My sql technical reference manual


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My sql technical reference manual

  1. 1. MySQL Technical Reference Manual.What is MySQL?MySQL is a SQL (Structured Query Language) database server. SQL is the most populardatabase language in the world. MySQL is a client server implementation that consists of a serverdaemon mysqld and many different client programs/libraries.The main goals of MySQL are speed and robustness.The base upon which MySQL is built is a set of routines that have been used in a highlydemanding production environment for many years.The main features in MySQL • Multi-threaded • C, C++, JAVA, Perl, Python and TCL API’s. • Lots of column types like: signed/unsigned integers 1,2,3,4,8 bytes long, FLOAT, CHAR,VARCHAR, TEXT, BLOB, DATE, SET and ENUM types. • Join optimizer with one-sweep multi-join (all joins made in one pass). • Full function support in the SELECT and WHERE parts. Example: select column1+column2 from table where column1/column2 > 0 • SQL functions are implemented through a very optimized class library and should be as fast as they can get! Usually there shouldn’t be any memory allocation at all after the query initialization. • Full support for SQL GROUP BY and ORDER BY. Support for group functions (SUM, MAX and MIN). • A privilege and password system with is very flexible and secure. Allows host based verification. • All password traffic on the net is encrypted. • Very fast B-tree disk tables with key compression. • Fixed and variable length records. • 16 indexes/table. Each index may consist of 1 to 15 columns/parts of columns. Max key length is 127 bytes. A key may be a prefix of a CHAR field. • ODBC Open-DataBase-Connectivity for Windows95 (with source). All ODBC 2.5 functions and lots of others. • In memory hash tables always used as temporary tables. • Can handle big databases (we are using MySQL with some databases that contain 50,000,000 records). • All columns have default values. One can always use INSERT on any subset of columns. • Uses GNU autoconf for portability. • Written in C and C++. Tested with gcc 2.7.2. • A thread based memory allocation system (very fast and no memory trashing). • No memory leaks. Tested with a commercial memory leakage detector (purify). • A very fast table check and repair utility (isamchk). • All data saved in ISO8859_1 format. All comparisons for normal string columns are case insensitive. 1
  2. 2. • Full ISO8859_1 (Latin1) support. For example Scandinavian åäö is allowed in table and column names. • Sorts by ISO8859_1 Latin1 (the Swedish way at the moment). It is possible to change this in the source by adding new sort order arrays. • Alias on tables and columns as in the SQL92 standard. avg & count). • INSERT,UPDATE and DELETE returns how many rows were affected. • Function names do not clash with table or column names. For example ABS is a valid column name. The only restriction is that space is not allowed between a function name and the ’(’ when using functions. • All MySQL commands have --help or -? for help. • The server currently supports error messages to clients in many languages. See section What languages are supported by MySQL.. • The clients uses a TCP connection or unix socket when connecting to the MySQL server. • User commands as show tables, show keys from table and show columns from TableProblems starting MySQLCheck the log file to see if mysqld started up <localstatedir default /usr/local/var>tail <your host name>.logTo verify that MySQL is working, run the following tests:> cd /usr/local/bin> ./mysqlshow+-----------+| Databases |+-----------+| mysql |+-----------+> ./mysqlshow mysqlDatabase: mysql+--------+| Tables |+--------+| db || host || user |+--------+> ./mysql -e "select host,db,user from db" mysql+------+--------+------+| host | db | user |+------+--------+------+| % | test | || % | test_% | |+------+--------+------+There is also a benchmark suite so you can compare how MySQL perform on different platforms.In the near future this will also be used to compare MySQL to other SQL databases.> cd bench> run-auto-increment-test 2
