started out with the intention of using mSQL
to connect to our
tables using our own fast low-level (ISAM) routines. However, after some
testing we came to the conclusion that mSQL
was not fast enough nor
flexible enough for our needs. This resulted in a new SQL interface to our
database but with almost the same API interface as mSQL
. This API was
chosen to ease porting of third-party code.
The derivation of the name MySQL
is not perfectly clear. Our base
directory and a large number of our libraries and tools have had the prefix
``my'' for well over 10 years. However, Monty's daughter (some years younger)
is also named My. Which of the two gave its name to MySQL
is
still a mystery, even for us.
The following list describes some of the important characteristics
of the MySQL Database Software
. See section 1.5 MySQL 4.x In A Nutshell.
MySQL
code gets tested with Purify
(a commercial memory leakage detector) as well as with Valgrind,
a GPL tool (http://developer.kde.org/~sewardj/).
FLOAT
, DOUBLE
, CHAR
, VARCHAR
,
TEXT
, BLOB
, DATE
, TIME
, DATETIME
,
TIMESTAMP
, YEAR
, SET
, and ENUM
types.
See section 6.2 Column Types.
INSERT
to insert a subset of a table's columns; those
columns that are not explicitly given values are set to their default
values.
SELECT
and WHERE
parts of queries. For example:
mysql> SELECT CONCAT(first_name, " ", last_name) -> FROM tbl_name -> WHERE income/dependents > 10000 AND age > 30;
GROUP BY
and
ORDER BY
clauses. Support
for group functions (COUNT()
,
COUNT(DISTINCT ...)
,
AVG()
, STD()
,
SUM()
, MAX()
, and MIN()
).
LEFT OUTER JOIN
and RIGHT OUTER JOIN
with ANSI
SQL and ODBC syntax.
DELETE
, INSERT
, REPLACE
, and UPDATE
return
the number of rows that were changed (affected). It is possible to return
the number of rows matched instead by setting a flag when connecting to the
server.
MySQL
-specific SHOW
command can be used to retrieve
information about databases, tables, and indexes. The EXPLAIN
command
can be used to determine how the optimiser resolves a query.
ABS
is a valid column name. The only restriction is that for a
function call, no spaces are allowed between the function name and the
`(' that follows it. See section 6.1.7 Is MySQL Picky About Reserved Words?.
MySQL Server
with some
databases that contain 50 million records and we know of users that
use MySQL Server
with 60,000 tables and about 5,000,000,000 rows.
MySQL Server
).
An index may use a prefix of a CHAR
or VARCHAR
field.
MySQL
server using TCP/IP Sockets,
Unix Sockets (Unix), or Named Pipes (NT).
ODBC
(Open-DataBase-Connectivity) support for Win32 (with source).
All ODBC 2.5 functions and many others. For example, you can use
MS Access to connect to your MySQL
server. See section 8.3 MySQL ODBC Support.
MySQL
server is started. To see an example of very advanced sorting, look
at the Czech sorting code. MySQL Server
supports many different
character sets that can be specified at compile and runtime.
myisamchk
, a very fast utility for table checking,
optimisation, and repair. All of the functionality of myisamchk
is also available through the SQL interface as well.
See section 4 Database Administration.
MySQL
programs can be invoked with the --help
or -?
options to obtain online assistance.
This section addresses the questions ``How stable is MySQL Server?'' and ``Can I depend on MySQL Server in this project?'' We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing list, which is very active in identifying problems as well as reporting types of use.
Original code stems back from the early '80s, providing a stable code
base, and the ISAM table format remains backward-compatible.
At TcX, the predecessor of MySQL AB
, MySQL
code has worked
in projects since mid-1996, without any problems.
When the MySQL Database Software
was released to a wider public,
we noticed that there were
some pieces of ``untested code'' that were quickly found by the new
users who made different types of queries from us. Each new release
has had fewer portability problems (even though each new release has
had many new features).
Each release of the MySQL Server
has been usable. There have only
been problems
when users try code from the ``gray zones.'' Naturally, new users
don't know what the gray zones are; this section attempts to indicate
those that are currently known.
The descriptions mostly deal with Version 3.23 of MySQL Server
.
All known and reported bugs are fixed in the latest version, with the
exception of those listed in the bugs section, which are things that
are design-related. See section 1.7.5 Known Errors and Design Deficiencies in MySQL.
The MySQL Server
design is multi-layered with independent modules.
Some of the newer modules are listed here with an indication of how
well-tested each of them is:
MySQL
4.x.
InnoDB
tables -- Stable (in 3.23 from 3.23.49)
InnoDB
transactional table handler has now been declared
stable in the MySQL
3.23 tree, starting from version 3.23.49.
InnoDB
is being used in large, heavy-load production systems.
BDB
tables -- Gamma
Berkeley DB
code is very stable, but we are still improving
the BDB
transactional table handler interface in
MySQL Server
, so it will take some time before this is as well
tested as the other table types.
FULLTEXT
-- Beta
MySQL
4.0.
MyODBC 2.50
(uses ODBC SDK 2.5) -- Gamma
MyISAM
tables -- Gamma
MyISAM
table
handler that checks if the table was closed properly on open and
executes an automatic check/repair of the table if it wasn't.
MyISAM
tables in MySQL
4.0 for faster
insert of many rows.
fcntl()
). In these cases, you should
run mysqld
with the --skip-external-locking
flag.
Problems are known to occur on some Linux systems, and on SunOS when
using NFS-mounted filesystems.
MySQL AB
provides high-quality support for paying customers,
but the MySQL
mailing list usually provides answers to common
questions. Bugs are usually fixed right away with a patch; for serious
bugs, there is almost always a new release.
MySQL
Version 3.22 has a 4G limit on table size. With the new
MyISAM
table type in MySQL
Version 3.23, the maximum table
size is pushed up to 8 million terabytes (2 ^ 63 bytes).
Note, however, that operating systems have their own file-size limits. Here are some examples:
Operating System | File-Size Limit |
Linux-Intel 32 bit | 2G, 4G or more, depends on Linux version |
Linux-Alpha | 8T (?) |
Solaris 2.5.1 | 2G (possible 4G with patch) |
Solaris 2.6 | 4G (can be changed with flag) |
Solaris 2.7 Intel | 4G |
Solaris 2.7 UltraSPARC | 512G |
On Linux 2.2 you can get bigger tables than 2G by using the LFS patch for the ext2 filesystem. On Linux 2.4 patches also exist for ReiserFS to get support for big files.
This means that the table size for MySQL
databases is normally
limited by the operating system.
By default, MySQL
tables have a maximum size of about 4G. You can
check the maximum table size for a table with the SHOW TABLE STATUS
command or with the myisamchk -dv table_name
.
See section 4.5.6 SHOW
Syntax.
If you need bigger tables than 4G (and your operating system supports
this), you should set the AVG_ROW_LENGTH
and MAX_ROWS
parameter when you create your table. See section 6.5.3 CREATE TABLE
Syntax. You can
also set these later with ALTER TABLE
. See section 6.5.4 ALTER TABLE
Syntax.
If your big table is going to be read-only, you could use
myisampack
to merge and compress many tables to one.
myisampack
usually compresses a table by at least 50%, so you can
have, in effect, much bigger tables. See section 4.7.4 myisampack
, The MySQL Compressed Read-only Table Generator.
You can go around the operating system file limit for MyISAM
data
files by using the RAID
option. See section 6.5.3 CREATE TABLE
Syntax.
Another solution can be the included MERGE
library, which allows
you to handle a collection of identical tables as one.
See section 7.2 MERGE
Tables.
The MySQL Server
itself has no problems with Year 2000 (Y2K)
compliance:
MySQL Server
uses Unix time functions and has no problems with dates
until 2069
; all 2-digit years are regarded to be in the range
1970
to 2069
, which means that if you store 01
in a
year
column, MySQL Server
treats it as 2001
.
MySQL
date functions are stored in one file, `sql/time.cc',
and are coded very carefully to be year 2000-safe.
MySQL
Version 3.22 and later, the new YEAR
column type
can store years 0
and 1901
to 2155
in 1 byte and
display them using 2 or 4 digits.
You may run into problems with applications that use MySQL Server
in a way that is not Y2K-safe. For example, many old applications store
or manipulate years using 2-digit values (which are ambiguous) rather than
4-digit values. This problem may be compounded by applications that use
values such as 00
or 99
as ``missing'' value indicators.
Unfortunately, these problems may be difficult to fix because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.
Here is a simple demonstration illustrating that MySQL Server
doesn't have any problems with dates until the year 2030:
mysql> DROP TABLE IF EXISTS y2k; Query OK, 0 rows affected (0.01 sec) mysql> CREATE TABLE y2k (date DATE, -> date_time DATETIME, -> time_stamp TIMESTAMP); Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO y2k VALUES -> ("1998-12-31","1998-12-31 23:59:59",19981231235959), -> ("1999-01-01","1999-01-01 00:00:00",19990101000000), -> ("1999-09-09","1999-09-09 23:59:59",19990909235959), -> ("2000-01-01","2000-01-01 00:00:00",20000101000000), -> ("2000-02-28","2000-02-28 00:00:00",20000228000000), -> ("2000-02-29","2000-02-29 00:00:00",20000229000000), -> ("2000-03-01","2000-03-01 00:00:00",20000301000000), -> ("2000-12-31","2000-12-31 23:59:59",20001231235959), -> ("2001-01-01","2001-01-01 00:00:00",20010101000000), -> ("2004-12-31","2004-12-31 23:59:59",20041231235959), -> ("2005-01-01","2005-01-01 00:00:00",20050101000000), -> ("2030-01-01","2030-01-01 00:00:00",20300101000000), -> ("2050-01-01","2050-01-01 00:00:00",20500101000000); Query OK, 13 rows affected (0.01 sec) Records: 13 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM y2k; +------------+---------------------+----------------+ | date | date_time | time_stamp | +------------+---------------------+----------------+ | 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 | | 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 | | 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 | | 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 | | 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 | | 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 | | 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 | | 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 | | 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 | | 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 | | 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 | | 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 | | 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 | +------------+---------------------+----------------+ 13 rows in set (0.00 sec)
This shows that the DATE
and DATETIME
types will not
give any problems with future dates (they handle dates until the year
9999).
The TIMESTAMP
type, which is used to store the current time, has a
range up to only 2030-01-01
. TIMESTAMP
has a range of
1970
to 2030
on 32-bit machines (signed value). On 64-bit
machines it handles times up to 2106
(unsigned value).
Even though MySQL Server
is Y2K-compliant, it is your responsibility
to provide unambiguous input. See section 6.2.2.1 Y2K Issues and Date Types for MySQL Server
's
rules for dealing with ambiguous date input data (data containing 2-digit
year values).
MySQL AB
is the company of the MySQL
founders and main
developers. MySQL AB
was originally established in Sweden by
David Axmark, Allan Larsson, and Michael Monty
Widenius.
All the developers of the MySQL
server are employed by the company.
We are a virtual organisation with people in a dozen countries around
the world. We communicate extensively over the Net every day with each
other and with our users, supporters and partners.
We are dedicated to developing the MySQL
software and spreading
our database to new users. MySQL AB
owns the copyright to the
MySQL
source code, the MySQL
logo and trademark, and this
manual. See section 1.2 What Is MySQL?.
The MySQL
core values show our dedication to MySQL
and
Open Source
.
We want the MySQL Database Software
to be:
MySQL AB
and the people at MySQL AB
:
Open Source
philosophy and support the
Open Source
community.
The MySQL
web site (http://www.mysql.com/)
provides the latest information about MySQL
and MySQL AB
.
One of the most common questions we encounter is: ``How can you make a living from something you give away for free?'' This is how.
MySQL AB
makes money on support, services, commercial licenses,
and royalties, and we use these revenues to fund product development
and to expand the MySQL
business.
The company has been profitable since its inception. In October 2001, we accepted venture financing from leading Scandinavian investors and a handful of business angels. This investment is used to solidify our business model and build a basis for sustainable growth.
