Hidden Features of MySQL

Question

I've been working with Microsoft SQL Server with many years now but have only just recently started to use MySQL with my web applications, and I'm hungry for knowledge.

To continue with the long line of "hidden feature" questions, I would like to know any hidden or handy features of MySQL which will hopefully improve my knowledge of this open source database.

Answer 1

One of the not so hidden feature of MySQL is that it's not really good at being SQL compliant, well, not bugs really, but, more gotchas... :-)

Answer 2

A command to find out who is doing what:

mysql> show processlist;
show processlist;
+----+-------------+-----------------+------+---------+------+----------------------------------+------------------+
| Id | User        | Host            | db   | Command | Time | State                            | Info             |
+----+-------------+-----------------+------+---------+------+----------------------------------+------------------+
|  1 | root        | localhost:32893 | NULL | Sleep   |    0 |                                  | NULL             |
|  5 | system user |                 | NULL | Connect |   98 | Waiting for master to send event | NULL             |
|  6 | system user |                 | NULL | Connect | 5018 | Reading event from the relay log | NULL             |
+-----+------+-----------+---------+---------+-------+-------+------------------+
3 rows in set (0.00 sec)

And you can kill a process with:

mysql>kill 5

Answer 3

A command to find out what tables are currently in the cache:

mysql> SHOW open TABLES FROM test;
+----------+-------+--------+-------------+
| DATABASE | TABLE | In_use | Name_locked |
+----------+-------+--------+-------------+
| test     | a     |      3 |           0 |
+----------+-------+--------+-------------+
1 row IN SET (0.00 sec)

(From MySQL performance blog)

Answer 4

I particularly like MySQL's built-in support for inet_ntoa() and inet_aton(). It makes handling of IP addresses in tables very straightforward (at least so long as they're only IPv4 addresses!)

Answer 5

Some things you may find interesting:

<query>\G -- \G in the CLI instead of the ; will show one column per row
explain <query>; -- this will show the execution plan for the query

Answer 6

Since you put up a bounty, I'll share my hard won secrets...

In general, all the SQLs I tuned today required using sub-queries. Having come from Oracle database world, things I took for granted weren’t working the same with MySQL. And my reading on MySQL tuning makes me conclude that MySQL is behind Oracle in terms of optimizing queries.

While the simple queries required for most B2C applications may work well for MySQL, most of the aggregate reporting type of queries needed for Intelligence Reporting seems to require a fair bit of planning and re-organizing the SQL queries to guide MySQL to execute them faster.

Administration:

max_connections is the number of concurrent connections. The default value is 100 connections - very small.

Note:

connections take memory and your OS might not be able to handle a lot of connections.

MySQL binaries for Linux/x86 allow you to have up to 4096 concurrent connections, but self compiled binaries often have less of a limit.

Set table_cache to match the number of your open tables and concurrent connections. Watch the open_tables value and if it is growing quickly you will need to increase its size.

Note:

The 2 previous parameters may require a lot of open files. 20+max_connections+table_cache*2 is a good estimate for what you need. MySQL on Linux has an open_file_limit option, set this limit.

If you have complex queries sort_buffer_size and tmp_table_size are likely to be very important. Values will depend on the query complexity and available resources, but 4Mb and 32Mb, respectively are recommended starting points.

Note: These are "per connection" values, among read_buffer_size, read_rnd_buffer_size and some others, meaning that this value might be needed for each connection. So, consider your load and available resource when setting these parameters. For example sort_buffer_size is allocated only if MySQL needs to do a sort. Note: be careful not to run out of memory.

If you have many connects established (i.e. a web site without persistent connections) you might improve performance by setting thread_cache_size to a non-zero value. 16 is good value to start with. Increase the value until your threads_created do not grow very quickly.

PRIMARY KEY:

There can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot have a DEFAULT value

KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.

A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL

If a PRIMARY KEY or UNIQUE index consists of only one column that has an integer type, you can also refer to the column as "_rowid" in SELECT statements.

In MySQL, the name of a PRIMARY KEY is PRIMARY

Currently, only InnoDB (v5.1?) tables support foreign keys.

Usually, you create all the indexes you need when you are creating tables. Any column declared as PRIMARY KEY, KEY, UNIQUE, or INDEX will be indexed.

NULL means "not having a value". To test for NULL, you cannot use the arithmetic comparison operators such as =, <, or <>. Use the IS NULL and IS NOT NULL operators instead:

NO_AUTO_VALUE_ON_ZERO suppresses auto increment for 0 so that only NULL generates the next sequence number. This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 is not a recommended practice, by the way.)

To change the value of the AUTO_INCREMENT counter to be used for new rows:

ALTER TABLE mytable AUTO_INCREMENT = value;

or SET INSERT_ID = value;

Unless otherwise specified, the value will begin with: 1000000 or specify it thus:

...) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=1

TIMESTAMPS:

Values for TIMESTAMP columns are converted from the current time zone to UTC for storage, and from UTC to the current time zone for retrieval.

http://dev.mysql.com/doc/refman/5.1/en/timestamp.html For one TIMESTAMP column in a table, you can assign the current timestamp as the default value and the auto-update value.

one thing to watch out for when using one of these types in a WHERE clause, it is best to do WHERE datecolumn = FROM_UNIXTIME(1057941242) and not WHERE UNIX_TIMESTAMP(datecolumn) = 1057941242. doing the latter won't take advantage of an index on that column.

http://dev.mysql.com/doc/refman/4.1/en/date-and-time-functions.html

 UNIX_TIMESTAMP() 
 FROM_UNIXTIME() 
 UTC_DATE()
 UTC_TIME()
 UTC_TIMESTAMP()

if you convert a datetime to unix timestamp in MySQL:
And then add 24 hours to it:
And then convert it back to a datetime it magically loses an hour!

Here's what's happening. When coverting the unix timestamp back to a datetime the timezone is taken into consideration and it just so happens that between the 28th and the 29th of October 2006 we went off daylight savings time and lost an hour.

Beginning with MySQL 4.1.3, the CURRENT_TIMESTAMP(), CURRENT_TIME(), CURRENT_DATE(), and FROM_UNIXTIME() functions return values in the connection's current time zone, which is available as the value of the time_zone system variable. In addition, UNIX_TIMESTAMP() assumes that its argument is a datetime value in the current time zone.

The current time zone setting does not affect values displayed by functions such as UTC_TIMESTAMP() or values in DATE, TIME, or DATETIME columns.

NOTE: ON UPDATE ONLY updates the DateTime if a field is changed If an UPDATE results in no fields being changed then the DateTime is NOT updated!

Addtionally, the First TIMESTAMP is always AUTOUPDATE by default even if not specified

When working with Dates, I almost always convet to Julian Date becuase Data math is then a simple matter of adding or subtracing integers, and Seconds since Midnight for the same reason. It is rare I need time resoultion of finer granularity than seconds.

Both these can be stored as a 4 byte integer, and if space is really tight can be combined into UNIX time (seconds since the epoch 1/1/1970) as an unsigned integer which will be good till around 2106 as:

' secs in 24Hrs = 86400

' Signed Integer max val = 2,147,483,647 - can hold 68 years of Seconds

' Unsigned Integer max val = 4,294,967,295 - can hold 136 years of Seconds

Binary Protocol:

MySQL 4.1 introduced a binary protocol that allows non-string data values to be sent and returned in native format without conversion to and from string format. (Very usefull)

Aside, mysql_real_query() is faster than mysql_query() because it does not call strlen() to operate on the statement string.

http://dev.mysql.com/tech-resources/articles/4.1/prepared-statements.html The binary protocol supports server-side prepared statements and allows transmission of data values in native format. The binary protocol underwent quite a bit of revision during the earlier releases of MySQL 4.1.

You can use the IS_NUM() macro to test whether a field has a numeric type. Pass the type value to IS_NUM() and it evaluates to TRUE if the field is numeric:

One thing to note is that binary data CAN be sent inside a regular query if you escape it and remember MySQL requires only that backslash and the quote character be escaped. So that is a really easy way to INSERT shorter binary strings like encrypted/Salted passwords for example.