MySQL AB
is run and owned by the founders and main developers of
the MySQL
database. The developers are committed to giving support
to customers and other users in order to stay in touch with their needs
and problems. All our support is given by qualified developers. Really
tricky questions are answered by Michael Monty
Widenius, principal
author of the MySQL Server
.
See section 1.4.1 Support Offered by MySQL AB.
For more information and ordering support at various levels, see http://www.mysql.com/support/ or contact our sales staff at [email protected].
MySQL AB
delivers MySQL
and related training worldwide.
We offer both open courses and in-house courses tailored to the
specific needs of your company. MySQL Training
is also available
through our partners, the Authorised MySQL Training Centers
.
Our training material uses the same example databases as our
documentation and our sample applications, and it is always updated
to reflect the latest MySQL
version. Our trainers are backed by
the development team to guarantee the quality of the training and the
continuous development of the course material. This also ensures
that no questions raised during the courses remain unanswered.
Attending our training courses will enable you to achieve your goals
related to your MySQL
applications. You will also:
MySQL Certification
.
If you are interested in our training as a potential participant or as a training partner, please visit the training section at http://www.mysql.com/training/ or contact us at: [email protected].
The MySQL Certification Program
is being released in the second
half of 2002. For details please see
http://www.mysql.com/training/certification.html.
MySQL AB
and its Authorised Partners
offer consulting
services to users of MySQL Server
and to those who embed
MySQL Server
in their own software, all over the world.
Our consultants can help you design and tune your databases, construct
efficient queries, tune your platform for optimal performance, resolve
migration issues, set up replication, build robust transactional
applications, and more.
We also help customers embed MySQL Server
in their products and
applications for large-scale deployment.
Our consultants work in close collaboration with our development team,
which ensures the technical quality of our professional services.
Consulting assignments range from 2-day power-start sessions to
projects that span weeks and months. Our expertise not only covers
MySQL Server
, but also extends into programming and scripting
languages such as PHP, Perl, and more.
If you are interested in our consulting services or want to become a consulting partner, please visit the consulting section of our web site at http://www.mysql.com/consulting/ or contact our consulting staff at [email protected].
The MySQL
database is released under the
GNU General Public License
(GPL
).
This means that the MySQL
software can be used free of charge
under the GPL
. If you do not want to be bound by the GPL
terms (like the requirement that your own application becomes GPL
as well), you may purchase a commercial license for the same product
from MySQL AB
.
See http://www.mysql.com/support/arrangements/price.html.
Since MySQL AB
owns the copyright to the MySQL
source code,
we are able to employ Dual Licensing
which means that the same
product is available under GPL
and under a commercial
license. This does not in any way affect the Open Source
commitment of MySQL AB
. For details about when a commercial
license is required, please see section 1.4.3 MySQL Licenses.
We also sell commercial licenses of third-party Open Source GPL
software that adds value to MySQL Server
. A good example is the
InnoDB
transactional table handler that offers ACID
support, row-level locking, crash recovery, multi-versioning, foreign
key support, and more. See section 7.5 InnoDB
Tables.
MySQL AB
has a worldwide partner programme that covers training
courses, consulting & support, publications plus reselling and
distributing MySQL
and related products. MySQL AB Partners
get visibility on the http://www.mysql.com/ web site and the right
to use special versions of the MySQL
trademarks to identify their
products and promote their business.
If you are interested in becoming a MySQL AB Partner
, please e-mail
[email protected].
The word MySQL
and the MySQL
dolphin logo are trademarks of
MySQL AB
. See section 1.4.4 MySQL AB Logos and Trademarks.
These trademarks represent a significant value that the MySQL
founders have built over the years.
The MySQL
web site (http://www.mysql.com/) is popular among
developers and users. In October 2001, we served 10 million page views.
Our visitors represent a group that makes purchase decisions and
recommendations for both software and hardware. Twelve percent of our
visitors authorise purchase decisions, and only nine percent are not
involved in purchase decisions at all. More than 65% have made one or
more online business purchase within the last half-year, and 70% plan
to make one in the next months.
If you are interested in placing banner ads on our web site, http://www.mysql.com/, please send an e-mail message to [email protected].
The MySQL
web site (http://www.mysql.com/)
provides the latest information about MySQL
and MySQL AB
.
For press service and inquiries not covered in our News releases (http://www.mysql.com/news/), please send e-mail to [email protected].
If you have a valid support contract with MySQL AB
, you will
get timely, precise answers to your technical questions about the
MySQL
software. For more information, see section 1.4.1 Support Offered by MySQL AB.
On our website, see http://www.mysql.com/support/, or send
an e-mail message to [email protected].
For information about MySQL
training, please visit the training
section at http://www.mysql.com/training/. If you have
restricted access to the Internet, please contact the MySQL AB
training staff at [email protected].
See section 1.3.1.2 Training and Certification.
For information on the MySQL Certification Program
, please see
http://www.mysql.com/training/certification.html.
See section 1.3.1.2 Training and Certification.
If you're interested in consulting, please visit the consulting
section at http://www.mysql.com/consulting/. If you have
restricted access to the Internet, please contact the MySQL AB
consulting staff at [email protected].
See section 1.3.1.3 Consulting.
Commercial licenses may be purchased online at
https://order.mysql.com/. There you will also find information
on how to fax your purchase order to MySQL AB
. More information
about licensing can be found at
http://www.mysql.com/support/arrangements/price.html.
If you have
questions regarding licensing or you want a quote for a high-volume
license deal, please fill in the contact form on our web site
(http://www.mysql.com/) or send an e-mail message
to [email protected] (for licensing questions) or to
[email protected] (for sales inquiries).
See section 1.4.3 MySQL Licenses.
If you represent a business that is interested in partnering with
MySQL AB
, please send e-mail to [email protected].
See section 1.3.1.5 Partnering.
If you are interested in placing a banner advertisement on the
MySQL
web site (http://www.mysql.com/), please send
e-mail to [email protected].
See section 1.3.1.6 Advertising.
For more information on the MySQL
trademark policy, refer to
http://www.mysql.com/company/trademark.html or send e-mail to
[email protected].
See section 1.4.4 MySQL AB Logos and Trademarks.
If you are interested in any of the MySQL AB
jobs listed in our
jobs section (http://www.mysql.com/company/jobs/),
please send an e-mail message to [email protected].
Please do not send your CV as an attachment, but rather as plain text
at the end of your e-mail message.
For general discussion among our many users, please direct your attention to the appropriate mailing list. See section 1.6.2 MySQL Mailing Lists.
Reports of errors (often called bugs), as well as questions and
comments, should be sent to the mailing list at
[email protected]. If you have found a sensitive
security bug in the MySQL Server
, please send an e-mail
to [email protected].
See section 1.6.2.3 How to Report Bugs or Problems.
If you have benchmark results that we can publish, please contact us at [email protected].
If you have any suggestions concerning additions or corrections to this manual, please send them to the manual team at [email protected].
For questions or comments about the workings or content of the
MySQL
web site (http://www.mysql.com/),
please send e-mail to [email protected].
Questions about the MySQL Portals
(http://www.mysql.com/portal/)
may be sent to [email protected].
MySQL AB
has a privacy policy, which can be read at
http://www.mysql.com/company/privacy.html.
For any queries regarding this policy, please e-mail
[email protected].
For all other inquires, please send e-mail to [email protected].
This section describes MySQL
support and licensing arrangements.
Technical support from MySQL AB
means individualised answers
to your unique problems direct from the software engineers who code
the MySQL
database engine.
We try to take a broad and inclusive view of technical support. Almost
any problem involving MySQL
software is important to us if it's
important to you.
Typically customers seek help on how to get different commands and
utilities to work, remove performance bottlenecks, restore crashed
systems, understand operating system or networking impacts on MySQL
,
set up best practices for backup and recovery, utilise API
s, etc.
Our support covers only the MySQL
server and our own utilities,
not third-party products that access the MySQL
server, though we
try to help with these where we can.
Detailed information about our various support options is given at http://www.mysql.com/support/, where support contracts can also be ordered online. If you have restricted access to the Internet, contact our sales staff at [email protected].
Technical support is like life insurance. You can live happily
without it for years, but when your hour arrives it becomes
critically important, yet it's too late to buy it!
If you use MySQL
Server for important applications and encounter
sudden troubles, it might take too long to figure out all the answers
yourself. You may need immediate access to the most experienced
MySQL
troubleshooters available, those employed by MySQL AB
.
MySQL AB
owns the copyright to the MySQL
source code,
the MySQL
logos and trademarks and this manual.
See section 1.3 What Is MySQL AB?.
Several different licenses are relevant to the MySQL
distribution:
MySQL
-specific source in the server, the mysqlclient
library and the client, as well as the GNU
readline
library
is covered by the GNU General Public License
.
See section H GNU General Public License.
The text of this license can also be found as the file `COPYING'
in the distributions.
GNU
getopt
library is covered by the
GNU Lesser General Public License
.
See section I GNU Lesser General Public License.
regexp
library) are covered
by a Berkeley-style copyright.
MySQL
(3.22 and earlier) are subject to a
more strict license
(http://www.mysql.com/support/arrangements/mypl.html).
See the documentation of the specific version for information.
GPL
-style license.
Use of the manual is subject to the following terms:
MySQL AB
is required.
For information about how the MySQL
licenses work in practice,
please refer to section 1.4.3 MySQL Licenses.
Also see section 1.4.4 MySQL AB Logos and Trademarks.
The MySQL
software is released under the
GNU General Public License
(GPL
),
which probably is the best known Open Source
license.
The formal terms of the GPL
license can be found at
http://www.gnu.org/licenses/.
See also http://www.gnu.org/licenses/gpl-faq.html and
http://www.gnu.org/philosophy/enforcing-gpl.html.
Since the MySQL
software is released under the GPL
,
it may often be used for free, but for certain uses you may want
or need to buy commercial licenses from MySQL AB
at
https://order.mysql.com/.
See http://www.mysql.com/support/arrangements.html for
more information.
Older versions of MySQL
(3.22 and earlier) are subject to a
more strict license
(http://www.mysql.com/support/arrangements/mypl.html).
See the documentation of the specific version for information.
Please note that the use of the MySQL
software under commercial
license, GPL
, or the old MySQL
license does not
automatically give you the right to use MySQL AB
trademarks.
See section 1.4.4 MySQL AB Logos and Trademarks.
The GPL
license is contagious in the sense that when a program
is linked to a GPL
program all the source code for all the parts
of the resulting product must also be released under the GPL
.
Otherwise you break the license terms and forfeit your right to use the
GPL
program altogether and also risk damages.
You need a commercial license:
GPL
code from the MySQL
software and don't want the resulting product to be GPL
, maybe
because you want to build a commercial product or keep the added
non-GPL
code closed source for other reasons. When purchasing
commercial licenses, you are not using the MySQL
software under
GPL
even though it's the same code.
GPL
application that only works with the
MySQL
software and ship it with the MySQL
software. This type
of solution is actually considered to be linking even if it's done over
a network.
MySQL
software without providing
the source code as required under the GPL
license.
MySQL
database even if you don't formally need a commercial license.
Purchasing support directly from MySQL AB
is another good way
of contributing to the development of the MySQL
software, with
immediate advantages for you.
See section 1.4.1 Support Offered by MySQL AB.
If you require a license, you will need one for each installation of the
MySQL
software. This covers any number of CPUs on a machine, and there
is no artificial limit on the number of clients that connect to the server
in any way.
For commercial licenses, please visit our website at http://www.mysql.com/support/arrangements/price.html. For support contracts, see http://www.mysql.com/support/. If you have special needs or you have restricted access to the Internet, please contact our sales staff at [email protected].
You can use the MySQL
software for free under the GPL
if
you adhere to the conditions of the GPL
.
For more complete coverage of the common questions about the GPL
see the generic FAQ from the Free Software Foundation at
http://www.gnu.org/licenses/gpl-faq.html.
Some common cases:
MySQL
source code under the GPL
with your product.
MySQL
source code bundled with other
programs that are not linked to or dependent on the MySQL
system
for their functionality even if you sell the distribution commercially.
This is called mere aggregation in the GPL
license.
MySQL
system, you can use it for free.
MySQL
servers for your customers.