Master Server:

http://www.experts-exchange.com/Database/MySQL/Q_22967482.html

http://www.databasejournal.com/features/mysql/article.php/10897_3355201_2

GRANT REPLICATION SLAVE ON . to slave_user IDENTIFIED BY 'slave_password'

#Master Binary Logging Config  STATEMENT causes replication 
              to be statement-based -  default

log-bin=Mike
binlog-format=STATEMENT
server-id=1            
max_binlog_size = 10M
expire_logs_days = 120    


#Slave Config
master-host=master-hostname
master-user=slave-user
master-password=slave-password
server-id=2

Binary Log File must read:

http://dev.mysql.com/doc/refman/5.0/en/binary-log.html

http://www.mydigitallife.info/2007/10/06/how-to-read-mysql-binary-log-files-binlog-with-mysqlbinlog/

http://dev.mysql.com/doc/refman/5.1/en/mysqlbinlog.html

http://dev.mysql.com/doc/refman/5.0/en/binary-log.html

http://dev.mysql.com/doc/refman/5.1/en/binary-log-setting.html

You can delete all binary log files with the RESET MASTER statement, or a subset of them with PURGE MASTER

--result-file=binlog.txt TrustedFriend-bin.000030

Normalization:

http://dev.mysql.com/tech-resources/articles/intro-to-normalization.html

UDF functions

http://www.koders.com/cpp/fid10666379322B54AD41AEB0E4100D87C8CDDF1D8C.aspx

http://souptonuts.sourceforge.net/readme_mysql.htm

DataTypes:

http://dev.mysql.com/doc/refman/5.1/en/storage-requirements.html

http://www.informit.com/articles/article.aspx?p=1238838&amp;seqNum=2

http://bitfilm.net/2008/03/24/saving-bytes-efficient-data-storage-mysql-part-1/

One thing to note is that on a mixed table with both CHAR and VARCHAR, mySQL will change the CHAR's to VARCHAR's

RecNum integer_type UNSIGNED NOT NULL AUTO_INCREMENT, PRIMARY KEY (RecNum)

MySQL always represents dates with the year first, in accordance with the standard SQL and ISO 8601 specifications

Misc:

Turing off some MySQl functionality will result in smaller data files and faster access. For example:

--datadir will specify the data directory and

--skip-innodb will turn off the inno option and save you 10-20M

More here http://dev.mysql.com/tech-resources/articles/mysql-c-api.html

Download Chapter 7 - Free

InnoDB is transactional but there is a performance overhead that comes with it. I have found MyISAM tables to be sufficient for 90% of my projects. Non-transaction-safe tables (MyISAM) have several advantages of their own, all of which occur because:

there is no transaction overhead:

Much faster

Lower disk space requirements

Less memory required to perform updates

Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format. The data file has an .MYD (MYData) extension. The index file has an .MYI (MYIndex) extension.

These Files can be copied to a storage location intact without using the MySQL Administrators Backup feature which is time consuming (so is the Restore)

The trick is make a copy of these files then DROP the table. When you put the files back MySQl will recognize them and update the table tracking.

If you must Backup/Restore,

Restoring a backup, or importing from an existing dump file can takes a long time depending on the number of indexes and primary keys you have on each table. You can speed this process up dramatically by modifying your original dump file by surrounding it with the following:

SET AUTOCOMMIT = 0;
SET FOREIGN_KEY_CHECKS=0;

.. your dump file ..

SET FOREIGN_KEY_CHECKS = 1;
COMMIT;
SET AUTOCOMMIT = 1;

To vastly increase the speed of the reload, add the SQL command SET AUTOCOMMIT = 0; at the beginning of the dump file, and add the COMMIT; command to the end.

By default, autocommit is on, meaning that each and every insert command in the dump file will be treated as a separate transaction and written to disk before the next one is started. If you don't add these commands, reloading a large database into InnoDB can take many hours...

The maximum size of a row in a MySQL table is 65,535 bytes

The effective maximum length of a VARCHAR in MySQL 5.0.3 and on = maximum row size (65,535 bytes)

VARCHAR values are not padded when they are stored. Trailing spaces are retained when values are stored and retrieved, in conformance with standard SQL.

CHAR and VARCHAR values in MySQL are compared without regard to trailing spaces.

Using CHAR will only speed up your access if the whole record is fixed size. That is, if you use any variable size object, you might as well make all of them variable size. You gain no speed by using a CHAR in a table that also contains a VARCHAR.

The VARCHAR limit of 255 characters was raised to 65535 characters as of MySQL 5.0.3

Full-text searches are supported for MyISAM tables only.

http://dev.mysql.com/doc/refman/5.0/en/fulltext-search.html

BLOB columns have no character set, and sorting and comparison are based on the numeric values of the bytes in column values

If strict SQL mode is not enabled and you assign a value to a BLOB or TEXT column that exceeds the column's maximum length, the value is truncated to fit and a warning is generated.

Useful Commands:

check strict mode: SELECT @@global.sql_mode;

turn off strict mode:

SET @@global.sql_mode= '';

SET @@global.sql_mode='MYSQL40'

or remove: sql-mode="STRICT_TRANS_TABLES,...

SHOW COLUMNS FROM mytable

SELECT max(namecount) AS virtualcolumn FROM mytable ORDER BY virtualcolumn

http://dev.mysql.com/doc/refman/5.0/en/group-by-hidden-fields.html

http://dev.mysql.com/doc/refman/5.1/en/information-functions.html#function_last-insert-id last_insert_id()

gets you the PK of the last row inserted in the current thread max(pkcolname) gets you last PK overall.

Note: if the table is empty max(pkcolname) returns 1 mysql_insert_id() converts the return type of the native MySQL C API function mysql_insert_id() to a type of long (named int in PHP).

If your AUTO_INCREMENT column has a column type of BIGINT, the value returned by mysql_insert_id() will be incorrect. Instead, use the internal MySQL SQL function LAST_INSERT_ID() in an SQL query.

http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_last-insert-id

Just a note that when you’re trying to insert data into a table and you get the error:

Unknown column ‘the first bit of data what you want to put into the table‘ in ‘field list’

using something like

INSERT INTO table (this, that) VALUES ($this, $that)

it’s because you’ve not got any apostrophes around the values you’re trying to stick into the table. So you should change your code to:

INSERT INTO table (this, that) VALUES ('$this', '$that')

reminder that `` are used to define MySQL fields, databases, or tables, not values ;)

Lost connection to server during query:

http://dev.mysql.com/doc/refman/5.1/en/gone-away.html

http://dev.mysql.com/doc/refman/5.1/en/packet-too-large.html

http://dev.mysql.com/doc/refman/5.0/en/server-parameters.html

http://dev.mysql.com/doc/refman/5.1/en/show-variables.html

http://dev.mysql.com/doc/refman/5.1/en/option-files.html

http://dev.mysql.com/doc/refman/5.1/en/error-log.html

Tuning Queries

http://www.artfulsoftware.com/infotree/queries.php?&amp;bw=1313

Well that should be enought ot earn the bonus I would think... The fruits of many hours and many projects with a great free database. I develop application data servers on windows platforms mostly with MySQL. The worst mess I had to straighten out was

http://stackoverflow.com/questions/101333/the-ultimate-mysql-legacy-database-nightmare

This required a series of appplications to process the tables into something usefull using many of the tricks mentioned here.

If you found this astoundingly helpfull, express your thanks by voting it up.

Answer 7

Here are some of my tips - I blogged about them in my blog (Link)

1. You don't need to use '@' sign when declaring variables.
2. You have to use a delimiter (the default is ';') to demarcate the end of a statement - Link
3. If you trying to move data between MS-SQL 2005 and mySQL there are a few hoops to jump through - Link
4. Doing case sensitive matches in mySQL - link

Answer 8

If you're going to be working with large and/or high transaction InnoDb databases learn and understand "SHOW INNODB STATUS" Mysql Performance Blog, it will become your friend.

Answer 9

Not a hidden feature, but useful nonetheless: http://mtop.sourceforge.net/

Answer 10

InnoDB by default stores all tables in one global tablespace that will never shrink.