However, we do encourage people to use ISPs that have MySQL support,
as this will give them the confidence that if they have some problem
with the MySQL
installation, their ISP will in fact have the
resources to solve the problem for them.
Note that even if an ISP does not have a commercial license for
MySQL Server
, they should at least give their customers read
access to the source of the MySQL
installation so that the
customers can verify that it is patched correctly.
MySQL
Database Software in conjunction with a
web server, you do not need a commercial license (so long as it is not
a product you distribute). This is true even if you run a commercial
web server that uses MySQL Server
, because you are not
distributing any part of the MySQL
system. However, in this
case we would like you to purchase MySQL
support because the
MySQL
software is helping your enterprise.
If your use of MySQL
database software does not require a commercial
license, we encourage you to purchase support from MySQL AB
anyway.
This way you contribute toward MySQL
development and also gain
immediate advantages for yourself. See section 1.4.1 Support Offered by MySQL AB.
If you use the MySQL
database software in a commercial context
such that you profit by its use, we ask that you further the development
of the MySQL
software by purchasing some level of support. We feel
that if the MySQL
database helps your business, it is reasonable to
ask that you help MySQL AB
.
(Otherwise, if you ask us support questions, you are not only using
for free something into which we've put a lot a work, you're asking
us to provide free support, too.)
Many users of the MySQL
database want to display the
MySQL AB
dolphin logo on their web sites, books, or
boxed products. We welcome and encourage this, although it should be
noted that the word MySQL
and the MySQL
dolphin logo
are trademarks of MySQL AB
and may only be used as stated in
our trademark policy at
http://www.mysql.com/company/trademark.html.
The MySQL
dolphin logo was designed by the Finnish advertising
agency Priority in 2001. The dolphin was chosen as a suitable symbol
for the MySQL
database since it is a smart, fast, and lean animal,
effortlessly navigating oceans of data. We also happen to like dolphins.
The original MySQL
logo may only be used by representatives of
MySQL AB
and by those having a written agreement allowing them
to do so.
We have designed a set of special Conditional Use logos that may be
downloaded from our web site at
http://www.mysql.com/press/logos.html
and used on third-party web sites without written permission from
MySQL AB
.
The use of these logos is not entirely unrestricted but, as the name
implies, subject to our trademark policy that is also available on our
web site. You should read through the trademark policy if you plan to
use them. The requirements are basically:
MySQL AB
, are the creator and
owner of the site that displays the MySQL
trademark.
MySQL AB
or to the value of MySQL AB
trademarks. We reserve the right to
revoke the right to use the MySQL AB
trademark.
MySQL
database under GPL
in an
application, your application must be Open Source
and
be able to connect to a MySQL
server.
Contact us at [email protected] to inquire about special arrangements to fit your needs.
In the following cases you need a written permission from MySQL AB
before using MySQL
logos:
MySQL AB
logo anywhere except on your web site.
MySQL AB
logo except the Conditional Use
logos mentioned previously on web sites or elsewhere.
Out of legal and commercial reasons we have to monitor the use of MySQL
trademarks on products, books, etc. We will usually require a fee for
displaying MySQL AB
logos on commercial products, since we think
it is reasonable that some of the revenue is returned to fund further
development of the MySQL
database.
MySQL
partnership logos may only be used by companies and persons
having a written partnership agreement with MySQL AB
. Partnerships
include certification as a MySQL
trainer or consultant.
Please see section 1.3.1.5 Partnering.
MySQL
in Printed Text or Presentations
MySQL AB
welcomes references to the MySQL
database, but
note that the word MySQL
is a trademark of MySQL AB
.
Because of this, you should append the trademark symbol (TM
) to
the first or most prominent use of the word MySQL
in a text and
where appropriate, state that MySQL
is a trademark of
MySQL AB
. Please refer to our trademark policy at
http://www.mysql.com/company/trademark.html for details.
MySQL
in Company and Product Names
Use of the word MySQL
in product or company names or in Internet
domain names is not allowed without written permission from MySQL AB
.
Long promised by MySQL AB
and long awaited by our users,
MySQL Server 4.0 is now available in beta version for download from
http://www.mysql.com/ and our mirrors.
Main new features of MySQL Server 4.0 are geared toward our existing business and community users, enhancing the MySQL database software as the solution for mission-critical, heavy-load database systems. Other new features target the users of embedded databases.
The rollout of MySQL Server 4.x comes in several steps, with the first version labelled 4.0.0-alpha already containing most of the new features. Additional features have since been incorporated into MySQL 4.0.1, 4.0.2, and onward; MySQL 4.0.3 has been labelled beta. Further new features are being added in MySQL 4.1, targeted for alpha release in third quarter 2002.
Users are not recommended to switch their production systems to a MySQL Server 4.x until it is released in beta version (such as 4.0.3 beta). However, even the initial release has passed our extensive test suite without any errors on any of the platforms we test on. Due to the large number of new features, we thus recommend MySQL Server 4.x even in alpha form for development use, with the release schedule of MySQL Server 4.x being such that it will reach stable state before the deployment of user applications now under development.
libmysqld
makes MySQL Server suitable for a vastly expanded realm of
applications. Using the embedded MySQL server library, one can
embed MySQL Server into various applications and electronics devices, where
the end user has no knowledge of there actually being an underlying
database. Embedded MySQL Server is ideal for use behind
the scenes in Internet appliances, public kiosks, turnkey
hardware/software combination units, high performance Internet
servers, self-contained databases distributed on CD-ROM, etc.
Many users of libmysqld
will benefit from the MySQL
Dual Licensing. For those not wishing to be bound by the GPL,
the software is also made available under a commercial license.
The embedded MySQL library uses the same interface as the normal
client library, so it is convenient and easy to use. See section 8.4.9 libmysqld, the Embedded MySQL Server Library.
INSERT
s, searching on packed indexes, creation of
FULLTEXT
indexes, as well as COUNT(DISTINCT)
.
InnoDB
table handler is now offered as a feature of the
standard MySQL server, including full support for transactions
and row-level locking
.
latin1_de
, which corrects the German sorting order,
placing German umlauts in the same order as German telephone books.
TRUNCATE TABLE
(like in Oracle) and IDENTITY
as a
synonym for automatically incremented keys (like in Sybase). Many users will
also be happy to learn that MySQL Server now supports the UNION
statement,
a long-awaited standard SQL feature.
DELETE
statements.
By adding support for symbolic linking
to MyISAM
on the table
level (and not just the database level as before), as well as by enabling symlink
handling by default on Windows, we hope to show that we take enhancement
requests seriously.
Functions like SQL_CALC_FOUND_ROWS
and FOUND_ROWS()
make it
possible to know how many rows a query would have returned without a
LIMIT
clause.
For the upcoming MySQL Server 4.x releases, expect the following features now still under development:
fail-safe replication
;
already existing in 4.0, the LOAD DATA FROM MASTER
command
will soon automate slave setup. The online backup
will make
it easy to add a new replication slave without taking down
the master, and have a very low performance penalty on
update-heavy systems.
mysqld
parameters (startup options) can soon be set without
taking down the servers.
FULLTEXT
search properties of MySQL Server 4.0 enable the
use of FULLTEXT
indexing of large text masses with both binary and
natural-language searching logic. Users can customise minimal word
length and define their own stop word lists in any human language,
enabling a new set of applications to be built on MySQL Server.
key cache
.
MySQL command help
in the client.
MySQL Server 4.0 lays the foundation for the new features of
MySQL Server 4.1 and onward,
such as nested subqueries
, stored procedures
, and
foreign key integrity rules
, which form the top of the
wish list for many of our customers. Along with those, we will
also include simpler additions, such as
multi-table UPDATE
statements.
After those additions, critics of the MySQL Database Server have to be more imaginative than ever in pointing out deficiencies in the MySQL Database Management System. For long already known for its stability, speed, and ease of use, MySQL Server will then match the requirement checklist of very demanding buyers.
The MySQL Portals
(http://www.mysql.com/portal/)
represent the ultimate resource to find MySQL AB Partners
,
as well as books, or other MySQL
-related solutions that you
may be looking for. Items are categorised and rated in order to
make it easy for you to locate information.
By registering as a user, you will have the ability to comment on and rate items presented in portals. You will also receive relevant newsletters according to your user profile that you may update at any time.
Some of the current MySQL Portal
categories include:
MySQL AB
partners worldwide.
MySQL
.
MySQL Server
for different purposes, with a description of each site.
This information can give you an idea of who uses the MySQL
database software and how MySQL Server
can fulfill
requirements.
Let us know about your site or success story, too!
Visit http://www.mysql.com/feedback/testimonial.php.
MySQL
server.
MySQL
software.
MySQL
-related services.
This section introduces you to the MySQL mailing lists, and gives some guidelines as to how to use them. By subscribing to a mailing list, you will receive as e-mail messages all other postings on the list, and you will be able to send in your own questions and answers.
To subscribe to the main MySQL mailing list, send a message to the electronic mail address [email protected].
To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address [email protected].
Only the address to which you send your messages is significant. The subject line and the body of the message are ignored.
If your reply address is not valid, you can specify your address
explicitly, by adding a hyphen to the subscribe or unsubscribe command
word, followed by your address with the `@' character in your
address replaced by a `='. For example, to subscribe
[email protected]
, send a message to
[email protected]
.
Mail to [email protected] or [email protected] is handled automatically by the ezmlm mailing list processor. Information about ezmlm is available at the ezmlm web site (http://www.ezmlm.org/).
To post a message to the list itself, send your message to
[email protected]
. However, please do not send mail about
subscribing or unsubscribing to [email protected] because any
mail sent to that address is distributed automatically to thousands of other
users.
Your local site may have many subscribers to [email protected].
If so, it may have a local mailing list, so messages sent from
lists.mysql.com
to your site are propagated to the local list. In such
cases, please contact your system administrator to be added to or dropped
from the local MySQL list.
If you wish to have traffic for a mailing list go to a separate mailbox in
your mail program, set up a filter based on the message headers. You can
use either the List-ID:
or Delivered-To:
headers to identify
list messages.
The following MySQL mailing lists exist:
[email protected] announce
[email protected] mysql
[email protected] mysql-digest
mysql
list in digest form. That means you get all individual
messages, sent as one large mail message once a day.
[email protected] bugs
mysqlbug
script (if you are running on Windows, you should
include a description of the operating system and the MySQL version).
Preferably, you should test the problem using the latest stable or development
version of MySQL Server before posting! Anyone should be able to repeat the
bug by just using mysql test < script
on the included test case. All
bugs posted on this list will be corrected or documented in the next
MySQL release! If only small code changes are needed, we
will also post a patch that fixes the problem.
[email protected] bugs-digest
bugs
list in digest form.
[email protected] internals
[email protected] internals-digest
internals
list.
[email protected] java
[email protected] java-digest
java
list.
[email protected] win32
[email protected] win32-digest
win32
list.
[email protected] myodbc
[email protected] myodbc-digest
myodbc
list.
[email protected] mycc
MyCC
graphical client.
[email protected] mycc-digest
mycc
list.
[email protected] plusplus
[email protected] plusplus-digest
plusplus
list.
[email protected] msql-mysql-modules
[email protected] msql-mysql-modules-digest
msql-mysql-modules
list.
You subscribe or unsubscribe to all lists in the same way as described
previously. In your subscribe or unsubscribe message, just put the appropriate
mailing list name rather than mysql
. For example, to subscribe to or
unsubscribe from the myodbc
list, send a message to
[email protected] or
[email protected].
If you can't get an answer for your questions from the mailing list, one option is to pay for support from MySQL AB, which will put you in direct contact with MySQL developers. See section 1.4.1 Support Offered by MySQL AB.
The following table shows some MySQL mailing in languages other than English. Note that these are not operated by MySQL AB, so we can't guarantee the quality on these.
[email protected] A French mailing list
[email protected] A Korean mailing list
subscribe mysql [email protected]
to this list.
[email protected] A German mailing list
subscribe mysql-de [email protected]
to this list.
You can find information about this mailing list at
http://www.4t2.com/mysql/.
[email protected] A Portugese mailing list
subscribe mysql-br [email protected]
to this list.