You can use innodb_file_per_table which will put each table in a separate tablespace that will be deleted when you drop the table or database.

Plan ahead for this since you have to dump and restore the database to reclaim space otherwise.

Using Per-Table Tablespaces

Answer 11

If using cmdline Mysq, you can interact with the command line (on Linux machines - not sure if there is an equivalent effect on Windows) by using the shriek/exclamation mark. For example:

\! cat file1.sql

will display the code for file1.sql. To save your statement and query to a file, use the tee facility

\T filename

to turn this off use \t

Lastly to run a script you've already saved, use "source filename". Of course, the normal alternative is to direct in the script name when starting mysql from the command line:

    mysql -u root -p < case1.sql

Hope that's of use to someone !

Edit: Just remembered another one - when invoking mysql from the command line you can use the -t switch so that output is in table format - a real boon with some queries (although of course terminating queries with \G as mentioned elsewhere here is also helpful in this respect). A lot more on various switches Command Line Tool

Just found out a neat way to change the order of a sort (normally use Case...) If you want to change the order of a sort (perhaps sort by 1, 4, 3 ,2 instead of 1, 2, 3,4) you can use the field function within the Order by clause. For example

Order By Field(sort_field,1,4,3,2)

• Found this out here http://stackoverflow.com/questions/1126961/order-by-dayofweek-in-mysql courtesey of user gms8994

Answer 12

I love on duplicate key (AKA upsert, merge) for all kinds of counters created lazily:

insert into occurances(word,count) values('foo',1),('bar',1) 
  on duplicate key cnt=cnt+1

You can insert many rows in one query, and immediately handle duplicate index for each of the rows.

Answer 13

The "pager" command in the client

If you've got, say, 10,000 rows in your result and want to view them (This assumes the "less" and "tee" commands available, which is normally the case under Linux; in Windows YMMV.)

pager less
select lots_of_stuff FROM tbl WHERE clause_which_matches_10k_rows;

And you'll get them in the "less" file viewer so you can page through them nicely, search etc.

Also

pager tee myfile.txt
select a_few_things FROM tbl WHERE i_want_to_save_output_to_a_file;

Will conveniently write to a file.

Answer 14

Again - not really hidden features, but really handy:

Feature

Easily grab DDL:

SHOW CREATE TABLE CountryLanguage

output:

CountryLanguage | CREATE TABLE countrylanguage (
  CountryCode char(3) NOT NULL DEFAULT '',
  Language char(30) NOT NULL DEFAULT '',
  IsOfficial enum('T','F') NOT NULL DEFAULT 'F',
  Percentage float(4,1) NOT NULL DEFAULT '0.0',
  PRIMARY KEY (CountryCode,Language)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

Feature: GROUP_CONCAT() aggregate function Creates a concatenated string of its arguments per detail, and aggregates by concatenating those per group.

Example 1: simple

SELECT   CountryCode
,        GROUP_CONCAT(Language) AS List
FROM     CountryLanguage
GROUP BY CountryCode             

Output:

+-------------+------------------------------------+
| CountryCode | List                               |
+-------------+------------------------------------+
| ABW         | Dutch,English,Papiamento,Spanish   |
. ...         . ...                                .
| ZWE         | English,Ndebele,Nyanja,Shona       |
+-------------+------------------------------------+

Example 2: multiple arguments

SELECT   CountryCode
,        GROUP_CONCAT(
             Language
,            IF(IsOfficial='T', ' (Official)', '')
         )               AS List
FROM     CountryLanguage
GROUP BY CountryCode

Output:

+-------------+---------------------------------------------+
| CountryCode | List                                        |
+-------------+---------------------------------------------+
| ABW         | Dutch (Official),English,Papiamento,Spanish |
. ...         . ...                                         .
| ZWE         | English (Official),Ndebele,Nyanja,Shona     |
+-------------+---------------------------------------------+

Example 3: Using a custom separator

SELECT   CountryCode
,        GROUP_CONCAT(Language SEPARATOR ' and ') AS List
FROM     CountryLanguage
GROUP BY CountryCode