[email protected] A Spanish mailing list
subscribe mysql [email protected]
to this list.
Before posting a bug report or question, please do the following:
If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to [email protected].
Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. A good bug report containing a full test case for the bug will make it very likely that we will fix it in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.
We encourage everyone to use the mysqlbug
script to generate a bug
report (or a report about any problem), if possible. mysqlbug
can be
found in the `scripts' directory in the source distribution, or for a
binary distribution, in the `bin' directory under your MySQL
installation directory. If you are unable to use mysqlbug
, you should
still include all the necessary information listed in this section.
The mysqlbug
script helps you generate a report by determining much
of the following information automatically, but if something important is
missing, please include it with your message! Please read this section
carefully and make sure that all the information described here is included
in your report.
The normal place to report bugs and problems is
[email protected]. If you can make a test case that clearly
demonstrates the bug, you should post it to the [email protected]
list. Note that on this list you should only post a full, repeatable bug
report using the mysqlbug
script. If you are running on Windows,
you should include a description of the operating system and the
MySQL version. Preferably, you should test the problem using
the latest stable or development version of MySQL Server before
posting! Anyone should be able to repeat the bug by just using
``mysql test < script
'' on the included test case or run the
shell or Perl script that is included in the bug report. All bugs
posted on the bugs
list will be corrected or documented in the next
MySQL release! If only small code changes are needed
to correct this problem, we will also post a patch that fixes the
problem.
If you have found a sensitive security bug in MySQL, you should send an e-mail to [email protected].
Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time.
The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have the MySQL server installed on (including the platform version number). This is highly relevant information, and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like, ``Why doesn't this work for me?'' Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform-dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform.
Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly.
It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem. See section E.1.6 Making a Test Case When You Experience Table Corruption.
If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report!
If you have a problem with MyODBC, you should try to generate a MyODBC trace file. See section 8.3.7 Reporting Problems with MyODBC.
Please remember that many of the people who will read your report will
do so using an 80-column display. When generating reports or examples
using the mysql
command-line tool, you should therefore use
the --vertical
option (or the \G
statement terminator)
for output that would exceed the available width for such a display
(for example, with the EXPLAIN SELECT
statement; see the
example later in this section).
Please include the following information in your report:
mysqladmin version
. mysqladmin
can be
found in the `bin' directory under your MySQL installation
directory.
uname -a
.
mysqld
died, you should also report the query that crashed
mysqld
. You can usually find this out by running mysqld
with
logging enabled. See section E.1.5 Using Log Files to Find Cause of Errors in mysqld.
mysqldump --no-data db_name tbl_name1 tbl_name2 ...
. This is very easy
to do and is a powerful way to get information about any table in a database
that will help us create a situation matching the one you have.
SELECT
statements, you
should always include the output of EXPLAIN SELECT ...
, and at
least the number of rows that the SELECT
statement produces. You
should also include the output from SHOW CREATE TABLE table_name
for each involved table. The more information you give about your
situation, the more likely it is that someone can help you! For
example, the following is an example of a very good bug report (it
should of course be posted with the mysqlbug
script):
Example run using the mysql
command-line tool (note the use of the
\G
statement terminator for statements whose output width would
otherwise exceed that of an 80-column display device):
mysql> SHOW VARIABLES; mysql> SHOW COLUMNS FROM ...\G <output from SHOW COLUMNS> mysql> EXPLAIN SELECT ...\G <output from EXPLAIN> mysql> FLUSH STATUS; mysql> SELECT ...; <A short version of the output from SELECT, including the time taken to run the query> mysql> SHOW STATUS; <output from SHOW STATUS>
mysqld
, try to provide an
input script that will reproduce the anomaly. This script should include any
necessary source files. The more closely the script can reproduce your
situation, the better. If you can make a reproduceable test case, you should
post this to [email protected] for a high-priority treatment!
If you can't provide a script, you should at least include the output
from mysqladmin variables extended-status processlist
in your mail to
provide some information of how your system is performing!
mysqldump
and create a `README' file
that describes your problem.
Create a compressed archive of your files using
tar
and gzip
or zip
, and use ftp
to transfer the
archive to ftp://support.mysql.com/pub/mysql/secret/. Then send a
short description of the problem to [email protected].
ftp
to transfer it to
ftp://support.mysql.com/pub/mysql/secret/. If the data is really top
secret and you don't want to show it even to us, then go ahead and provide
an example using other names, but please regard this as the last choice.
mysqld
daemon and that you use to run any MySQL client programs. The
options to programs like mysqld
and mysql
, and to the
configure
script, are often keys to answers and are very relevant!
It is never a bad idea to include them anyway! If you use any modules, such
as Perl or PHP, please include the version number(s) of those as well.
mysqlaccess
, the output of mysqladmin reload
, and all
the error messages you get when trying to connect! When you test your
privileges, you should first run mysqlaccess
. After this, execute
mysqladmin reload version
and try to connect with the program that
gives you trouble. mysqlaccess
can be found in the `bin'
directory under your MySQL installation directory.
parse error
, please check your syntax closely! If
you can't find something wrong with it, it's extremely likely that your
current version of MySQL Server doesn't support the query you are
using. If you are using the current version and the manual at
http://www.mysql.com/doc/ doesn't cover the
syntax you are using, MySQL Server doesn't support your query. In this
case, your only options are to implement the syntax yourself or e-mail
[email protected] and ask for an offer to implement it!
If the manual covers the syntax you are using, but you have an older version
of MySQL Server, you should check the MySQL change history to see
when the syntax was implemented. In this case, you have the option of
upgrading to a newer version of MySQL Server. See section D MySQL Change History.
myisamchk
or CHECK TABLE
and
REPAIR TABLE
. See section 4 Database Administration.
mysqld
should never crash a table if nothing killed it in the
middle of an update! If you can find the cause of mysqld
dying,
it's much easier for us to provide you with a fix for the problem.
See section A.1 How to Determine What Is Causing Problems.
If you are a support customer, please cross-post the bug report to [email protected] for higher-priority treatment, as well as to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.
For information on reporting bugs in MyODBC
, see section 8.3.4 How to Report Problems with MyODBC.
For solutions to some common problems, see section A Problems and Common Errors.
When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarise the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem!
If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.
Try to summarise the essential part of the question in your reply; don't feel obliged to quote the entire original message.
Please don't post mail messages from your browser with HTML mode turned on! Many users don't read mail with a browser!
This section describes how MySQL relates to the ANSI SQL standards. MySQL Server has many extensions to the ANSI SQL standards, and here you will find out what they are and how to use them. You will also find information about functionality missing from MySQL Server, and how to work around some differences.
Our goal is to not, without a very good reason, restrict MySQL Server usability for any usage. Even if we don't have the resources to do development for every possible use, we are always willing to help and offer suggestions to people who are trying to use MySQL Server in new territories.
One of our main goals with the product is to continue to work toward
ANSI 99 compliancy, but without sacrificing speed or reliability.
We are not afraid to add extensions to SQL or support for non-SQL
features if this greatly increases the usability of MySQL Server for a big
part of our users. (The new HANDLER
interface in MySQL Server 4.0
is an example of this strategy. See section 6.4.2 HANDLER
Syntax.)
We will continue to support transactional and non-transactional databases to satisfy both heavy web/logging usage and mission-critical 24/7 usage.
MySQL Server was designed from the start to work with medium size databases (10-100 million rows, or about 100 MB per table) on small computer systems. We will continue to extend MySQL Server to work even better with terabyte-size databases, as well as to make it possible to compile a reduced MySQL version that is more suitable for hand-held devices and embedded usage. The compact design of the MySQL server makes both of these directions possible without any conflicts in the source tree.
We are currently not targeting realtime support or clustered databases (even if you can already do a lot of things with our replication services).
We don't believe that one should have native XML support in the database, but will instead add the XML support our users request from us on the client side. We think it's better to keep the main server code as ``lean and clean'' as possible and instead develop libraries to deal with the complexity on the client side. This is part of the strategy mentioned previously of not sacrificing speed or reliability in the server.
Entry-level SQL92. ODBC levels 0-3.51.
We are aiming toward supporting the full ANSI SQL99 standard, but without concessions to speed and quality of the code.
If you start mysqld
with the --ansi
option, the following
behaviour of MySQL Server changes:
||
is string concatenation instead of OR
.
REAL
will be a synonym for FLOAT
instead of a synonym for
DOUBLE
.
SERIALIZABLE
.
See section 6.7.3 SET TRANSACTION
Syntax.
This is the same as using
--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY
.
MySQL Server includes some extensions that you probably will not find in
other SQL databases. Be warned that if you use them, your code will not be
portable to other SQL servers. In some cases, you can write code that
includes MySQL extensions, but is still portable, by using comments
of the form /*! ... */
. In this case, MySQL Server will parse and
execute the code within the comment as it would any other MySQL
statement, but other SQL servers will ignore the extensions. For example:
SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...
If you add a version number after the '!'
, the syntax will be
executed only if the MySQL version is equal to or newer than the used
version number:
CREATE /*!32302 TEMPORARY */ TABLE t (a int);
This means that if you have Version 3.23.02 or newer, MySQL
Server will use the TEMPORARY
keyword.
The following is a list of MySQL extensions:
MEDIUMINT
, SET
, ENUM
, and the
different BLOB
and TEXT
types.
AUTO_INCREMENT
, BINARY
, NULL
,
UNSIGNED
, and ZEROFILL
.
BINARY
attribute or use the BINARY
cast, which causes
comparisons to be done according to the ASCII order used on the
MySQL server host.
db_name.tbl_name
syntax. Some SQL servers provide
the same functionality but call this User space
.
MySQL Server doesn't support tablespaces as in:
create table ralph.my_table...IN my_tablespace
.
LIKE
is allowed on numeric columns.
INTO OUTFILE
and STRAIGHT_JOIN
in a SELECT
statement. See section 6.4.1 SELECT
Syntax.
SQL_SMALL_RESULT
option in a SELECT
statement.
EXPLAIN SELECT
to get a description on how tables are joined.
INDEX
or KEY
in a CREATE TABLE
statement. See section 6.5.3 CREATE TABLE
Syntax.
TEMPORARY
or IF NOT EXISTS
with CREATE TABLE
.
COUNT(DISTINCT list)
where list
is more than one element.
CHANGE col_name
, DROP col_name
, or DROP
INDEX
, IGNORE
or RENAME
in an ALTER TABLE
statement. See section 6.5.4 ALTER TABLE
Syntax.
RENAME TABLE
. See section 6.5.5 RENAME TABLE
Syntax.
ADD
, ALTER
, DROP
, or CHANGE
clauses in an ALTER TABLE
statement.
DROP TABLE
with the keywords IF EXISTS
.
DROP TABLE
statement.
LIMIT
clause of the DELETE
statement.
DELAYED
clause of the INSERT
and REPLACE
statements.
LOW_PRIORITY
clause of the INSERT
, REPLACE
,
DELETE
, and UPDATE
statements.
LOAD DATA INFILE
. In many cases, this syntax is compatible with
Oracle's LOAD DATA INFILE
. See section 6.4.9 LOAD DATA INFILE
Syntax.
ANALYZE TABLE
, CHECK TABLE
, OPTIMIZE TABLE
, and
REPAIR TABLE
statements.
SHOW
statement.
See section 4.5.6 SHOW
Syntax.
SET
statement. See section 5.5.6 SET
Syntax.
GROUP BY
part.
This gives better performance for some very specific, but quite normal
queries.
See section 6.3.7 Functions for Use with GROUP BY
Clauses.
ASC
and DESC
with GROUP BY
.
||
and &&
operators to mean
logical OR and AND, as in the C programming language. In MySQL Server,
||
and OR
are synonyms, as are &&
and AND
.
Because of this nice syntax, MySQL Server doesn't support
the ANSI SQL ||
operator for string concatenation; use
CONCAT()
instead. Because CONCAT()
takes any number
of arguments, it's easy to convert use of the ||
operator to
MySQL Server.
CREATE DATABASE
or DROP DATABASE
.
See section 6.5.1 CREATE DATABASE
Syntax.
%
operator is a synonym for MOD()
. That is,
N % M
is equivalent to MOD(N,M)
. %
is supported
for C programmers and for compatibility with PostgreSQL.
=
, <>
, <=
,<
, >=
,>
,
<<
, >>
, <=>
, AND
, OR
, or LIKE
operators may be used in column comparisons to the left of the
FROM
in SELECT
statements. For example:
mysql> SELECT col1=1 AND col2=2 FROM tbl_name;
LAST_INSERT_ID()
function.
See section 8.4.3.126 mysql_insert_id()
.
REGEXP
and NOT REGEXP
extended regular expression
operators.
CONCAT()
or CHAR()
with one argument or more than two
arguments. (In MySQL Server, these functions can take any number of
arguments.)
BIT_COUNT()
, CASE
, ELT()
,
FROM_DAYS()
, FORMAT()
, IF()
, PASSWORD()
,
ENCRYPT()
, MD5()
, ENCODE()
, DECODE()
,
PERIOD_ADD()
, PERIOD_DIFF()
, TO_DAYS()
, or
WEEKDAY()
functions.
TRIM()
to trim substrings. ANSI SQL only supports removal
of single characters.
GROUP BY
functions STD()
, BIT_OR()
, and
BIT_AND()
.
REPLACE
instead of DELETE
+ INSERT
.
See section 6.4.8 REPLACE
Syntax.
FLUSH
, RESET
and DO
statements.
:=
:
SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg FROM test_table; SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL Server does things differently:
VARCHAR
columns, trailing spaces are removed when the value is
stored. See section 1.7.5 Known Errors and Design Deficiencies in MySQL.
CHAR
columns are silently changed to VARCHAR
columns. See section 6.5.3.1 Silent Column Specification Changes.
REVOKE
to revoke privileges for
a table. See section 4.3.1 GRANT
and REVOKE
Syntax.
NULL AND FALSE
will evaluate to NULL
and not to FALSE
.
This is because we don't think it's good to have to evaluate a lot of
extra conditions in this case.
For a prioritised list indicating when new extensions will be added to MySQL Server, you should consult the online MySQL TODO list at http://www.mysql.com/documentation/manual.php?section=TODO. That is the latest version of the TODO list in this manual. See section 1.8 MySQL and The Future (The TODO).
SELECT
s
MySQL Server currently only supports nested queries of the form
INSERT ... SELECT ...
and REPLACE ... SELECT ...
.
You can, however, use the function IN()
in other contexts.
Subselects are currently being implemented in the 4.1 development tree.
Meanwhile, you can often rewrite the query without a subselect:
SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);
This can be rewritten as:
SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;
The queries:
SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2); SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2 WHERE table1.id=table2.id);
Can be rewritten as:
SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id WHERE table2.id IS NULL;
For more complicated subqueries you can often create temporary tables
to hold the subquery. In some cases, however, this option will not
work. The most frequently encountered of these cases arises with
DELETE
statements, for which standard SQL does not support joins
(except in subselects). For this situation there are two options
available until subqueries are supported by MySQL Server.
The first option is to use a procedural programming language (such as
Perl or PHP) to submit a SELECT
query to obtain the primary keys
for the records to be deleted, and then use these values to construct
the DELETE
statement (DELETE FROM ... WHERE ... IN (key1,
key2, ...)
).
The second option is to use interactive SQL to construct a set of
DELETE
statements automatically, using the MySQL
extension CONCAT()
(in lieu of the standard ||
operator).
For example:
SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', "'", tab1.pkid, "'", ';') FROM tab1, tab2 WHERE tab1.col1 = tab2.col2;
You can place this query in a script file and redirect input from it to
the mysql
command-line interpreter, piping its output back to a
second instance of the interpreter:
shell> mysql --skip-column-names mydb < myscript.sql | mysql mydb
MySQL Server 4.0 supports multi-table deletes that can be used to efficiently delete rows based on information from one table or even from many tables at the same time.
SELECT INTO TABLE
MySQL Server doesn't yet support the Oracle SQL extension:
SELECT ... INTO TABLE ...
. MySQL Server supports instead the
ANSI SQL syntax INSERT INTO ... SELECT ...
, which is basically
the same thing. See section 6.4.3.1 INSERT ... SELECT
Syntax.
INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE tblTemp1.fldOrder_ID > 100;
Alternatively, you can use SELECT INTO OUTFILE...
or CREATE
TABLE ... SELECT
.
MySQL Server supports transactions with the InnoDB
and BDB
Transactional table handlers
. See section 7 MySQL Table Types.
InnoDB
provides ACID
compliancy.
However, the non-transactional table types in MySQL Server such as
MyISAM
follow another paradigm for data integrity called
``Atomic Operations
.'' Atomic operations often offer equal or
even better integrity with much better performance.
With MySQL Server supporting both paradigms, the user is able to decide if
he needs the speed of atomic operations or if he need to use
transactional features in his applications. This choice can be made
on a per-table basis.
How does one use the features of MySQL Server to maintain rigorous integrity and how do these features compare with the transactional paradigm?
ROLLBACK
instead of
COMMIT
in critical situations, transactions are more
convenient. Transactions also ensure that unfinished updates or
corrupting activities are not committed to the database; the server is
given the opportunity to do an automatic rollback and your database is
saved.
MySQL Server, in almost all cases, allows you to resolve potential problems
by including simple checks before updates and by running simple scripts
that check the databases for inconsistencies and automatically repair
or warn if such an inconsistency occurs. Note that just by using the
MySQL log or even adding one extra log, one can normally fix tables
perfectly with no data integrity loss.
LOCK TABLES
or atomic updates, ensuring
that you never will get an automatic abort from the database, which is
a common problem with transactional databases.
The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that non-transactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.
In situations where integrity is of highest importance, MySQL Server offers
transaction-level or better reliability and integrity even for
non-transactional tables.
If you lock tables with LOCK TABLES
, all updates will stall
until any integrity checks are made. If you only obtain a read lock
(as opposed to a write lock), reads and inserts are still allowed
to happen. The new inserted records will not be seen by any of the
clients that have a read lock until they release their read
locks. With INSERT DELAYED
you can queue inserts into a local
queue, until the locks are released, without having the client wait
for the insert to complete. See section 6.4.4 INSERT DELAYED
Syntax.
``Atomic,'' in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen with transactional tables if you are not very careful). MySQL Server also guarantees that there will not be any dirty reads.
Following are some techniques for working with non-transactional tables:
LOCK TABLES
, and you don't need cursors when you can update
records on the fly.
ROLLBACK
, you can use the following strategy:
LOCK TABLES ...
to lock all the tables you want to access.
UNLOCK TABLES
to release your locks.
ROLLBACK
s, although not always. The only situation
this solution doesn't handle is when someone kills the threads in the
middle of an update. In this case, all locks will be released but some
of the updates may not have been executed.
WHERE
clause in the UPDATE
statement. If the record wasn't
updated, we give the client a message: "Some of the data you have changed
has been changed by another user." Then we show the old row versus the new
row in a window, so the user can decide which version of the customer record
he should use.
This gives us something that is similar to column locking but is actually
even better because we only update some of the columns, using values that
are relative to their current values. This means that typical UPDATE
statements look something like these:
UPDATE tablename SET pay_back=pay_back+'relative change'; UPDATE customer SET customer_date='current_date', address='new address', phone='new phone', money_he_owes_us=money_he_owes_us+'new_money' WHERE customer_id=id AND address='old address' AND phone='old phone';As you can see, this is very efficient and works even if another client has changed the values in the
pay_back
or money_he_owes_us
columns.
ROLLBACK
and/or LOCK
TABLES
for the purpose of managing unique identifiers for some tables.
This can be handled much more efficiently by using an
AUTO_INCREMENT
column and either the SQL function
LAST_INSERT_ID()
or the C API function mysql_insert_id()
.
See section 8.4.3.126 mysql_insert_id()
.
You can generally code around row-level locking. Some situations really
need it, but they are very few. InnoDB
tables support row-level
locking. With MyISAM, you can use a flag column in the table and do
something like the following:
UPDATE tbl_name SET row_flag=1 WHERE id=ID;MySQL returns 1 for the number of affected rows if the row was found and
row_flag
wasn't already 1 in the original row.
You can think of it as though MySQL Server changed the preceding query to:
UPDATE tbl_name SET row_flag=1 WHERE id=ID AND row_flag <> 1;
A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep re-issuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.
A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.
The planned update language will be able to handle stored procedures. Our aim is to have stored procedures implemented in MySQL Server around version 5.0. We are also looking at triggers.
Note that foreign keys in SQL are not used to join tables, but are used
mostly for checking referential integrity (foreign key constraints). If
you want to get results from multiple tables from a SELECT
statement, you do this by joining tables:
SELECT * FROM table1,table2 WHERE table1.id = table2.id;
See section 6.4.1.1 JOIN
Syntax. See section 3.5.6 Using Foreign Keys.
In MySQL Server 3.23.44 and up, InnoDB
tables support checking of
foreign key constraints. See section 7.5 InnoDB
Tables. For other table types, MySQL Server
does parse the FOREIGN KEY
syntax in CREATE TABLE
commands, but without further action being taken.
The FOREIGN KEY
syntax without ON DELETE ...
is mostly
used for documentation purposes. Some ODBC applications may use this
to produce automatic WHERE
clauses, but this is usually easy to
override. FOREIGN KEY
is sometimes used as a constraint check,
but this check is unnecessary in practice if rows are inserted into the
tables in the right order.
In MySQL Server, you can work around the problem of ON DELETE ...
not
being implemented by adding the appropriate DELETE
statement to
an application when you delete records from a table that has a foreign
key. In practice this is as quick (in some cases quicker) and much more
portable than using foreign keys.
In MySQL Server 4.0 you can use multi-table delete to delete rows from many
tables with one command. See section 6.4.6 DELETE
Syntax.
In the near future we will extend the FOREIGN KEY
implementation
so that the information will be saved in the table specification file
and may be retrieved by mysqldump
and ODBC. At a later stage we
will implement the foreign key constraints for applications that can't
easily be coded to avoid them.
Do keep in mind that foreign keys are often misused, which can cause severe problems. Even when used properly, it is not a magic solution for the referential integrity problem, although it does make things easier in some cases.
Some advantages of foreign key enforcement:
Disadvantages:
It is planned to implement views in MySQL Server around version 5.0.
Views are mostly useful for letting users access a set of relations as one table (in read-only mode). Many SQL databases don't allow one to update any rows in a view, but you have to do the updates in the separate tables.
As MySQL Server is mostly used in applications and on web systems where the application writer has full control on the database usage, most of our users haven't regarded views to be very important. (At least no one has been interested enough in this to be prepared to finance the implementation of views.)
One doesn't need views in MySQL Server to restrict access to columns, as MySQL Server has a very sophisticated privilege system. See section 4.2 General Security Issues and the MySQL Access Privilege System.
Some other SQL databases use `--' to start comments.
MySQL Server has `#' as the start comment character. You can also use
the C comment style /* this is a comment */
with MySQL Server.
See section 6.1.6 Comment Syntax.
MySQL Server Version 3.23.3 and above support the `--' comment style,
provided the comment is followed by a space. This is because this
comment style has caused many problems with automatically generated
SQL queries that have used something like the following code, where
we automatically insert the value of the payment for
!payment!
:
UPDATE tbl_name SET credit=credit-!payment!
Think about what happens if the value of payment
is negative.
Because 1--1
is legal in SQL, the consequences of allowing
comments to start with `--' are terrible.
Using our implementation of this method of commenting in MySQL Server
Version 3.23.3 and up, 1-- This is a comment
is actually safe.
Another safe feature is that the mysql
command-line client
removes all lines that start with `--'.
The following information is relevant only if you are running a MySQL version earlier than 3.23.3:
If you have a SQL program in a text file that contains `--' comments you should use:
shell> replace " --" " #" < text-file-with-funny-comments.sql \ | mysql database
instead of the usual:
shell> mysql database < text-file-with-funny-comments.sql
You can also edit the command file ``in place'' to change the `--' comments to `#' comments:
shell> replace " --" " #" -- text-file-with-funny-comments.sql
Change them back with this command:
shell> replace " #" " --" -- text-file-with-funny-comments.sql
The following problems are known and have a very high priority to get fixed:
ANALYZE TABLE
on a BDB table may in some case make the table
unusable until one has restarted mysqld
. When this happens you will
see errors like the following in the MySQL error file:
001207 22:07:56 bdb: log_flush: LSN past current end-of-log
ALTER TABLE
on a BDB
table on which you are
running multi-statement transactions until all those transactions complete.