Output:

+-------------+----------------------------------------------+
| CountryCode | List                                         |
+-------------+----------------------------------------------+
| ABW         | Dutch and English and Papiamento and Spanish |
. ...         . ...                                          .
| ZWE         | English and Ndebele and Nyanja and Shona     |
+-------------+----------------------------------------------+

Example 4: Controlling the order of the list elements

SELECT   CountryCode
,        GROUP_CONCAT(
         Language
         ORDER BY CASE IsOfficial WHEN 'T' THEN 1 ELSE 2 END DESC
         ,        Language
         )               AS List
FROM     CountryLanguage
GROUP BY CountryCode

Output:

+-------------+------------------------------------+
| CountryCode | List                               |
+-------------+------------------------------------+
| ABW         | English,Papiamento,Spanish,Dutch,  |
. ...         . ...                                .
| ZWE         | Ndebele,Nyanja,Shona,English       |
+-------------+------------------------------------+

Feature: COUNT(DISTINCT ) with multiple expressions

You can use multiple expressions in a COUNT(DISTINCT ...) expression to count the number of combinations.

SELECT COUNT(DISTINCT CountryCode, Language) FROM CountryLanguage

Feature / Gotcha: No need to include non-aggregated expressions in the GROUP BY list

Most RDBMS-es enforce a SQL92 compliant GROUP BY which requires all non-aggregated expressions in the SELECT list to appear in the GROUP BY. In these RDBMS-es, this statement:

SELECT     Country.Code, Country.Continent, COUNT(CountryLanguage.Language)
FROM       CountryLanguage 
INNER JOIN Country 
ON         CountryLanguage.CountryCode = Country.Code
GROUP BY   Country.Code

is not valid, because the SELECT list contains the non-aggregated column Country.Continent which does not appear in the GROUP BY list. In these RDBMS-es, you must either modify the GROUP BY list to read

GROUP BY   Country.Code, Country.Continent

or you must add some non-sense aggregate to Country.Continent, for example

SELECT     Country.Code, MAX(Country.Continent), COUNT(CountryLanguage.Language)

Now, the thing is, logically there is nothing that demands that Country.Continent should be aggreagated. See, Country.Code is the primary key of the Country table. Country.Continent is also a column from the Country table and is thus by definitions functionally dependent upon the primary key Country.Code. So, there must exist exactly one value in Country.Continent for each distinct Country.Code. If you realize that, than you realize that it does not make sense to aggregate it (there is just one value, right) nor to group by it (as it won't make the result more unique as you're already grouping by on the pk)

Anyway - MySQL lets you include non-aggregated columns in the SELECT list without requiring you to also add them to the GROUP BY clause.

The gotcha with this is that MySQL does not protect you in case you happen to use a non-aggregated column. So, a query like this:

SELECT     Country.Code, COUNT(CountryLanguage.Language), CountryLanguage.Percentage
FROM       CountryLanguage 
INNER JOIN Country 
ON         CountryLanguage.CountryCode = Country.Code
GROUP BY   Country.Code

Will be executed without complaint, but the CountryLanguage.Percentage column will contain non-sense (that is to say, of all languages percentages, one of the available values for the percentage will be picked at random or at least outside your control.

See: Debunking Group By Myths

Answer 15

mysqlsla - One of the very commonly used slow query log analysis tool. You can see top 10 worsts queries since u last rolled out slow query logs. It can also tell you the number of times that BAD query was fired and how much total time it took on the server.

Answer 16

Actually documented, but very annoying: automatic conversions for incorrect dates and other incorrect input.

Before MySQL 5.0.2, MySQL is forgiving of illegal or improper data values and coerces them to legal values for data entry. In MySQL 5.0.2 and up, that remains the default behavior, but you can change the server SQL mode to select more traditional treatment of bad values such that the server rejects them and aborts the statement in which they occur.

As for dates: sometimes you'll be "lucky" when MySQL doesn't adjust the input to nearby valid dates, but instead stores it as 0000-00-00 which by definition is invalid. However, even then you might have wanted MySQL to fail rather than silently storing this value for you.