(The transaction will probably be ignored.)
ANALYZE TABLE
, OPTIMIZE TABLE
, and REPAIR TABLE
may
cause problems on tables for which you are using INSERT DELAYED
.
LOCK TABLE ...
and FLUSH TABLES ...
doesn't
guarantee that there isn't a half-finished transaction in progress on the
table.
mysql
client on the
database if you are not using the -A
option or if you are using
rehash
. This is especially notable when you have a big table
cache.
The following problems are known and will be fixed in due time:
SET CHARACTER SET
, one can't use translated
characters in database, table, and column names.
_
or %
with ESCAPE
in LIKE
... ESCAPE
.
DECIMAL
column with a number stored in different
formats (+01.00, 1.00, 01.00), GROUP BY
may regard each value
as a different value.
DELETE FROM merge_table
used without a WHERE
will only clear the mapping for the table, not delete everything in the
mapped tables.
BLOB
values can't ``reliably'' be used in GROUP BY
or
ORDER BY
or DISTINCT
. Only the first max_sort_length
bytes (default 1024) are used when comparing BLOB
s in these cases.
This can be changed with the -O max_sort_length
option to
mysqld
. A workaround for most cases is to use a substring:
SELECT DISTINCT LEFT(blob,2048) FROM tbl_name
.
BIGINT
or DOUBLE
(both are
normally 64 bits long). It depends on the function which precision one
gets. The general rule is that bit functions are done with BIGINT
precision, IF
, and ELT()
with BIGINT
or DOUBLE
precision and the rest with DOUBLE
precision. One should try to
avoid using unsigned long long values if they resolve to be bigger than
63 bits (9223372036854775807) for anything else than bit fields!
MySQL Server 4.0 has better BIGINT
handling than 3.23.
BLOB
and TEXT
columns, automatically
have all trailing spaces removed when retrieved. For CHAR
types this
is okay, and may be regarded as a feature according to ANSI SQL92. The bug is
that in MySQL Server, VARCHAR
columns are treated the same way.
ENUM
and SET
columns in one table.
MIN()
, MAX()
and other aggregate functions, MySQL
currently compares ENUM
and SET
columns by their string
value rather than by the string's relative position in the set.
safe_mysqld
redirects all messages from mysqld
to the
mysqld
log. One problem with this is that if you execute
mysqladmin refresh
to close and reopen the log,
stdout
and stderr
are still redirected to the old log.
If you use --log
extensively, you should edit safe_mysqld
to
log to `'hostname'.err' instead of `'hostname'.log' so you can
easily reclaim the space for the old log by deleting the old one and
executing mysqladmin refresh
.
UPDATE
statement, columns are updated from left to right. If
you refer to an updated column, you will get the updated value instead of the
original value. For example:
mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1;This will update
KEY
with 2
instead of with 1
.
mysql> SELECT * FROM temporary_table, temporary_table AS t2;
RENAME
doesn't work with TEMPORARY
tables or tables used in a
MERGE
table.
DISTINCT
differently if you are using
'hidden' columns in a join or not. In a join, hidden columns are
counted as part of the result (even if they are not shown) while in
normal queries hidden columns don't participate in the DISTINCT
comparison. We will probably change this in the future to never compare
the hidden columns when executing DISTINCT
.
An example of this is:
SELECT DISTINCT mp3id FROM band_downloads WHERE userid = 9 ORDER BY id DESC;and
SELECT DISTINCT band_downloads.mp3id FROM band_downloads,band_mp3 WHERE band_downloads.userid = 9 AND band_mp3.id = band_downloads.mp3id ORDER BY band_downloads.id DESC;In the second case you may in MySQL Server 3.23.x get two identical rows in the result set (because the hidden
id
column may differ).
Note that this happens only for queries where you don't have the
ORDER BY columns in the result, something that you are not allowed
to do in ANSI SQL.
rollback
data, some things
behave a little differently in MySQL Server than in other SQL servers.
This is just to ensure that MySQL Server never needs to do a rollback
for a SQL command. This may be a little awkward at times as column
values must be checked in the application, but this will actually give
you a nice speed increase as it allows MySQL Server to do some
optimisations that otherwise would be very hard to do.
If you set a column to an incorrect value, MySQL Server will, instead of
doing a rollback, store the best possible value
in the column:
NULL
into a column that doesn't take
NULL
values, MySQL Server will store 0 or ''
(empty
string) in it instead. (This behaviour can, however, be changed with the
-DDONT_USE_DEFAULT_FIELDS compile option.)
DATE
and
DATETIME
columns (like 2000-02-31 or 2000-02-00). The idea is
that it's not the SQL server job to validate date. If MySQL can store a
date and retrieve exactly the same date, then MySQL will store the
date. If the date is totally wrong (outside the server's ability to store
it), then the special date value 0000-00-00 will be stored in the column.
ENUM
column to an unsupported value, it will be set to
the error value empty string
, with numeric value 0.
SET
column to an unsupported value, the value will
be ignored.
PROCEDURE
on a query that returns an empty set,
in some cases the PROCEDURE
will not transform the columns.
MERGE
doesn't check if the underlying
tables are of compatible types.
NaN
, -Inf
, and Inf
values in double. Using these will cause problems when trying to export
and import data. We should as an intermediate solution change NaN
to
NULL
(if possible) and -Inf
and Inf
to the
minimum respective maximum possible double
value.
LIMIT
on negative numbers are treated as big positive numbers.
ALTER TABLE
to first add a UNIQUE
index to a
table used in a MERGE
table and then use ALTER TABLE
to
add a normal index on the MERGE
table, the key order will be
different for the tables if there was an old key that was not unique in the
table. This is because ALTER TABLE
puts UNIQUE
keys before
normal keys to be able to detect duplicate keys as early as possible.
The following are known bugs in earlier versions of MySQL:
DROP TABLE
on a table that is
one among many tables that is locked with LOCK TABLES
.
LOCK table
with WRITE
.
FLUSH TABLES
.
UPDATE
that updated a key with
a WHERE
on the same key may have failed because the key was used to
search for records and the same row may have been found multiple times:
UPDATE tbl_name SET KEY=KEY+1 WHERE KEY > 100;A workaround is to use:
mysql> UPDATE tbl_name SET KEY=KEY+1 WHERE KEY+0 > 100;This will work because MySQL Server will not use an index on expressions in the
WHERE
clause.
For platform-specific bugs, see the sections about compiling and porting.
This section lists the features that we plan to implement in MySQL Server.
Everything in this list is approximately in the order it will be done. If you want to affect the priority order, please register a license or support us and tell us what you want to have done more quickly. See section 1.4 MySQL Support and Licensing.
The plan is that we in the future will support the full ANSI SQL99 standard, but with a lot of useful extensions. The challenge is to do this without sacrificing the speed or compromising the code.
All done. We now only do bug fixes for MySQL 4.0. See section D.1 Changes in release 4.0.x (Beta). Development has shifted to 4.1
The following features are planned for inclusion into MySQL 4.1. Note that because we have many developers that are working on different projects, there will also be many additional features. There is also a small chance that some of these features will be added to MySQL 4.0. Some of the work on MySQL 4.1 is already in progress.
SELECT id FROM t WHERE grp IN (SELECT grp FROM g WHERE u > 100);
FOREIGN KEY
definitions.
SHOW COLUMNS FROM table_name
(used by mysql
client to allow
expansions of column names) should not open the table, only the
definition file. This will require less memory and be much faster.
MyISAM
tables, including cascading delete.
RAND()
and user variables @var
.
SELECT a.col1, b.col2 FROM (SELECT MAX(col1) AS col1 FROM root_table) a, other_table b WHERE a.col1=b.col1;This could be done by automatically creating temporary tables for the derived tables for the duration of the query.
ROLLUP
and CUBE
OLAP (Online Analytical Processing) grouping
options for data warehousing applications.
DELETE
on MyISAM
tables to use the record cache.
To do this, we need to update the threads record cache when we update
the `.MYD' file.
SET CHARACTER SET
we should translate the whole query
at once and not only strings. This will enable users to use the translated
characters in database, table, and column names.
record_in_range()
method to MERGE
tables to be
able to choose the right index when there are many to choose from. We should
also extend the info interface to get the key distribution for each index,
if analyze
is run on all subtables.
RENAME TABLE
on a table used in an active
MERGE
table possibly corrupting the table.
UNICODE
.
RENAME DATABASE
. To make this safe for all table handlers,
it should work as follows:
RENAME
command.
VARCHAR
support (there is already support for this in
MyISAM
).
BIT
type to take 1 bit (now BIT
takes 1 char).
HEAP
) tables:
UPDATE items,month SET items.price=month.price WHERE items.id=month.id;
INSERT ... SELECT
to optionally use concurrent inserts.
SELECT MIN(column)
... GROUP BY
.
long_query_time
with a granularity
in microseconds.
myisampack
code into the server.
INSERT/DELETE/UPDATE
so that we
can gracefully recover if the index file gets full.
ALTER TABLE
on a table that is symlinked to another
disk, create temporary tables on this disk.
DATE/DATETIME
type that handles time zone information
properly so that dealing with dates in different time zones is easier.
MyISAM
)
without threads.
INSERT SQL_CONCURRENT
and mysqld --concurrent-insert
to do
a concurrent insert at the end of the file if the file is read-locked.
lockd
works with modern Linux kernels; if not, we have
to fix lockd
! To test this, start mysqld
with
--enable-locking
and run the different fork* test suits. They shouldn't
give any errors if lockd
works.
LIMIT
, like in LIMIT @a,@b
.
UPDATE
statements. For example:
UPDATE TABLE foo SET @a=a+b,a=@a, b=@a+c
.
GROUP BY
, as in the following example:
SELECT id, @a:=COUNT(*), SUM(sum_col)/@a FROM table_name GROUP BY id
.
DEFAULT
values to columns. Give an error when using
an INSERT
that doesn't contain a column that doesn't have a
DEFAULT
.
mysql_query()
commands in a row
without reading results or give a nice error message when one does this.
ctime()
doesn't work on some FreeBSD systems.
IMAGE
option to LOAD DATA INFILE
to not update
TIMESTAMP
and AUTO_INCREMENT
fields.
LOAD DATE INFILE ... UPDATE
syntax.
LOAD DATA INFILE ... REPLACE INTO
now.
LOAD DATA INFILE
understand syntax like:
LOAD DATA INFILE 'file_name.txt' INTO TABLE tbl_name TEXT_FIELDS (text_field1, text_field2, text_field3) SET table_field1=CONCAT(text_field1, text_field2), table_field3=23 IGNORE text_field3This can be used to skip over extra columns in the text file, or update columns based on expressions of the read data.
LOAD DATA INFILE 'file_name' INTO TABLE 'table_name' ERRORS TO err_table_name
.
This would cause any errors and warnings to be logged into the
err_table_name
table. That table would have a structure like:
line_number - line number in datafile error_message - the error/warning message and maybe data_line - the line from the datafile
mysql
to Netscape.
LOCK DATABASES
(with various options.)
t1 JOIN t2 ON ...
and t1 JOIN t2 USING ...
Currently, you can only use this syntax with LEFT JOIN
.
show status
. Records reads and
updates. Selects on 1 table and selects with joins. Mean number of
tables in select. Number of ORDER BY
and GROUP BY
queries.
mysql
in the middle of a query, you should open
another connection and kill the old running query.
Alternatively, an attempt should be made to detect this in the server.
SHOW INFO FROM tbl_name
for basic table information
should be implemented.
NATURAL JOIN
.
SELECT a FROM crash_me LEFT JOIN crash_me2 USING (a)
; in this
case a
is assumed to come from the crash_me
table.
ON
and USING
works with the JOIN
join type.
CONNECT BY PRIOR ...
to search hierarchy structures.
mysqladmin copy database new-database
; requires COPY
command to be added to mysqld
.
SHOW HOSTS
for printing information about the hostname cache.
DELETE
and REPLACE
options to the UPDATE
statement
(this will delete rows when one gets a duplicate key error while updating).
DATETIME
to store fractions of seconds.
NULL
for calculated columns.
Item_copy_string
on numerical values to avoid
number->string->number conversion in case of:
SELECT COUNT(*)*(id+0) FROM table_name GROUP BY id
ALTER TABLE
doesn't abort clients
that execute INSERT DELAYED
.
UPDATE
clause,
they contain the old values from before the update started.
pread()
/pwrite()
on Windows to enable
concurrent inserts.
SUM(DISTINCT)
.
ANY()
, EVERY()
, and SOME()
group functions. In
ANSI SQL these work only on boolean columns, but we can extend these to
work on any columns/expressions by applying: value == 0 -> FALSE and
value <> 0 -> TRUE.
MAX(column)
is the same as the column type:
mysql> CREATE TABLE t1 (a DATE); mysql> INSERT INTO t1 VALUES (NOW()); mysql> CREATE TABLE t2 SELECT MAX(a) FROM t1; mysql> SHOW COLUMNS FROM t2;
UPDATE
the row
if it exists and INSERT
a new row if the row didn't exist
(like REPLACE
works with INSERT
/ DELETE
).
get_changed_tables(timeout,table1,table2,...)
.
SET TIMESTAMP=#;
.
MyISAM
tables, probably after
the implementation of stored procedures with triggers.
MINUS
, INTERSECT
, and FULL OUTER JOIN
.
(Currently UNION
[in 4.0] and LEFT OUTER JOIN
are supported.)
SQL_OPTION MAX_SELECT_TIME=#
to put a time limit on a query.
LIMIT
to allow retrieval of data from the end of a result set.
safe_mysqld
: according to FSSTND (which
Debian tries to follow) PID files should go into `/var/run/<progname>.pid'
and log files into `/var/log'. It would be nice if you could put the
"DATADIR" in the first declaration of "pidfile" and "log", so the
placement of these files can be changed with a single statement.
zlib()
for gzip
-ed files to LOAD DATA INFILE
.
BLOB
columns (partly solved now).
AUTO_INCREMENT
value when one sets a column to 0.
Use NULL
instead.
JOIN
with parentheses.
GET_LOCK
. When doing this,
one must also handle the possible deadlocks this change will introduce.
Time is given according to amount of work, not real time.
Our users have successfully run their own benchmarks against a number
of Open Source
and traditional database servers.
We are aware of tests against Oracle
server, DB/2
server,
Microsoft SQL Server
, and other commercial products.
Due to legal reasons we are restricted from publishing some of those
benchmarks in our reference manual.
This section includes a comparison with mSQL
for historical
reasons and with PostgreSQL
as it is also an Open Source
database. If you have benchmark results that we can publish, please
contact us at [email protected].
For comparative lists of all supported functions and types as well
as measured operational limits of many different database systems,
see the crash-me
web page at
http://www.mysql.com/information/crash-me.php.
mSQL
mSQL
should be quicker at:
INSERT
operations into very simple tables with few columns and keys.
CREATE TABLE
and DROP TABLE
.
SELECT
on something that isn't an index. (A table scan is very
easy.)
mSQL
(and
most other SQL implementations) on the following:
SELECT
operations.
VARCHAR
columns.
SELECT
with many expressions.
SELECT
on large tables.
mSQL
, once one
connection is established, all others must wait until the first has
finished, regardless of whether the connection is running a query
that is short or long. When the first connection terminates, the
next can be served, while all the others wait again, etc.
mSQL
can become pathologically slow if you change the order of
tables in a SELECT
. In the benchmark suite, a time more than
15,000 times slower than MySQL Server was seen. This is due to mSQL
's
lack of a join optimiser to order tables in the optimal order.
However, if you put the tables in exactly the right order in
mSQL
2 and the WHERE
is simple and uses index columns,
the join will be relatively fast!
See section 5.1.4 The MySQL Benchmark Suite.
ORDER BY
and GROUP BY
.
DISTINCT
.
TEXT
or BLOB
columns.
GROUP BY
and HAVING
.
mSQL
does not support GROUP BY
at all.
MySQL Server supports a full GROUP BY
with both HAVING
and
the following functions: COUNT()
, AVG()
, MIN()
,
MAX()
, SUM()
, and STD()
. COUNT(*)
is
optimised to return very quickly if the SELECT
retrieves from
one table, no other columns are retrieved, and there is no
WHERE
clause. MIN()
and MAX()
may take string
arguments.
INSERT
and UPDATE
with calculations.
MySQL Server can do calculations in an INSERT
or UPDATE
.
For example:
mysql> UPDATE SET x=x*10+y WHERE x<20;
SELECT
with functions.
MySQL Server has many functions (too many to list here; see section 6.3 Functions for Use in SELECT
and WHERE
Clauses).
MEDIUMINT
that is 3 bytes long. If you have 100 million
records, saving even 1 byte per record is very important.
mSQL2
has a more limited set of column types, so it is
more difficult to get small tables.
mSQL
stability, so we cannot say
anything about that.
mSQL
, and is also less expensive
than mSQL
. Whichever product you choose to use, remember to
at least consider paying for a license or e-mail support.
mSQL
with
some added features.
mSQL
has a JDBC driver, but we have too little
experience with it to compare.
GROUP BY
, and so on are still not implemented in mSQL
, it
has a lot of catching up to do. To get some perspective on this, you
can view the mSQL
`HISTORY' file for the last year and
compare it with the News section of the MySQL Reference Manual
(see section D MySQL Change History). It should be pretty obvious which one has developed
most rapidly.
mSQL
and MySQL Server have many interesting third-party
tools. Because it is very easy to port upward (from mSQL
to
MySQL Server), almost all the interesting applications that are available for
mSQL
are also available for MySQL Server.
MySQL Server comes with a simple msql2mysql
program that fixes
differences in spelling between mSQL
and MySQL Server for the
most-used C API functions.
For example, it changes instances of msqlConnect()
to
mysql_connect()
. Converting a client program from mSQL
to
MySQL Server usually requires only minor effort.
mSQL
Tools for MySQL
According to our experience, it doesn't take long to convert tools
such as msql-tcl
and msqljava
that use the
mSQL
C API so that they work with the MySQL C API.
The conversion procedure is:
msql2mysql
on the source. This requires
the replace
program, which is distributed with MySQL Server.
Differences between the mSQL
C API and the MySQL C API are:
MYSQL
structure as a connection type (mSQL
uses an int
).
mysql_connect()
takes a pointer to a MYSQL
structure as a
parameter. It is easy to define one globally or to use malloc()
to get one. mysql_connect()
also takes two parameters for
specifying the user and password. You may set these to
NULL, NULL
for default use.
mysql_error()
takes the MYSQL
structure as a parameter.
Just add the parameter to your old msql_error()
code if you are
porting old code.
mSQL
returns only a text error message.
mSQL
and MySQL Client/Server Communications Protocols DifferThere are enough differences that it is impossible (or at least not easy) to support both.
The most significant ways in which the MySQL protocol differs
from the mSQL
protocol are listed here:
mSQL
2.0 SQL Syntax Differs from MySQLColumn types
MySQL Server
CREATE TABLE
Syntax):
ENUM
type for one of a set of strings.
SET
type for many of a set of strings.
BIGINT
type for 64-bit integers.
UNSIGNED
option for integer and floating-point columns.
ZEROFILL
option for integer columns.
AUTO_INCREMENT
option for integer columns that are a
PRIMARY KEY
.
See section 8.4.3.126 mysql_insert_id()
.
DEFAULT
value for all columns.
mSQL2
mSQL
column types correspond to the MySQL types shown in the following table:
mSQL type | Corresponding MySQL type |
CHAR(len) | CHAR(len)
|
TEXT(len) | TEXT(len) . len is the maximal length.
And LIKE works.
|
INT | INT . With many more options!
|
REAL | REAL . Or FLOAT . Both 4- and 8-byte versions are available.
|
UINT | INT UNSIGNED
|
DATE | DATE . Uses ANSI SQL format rather than mSQL 's own format.
|
TIME | TIME
|
MONEY | DECIMAL(12,2) . A fixed-point value with two decimals.
|
Index Creation
MySQL Server
CREATE TABLE
statement.
mSQL
CREATE INDEX
statements.
To Insert a Unique Identifier into a Table
MySQL Server
AUTO_INCREMENT
as a column type
specifier.
See section 8.4.3.126 mysql_insert_id()
.
mSQL
SEQUENCE
on a table and select the _seq
column.
To Obtain a Unique Identifier for a Row
MySQL Server
PRIMARY KEY
or UNIQUE
key to the table and use this.
New in Version 3.23.11: If the PRIMARY
or UNIQUE
key consists of only one
column and this is of type integer, one can also refer to it as
_rowid
.
mSQL
_rowid
column. Observe that _rowid
may change over time
depending on many factors.
To Get the Time a Column Was Last Modified
MySQL Server
TIMESTAMP
column to the table. This column is automatically set
to the current date and time for INSERT
or UPDATE
statements if
you don't give the column a value or if you give it a NULL
value.
mSQL
_timestamp
column.
NULL
Value Comparisons
MySQL Server
NULL
is always NULL
.
mSQL
mSQL
, NULL = NULL
is TRUE. You
must change =NULL
to IS NULL
and <>NULL
to
IS NOT NULL
when porting old code from mSQL
to MySQL Server.
String Comparisons
MySQL Server
BINARY
attribute, which causes comparisons to be done according to the
ASCII order used on the MySQL server host.
mSQL
Case-insensitive Searching
MySQL Server
LIKE
is a case-insensitive or case-sensitive operator, depending on
the columns involved. If possible, MySQL uses indexes if the
LIKE
argument doesn't start with a wildcard character.
mSQL
CLIKE
.
Handling of Trailing Spaces
MySQL Server
CHAR
and VARCHAR
columns. Use a TEXT
column if this behaviour is not desired.
mSQL
WHERE
Clauses
MySQL Server
AND
is evaluated
before OR
). To get mSQL
behaviour in MySQL Server, use
parentheses (as shown in an example later in this section).
mSQL
mSQL
query:
mysql> SELECT * FROM table WHERE a=1 AND b=2 OR a=3 AND b=4;To make MySQL Server evaluate this the way that
mSQL
would,
you must add parentheses:
mysql> SELECT * FROM table WHERE (a=1 AND (b=2 OR (a=3 AND (b=4))));
Access Control
MySQL Server
mSQL
PostgreSQL
When reading the following, please note that both products are continually evolving. We at MySQL AB and the PostgreSQL developers are both working on making our respective databases as good as possible, so we are both a serious alternative to any commercial database.
The following comparison is made by us at MySQL AB. We have tried to be as accurate and fair as possible, but although we know MySQL Server thoroughly, we don't have a full knowledge of all PostgreSQL features, so we may have got some things wrong. We will, however, correct these when they come to our attention.
We would first like to note that PostgreSQL and MySQL Server are both widely used
products, but with different design goals, even if we are both striving
toward ANSI SQL compliancy. This means that for some applications MySQL Server
is more suited, while for others PostgreSQL is more suited. When choosing
which database to use, you should first check if the database's feature set
satisfies your application. If you need raw speed, MySQL Server is probably your
best choice. If you need some of the extra features that only PostgreSQL
can offer, you should use PostgreSQL
.
When adding things to MySQL Server we take pride to do an optimal, definite solution. The code should be so good that we shouldn't have any need to change it in the foreseeable future. We also do not like to sacrifice speed for features but instead will do our utmost to find a solution that will give maximal throughput. This means that development will take a little longer, but the end result will be well worth this. This kind of development is only possible because all server code are checked by one of a few (currently two) persons before it's included in the MySQL server.
We at MySQL AB believe in frequent releases to be able to push out new features quickly to our users. Because of this we do a new small release about every three weeks, and a major branch every year. All releases are thoroughly tested with our testing tools on a lot of different platforms.
PostgreSQL is based on a kernel with lots of contributors. In this setup it makes sense to prioritise adding a lot of new features, instead of implementing them optimally, because one can always optimise things later if there arises a need for this.
Another big difference between MySQL Server and PostgreSQL is that nearly all of the code in the MySQL server is coded by developers that are employed by MySQL AB and are still working on the server code. The exceptions are the transaction engines and the regexp library.
This is in sharp contrast to the PostgreSQL code, the majority of which is coded by a big group of people with different backgrounds. It was only recently that the PostgreSQL developers announced that their current developer group had finally had time to take a look at all the code in the current PostgreSQL release.
Both of the aforementioned development methods have their own merits and drawbacks. We here at MySQL AB think, of course, that our model is better because our model gives better code consistency, more optimal and reusable code, and in our opinion, fewer bugs. Because we are the authors of the MySQL server code, we are better able to coordinate new features and releases.
On the crash-me
page
(http://www.mysql.com/information/crash-me.php)
you can find a list of those database constructs and limits that
one can detect automatically with a program. Note, however, that a lot of
the numerical limits may be changed with startup options for their respective
databases. This web page is, however, extremely useful when you want to
ensure that your applications work with many different databases or
when you want to convert your application from one database to another.
MySQL Server offers the following advantages over PostgreSQL:
MySQL
Server is generally much faster than PostgreSQL. MySQL
4.0.1 also has a query cache that can boost up the query speed for
mostly-read-only sites many times.
Cygwin
emulation. We have
heard that PostgreSQL is not yet that stable on Windows but we haven't
been able to verify this ourselves.
VACUUM
once in a while to reclaim space from UPDATE
and DELETE
commands and to perform statistics analyses that are critical to get
good performance with PostgreSQL. VACUUM
is also needed after
adding a lot of new rows to a table. On a busy system with lots of changes,
VACUUM
must be run very frequently, in the worst cases even
many times a day. During the VACUUM
run, which may take hours
if the database is big, the database is, from a production standpoint,
practically dead. Please note: in PostgreSQL version 7.2, basic vacuuming
no longer locks tables, thus allowing normal user access during the vacuum.
A new VACUUM FULL
command does old-style vacuum by locking the table
and shrinking the on-disk copy of the table.
crash-me
(http://www.mysql.com/information/crash-me.php), as well
as a benchmark suite. The test system is actively updated with code to
test each new feature and almost all reproduceable bugs that have come to
our attention. We test MySQL Server with these on a lot of platforms before
every release. These tests are more sophisticated than anything we have
seen from PostgreSQL, and they ensure that the MySQL Server is kept to a high
standard.
PostgreSQL
.
ALTER TABLE
.
HEAP
tables or disk based MyISAM
. See section 7 MySQL Table Types.
InnoDB
, and BerkeleyDB
. Because every
transaction engine performs differently under different conditions, this
gives the application writer more options to find an optimal solution for
his or her setup, if need be per individual table. See section 7 MySQL Table Types.
MERGE
tables gives you a unique way to instantly make a view over
a set of identical tables and use these as one. This is perfect for
systems where you have log files that you order, for example, by month.
See section 7.2 MERGE
Tables.
myisampack
, The MySQL Compressed Read-only Table Generator.
INSERT
,
SELECT
, and UPDATE/DELETE
grants per user on a database or
a table, MySQL Server allows you to define a full set of different
privileges on the database, table, and column level. MySQL Server also
allows you to specify the privilege on host and user combinations.
See section 4.3.1 GRANT
and REVOKE
Syntax.
InnoDB
) are implemented as files
(one table per file), which makes it really easy to back up, move, delete,
and even symlink databases and tables, even when the server is down.
MyISAM
tables (the most common
MySQL table type). A repair tool is only needed when a physical corruption
of a datafile happens, usually from a hardware failure. It allows a
majority of the data to be recovered.
Drawbacks with MySQL Server compared to PostgreSQL:
MyISAM
tables, is
in many cases faster than page locks, row locks, or versioning. The
drawback, however, is that if one doesn't take into account how table
locks work, a single long-running query can block a table for updates
for a long time. This can usually be avoided when designing the
application. If not, one can always switch the trouble table to use one
of the transactional table types. See section 5.3.2 Table Locking Issues.
UPDATE
and in MySQL Server 4.1 with subselects. In MySQL Server
4.0 one can use multi-table deletes to delete from many tables at the
same time. See section 6.4.6 DELETE
Syntax.
PostgreSQL currently offers the following advantages over MySQL Server:
Note that because we know the MySQL road map, we have included in the following table the version when MySQL Server should support this feature. Unfortunately we couldn't do this for previous comparisons, because we don't know the PostgreSQL roadmap.
Feature | MySQL version |
Subselects | 4.1 |
Foreign keys | 4.1 (3.23 with InnoDB) |
Views | 5.0 |
Stored procedures | 5.0 |
Triggers | 5.0 |
Unions | 4.0 |
Full join | 4.1 |
Constraints | 4.1 or 5.0 |
Cursors | 4.1 or 5.0 |
R-trees | 4.1 (for MyISAM tables) |
Inherited tables | Not planned |
Extensible type system | Not planned |
Other reasons someone may consider using PostgreSQL:
Drawbacks with PostgreSQL compared to MySQL Server:
VACUUM
makes PostgreSQL hard to use in a 24/7 environment.
INSERT
, DELETE
, and UPDATE
.
For a complete list of drawbacks, you should also examine the first table in this section.
The only Open Source
benchmark that we know of that can be used to
benchmark MySQL Server and PostgreSQL (and other databases) is our own. It can
be found at http://www.mysql.com/information/benchmarks.html.
We have many times asked the PostgreSQL developers and some PostgreSQL users to help us extend this benchmark to make it the definitive benchmark for databases, but unfortunately we haven't gotten any feedback for this.
We, the MySQL developers, have, because of this, spent a lot of hours to get maximum performance from PostgreSQL for the benchmarks, but because we don't know PostgreSQL intimately, we are sure that there are things that we have missed. We have on the benchmark page documented exactly how we did run the benchmark so that it should be easy for anyone to repeat and verify our results.
The benchmarks are usually run with and without the --fast
option.
When run with --fast
we are trying to use every trick the server can
do to get the code to execute as fast as possible. The idea is that the
normal run should show how the server would work in a default setup and
the --fast
run shows how the server would do if the application
developer would use extensions in the server to make his application run
faster.
When running with PostgreSQL and --fast
we do a VACUUM
after every major table UPDATE
and DROP TABLE
to make the
database in perfect shape for the following SELECT
s. The time for
VACUUM
is measured separately.
When running with PostgreSQL 7.1.1 we could, however, not run with
--fast
because during the INSERT
test, the postmaster (the
PostgreSQL deamon) died and the database was so corrupted that it was
impossible to restart postmaster. After this happened twice, we decided
to postpone the --fast
test until the next PostgreSQL release. The
details about the machine we run the benchmark on can be found on the
benchmark page.
Before going to the other benchmarks we know of, we would like to give some background on benchmarks.
It's very easy to write a test that shows any database to be the best database in the world, by just restricting the test to something the database is very good at and not testing anything that the database is not good at. If one, after doing this, summarises the result as a single figure, things are even easier.
This would be like us measuring the speed of MySQL Server compared to PostgreSQL by looking at the summary time of the MySQL benchmarks on our web page. Based on this MySQL Server would be more than 40 times faster than PostgreSQL, something that is, of course, not true. We could make things even worse by just taking the test where PostgreSQL performs worst and claim that MySQL Server is more than 2000 times faster than PostgreSQL.
The case is that MySQL does a lot of optimisations that PostgreSQL doesn't do. This is, of course, also true the other way around. An SQL optimiser is a very complex thing, and a company could spend years just making the optimiser faster and faster.
When looking at the benchmark results you should look for things that you do in your application and just use these results to decide which database would be best suited for your application. The benchmark results also show things a particular database is not good at and should give you a notion about things to avoid and what you may have to do in other ways.
We know of two benchmark tests that claim that PostgreSQL performs better than MySQL Server. These both where multi-user tests, a test that we here at MySQL AB haven't had time to write and include in the benchmark suite, mainly because it's a big task to do this in a manner that is fair to all databases.
One is the benchmark paid for by Great Bridge, the company that for 16 months attempted to build a business based on PostgreSQL but now has ceased operations. This is probably the worst benchmark we have ever seen anyone conduct. This was not only tuned to only test what PostgreSQL is absolutely best at, but it was also totally unfair to every other database involved in the test.
Note: We know that even some of the main PostgreSQL developers did not like the way Great Bridge conducted the benchmark, so we don't blame the PostgreSQL team for the way the benchmark was done.
This benchmark has been condemned in a lot of postings and newsgroups, so here we will just briefly repeat some things that were wrong with it.
Open Source
company like us to verify the benchmarks,
or even check how the benchmarks were really done. The tool is not even
a true benchmark tool, but an application/setup testing tool. To refer
to this as a ``standard'' benchmark tool is to stretch the truth a long way.
VACUUM
before the test) and tuned the startup for the tests,
something they hadn't done for any of the other databases involved. They
say ``This process optimises indexes and frees up disk space a bit. The
optimised indexes boost performance by some margin.'' Our benchmarks
clearly indicate that the difference in running a lot of selects on a
database with and without VACUUM
can easily differ by a factor
of 10.
SELECT
s and JOIN
s (especially
after a VACUUM
), but doesn't perform as well on INSERT
s or
UPDATE
s. The benchmarks seem to indicate that only SELECT
s
were done (or very few updates). This could easily explain the good results
for PostgreSQL in this test. The bad results for MySQL will be obvious a
bit down in this document.
Tim Perdue, a long-time PostgreSQL fan and a reluctant MySQL user, published a comparison on PHPbuilder (http://www.phpbuilder.com/columns/tim20001112.php3).
When we became aware of the comparison, we phoned Tim Perdue about this because there were a lot of strange things in his results. For example, he claimed that MySQL Server had a problem with five users in his tests, when we know that there are users with similar machines as his that are using MySQL Server with 2000 simultaneous connections doing 400 queries per second. (In this case the limit was the web bandwidth, not the database.)
It sounded like he was using a Linux kernel that either had some problems with many threads, such as kernels before 2.4, which had a problem with many threads on multi-CPU machines. We have documented in this manual how to fix this and Tim should be aware of this problem.
The other possible problem could have been an old glibc library and that Tim didn't use a MySQL binary from our site, which is linked with a corrected glibc library, but had compiled a version of his own. In any of these cases, the symptom would have been exactly what Tim had measured.
We asked Tim if we could get access to his data so that we could repeat the benchmark and if he could check the MySQL version on the machine to find out what was wrong and he promised to come back to us about this. He has not done that yet.
Because of this we can't put any trust in this benchmark either. :(
Over time things also change and the preceding benchmarks are not that relevant anymore. MySQL Server now has a couple of different table handlers with different speed/concurrency tradeoffs. See section 7 MySQL Table Types. It would be interesting to see how the above tests would run with the different transactional table types in MySQL Server. PostgreSQL has, of course, also got new features since the test was made. As these tests are not publicly available there is no way for us to know how the database would perform in the same tests today.
Conclusion:
The only benchmarks that exist today that anyone can download and run
against MySQL Server and PostgreSQL are the MySQL benchmarks.
We here at MySQL AB
believe that Open Source
databases should be tested with Open Source
tools!
This is the only way to ensure that no one does tests that nobody can
reproduce and use this to claim that one database is better than another.
Without knowing all the facts it's impossible to answer the claims of the
tester.
The thing we find strange is that every test we have seen about PostgreSQL, that is impossible to reproduce, claims that PostgreSQL is better in most cases while our tests, which anyone can reproduce, clearly show otherwise. With this we don't want to say that PostgreSQL isn't good at many things (it is!) or that it isn't faster than MySQL Server under certain conditions. We would just like to see a fair test where PostgreSQL performs very well, so that we could get some friendly competition going!
For more information about our benchmark suite, see section 5.1.4 The MySQL Benchmark Suite.
We are working on an even better benchmark suite, including multi-user tests, and a better documentation of what the individual tests really do and how to add more tests to the suite.
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