  3. 3. You can also run the tests in the test subdirectory. To run ‘auto_increment.tst’:./mysql -vf test < ./tests/auto_increment.tst24Expected results are shown in the file ‘./tests/auto_increment.res’.Automatic start/stop of MySQLTo start or stop MySQL use the following commands:scripts/mysql.server stopscripts/mysql.server startYou might want to add these start and stop commands in the appropriate places in your /etc/rc*fileswhen you start using MySQL for production applications.How to debug MySQLIf you are porting MySQL to an new system you should first try to get mysys/thr_lock andmysys/thr_alarm to work. They shouldn’t core dump and not print any error (they also print a lotofother information). Also see the file ‘PORTING’ in the distribution.By starting bin/safe_mysqld with --log you will get a log ininstall-directory/var/’hostname’.log (the top level database directory). This log willcontain all successful connections and all commands issued to the MySQL server.If you have compiled MySQL with --with-debug=yes you can can also get a very informative logwith:libexec/mysqld --debugwhich makes a large log in /tmp/mysql.trace. The default debug option isd:i:t:o,/tmp/mysql.trace.You can get a smaller log with:libexec/mysqld --debug=d,info,query,general:o,/tmp/mysql.traceor an even smaller (on stdout):libexec/mysqld --debug=d,general,queryYou can get more information about the debug switches by examining the file dbug/dbug.c.If you have a problem with mysqld that it crashes and you want this quickly solved, you shouldinclude a trace file with your mail if possible. Trace files can be posted directly to@email{} to avoid long messages to the standard mail list. If the trace fileis bigyou should use ftp and send it to together with amysqldump of any tables that you think can help to solve the problem. The context of the abovedirectory is hidden for outside users so no one except the TCX staff can look at what you sendinto it.Go to the first [p 5] , previous [p 19] , next [p 26] , last [p 87] section, table of contents [p 1] .25Go to the first [p 5] , previous [p 24] , next [p 30] , last [p 87] section, table of contents [p 1] .How does MySQL privileges workMySQL has a advanded but non standard security/privileges system.How does the privilege system work?The MySQL privilege system makes sure that each user may do exactly the things that they are 3
  4. 4. supposed to be allowed to do. The system decides to grant different privileges depending onwhichuser connects from which host to which database. The decision is based on the contents ofthe threetables in the MySQL database: user, host and db.The grant tables privileges on rows are select, insert, update and delete.The table and database privileges are create and drop. Create and drop are for both tables anddatabases. Since a user with a drop grant can delete any table, this is the same thing as a dropgrant forthe database.Other privileges give the right to use files (for LOAD DATA INFILE and SELECT INTOOUTFILE)and to use the administrative commands shutdown, reload, refresh and process, to get thecurrentprocess list.The privilege tables are read into mysqld with mysqladmin reload. If the privilege tables areempty or non-existent or if the server is started with --skip-grant-tables, full access isgranted to everyone.You can always test your privileges with the script mysqlaccess, which Yves Carlier hasprovidedfor the MySQL distribution. See section Why do I get ’Access denied’? [p 78]The host and db fields may contain a SQL regexp with chars % and _. Leaving any of thesefieldsempty is equivalent to setting it to ’%’.A host may be localhost, a hostname, an IP number or an SQL expression. An empty host inthe db table means any host in the host table. An empty host in the host or user table means anyhost that can create a TCP connection to your server.A db is the name of a database or an SQL regexp.An empty user field means any username will do.An empty password means that the entry will only be used if no password is supplied.The privileges from the user table are OR’ed to the db table. This means that a superuser onlyneeds to be in the user table with all privilege-flags set to Y.Everything granted in the user table is valid for every database that cannot be found in the dbtable.For this reason, it might be wise to grant users (apart from superusers) privileges on a per-databasebasis only.The host table is mainly there to maintain a list of "secure" servers. At TcX host contains a list ofall machines on the local network. These are granted all privileges.26The connecting user’s privileges are calculated by the following algorithm:1. First sort all three tables by host by putting hosts without wildcards first, followed by hostswithwildcards and entries with host = "". Within each host, di.e. very much like GROUP BYHost, sort by user using the same rules. Finally, in the db table, sort by db using the same rules.In the steps below, we will look through the sorted tables and always use the first match found.2. Get the privileges for the connecting user from the db table using the first match found. Callthisset of privileges P. 4
  5. 5. 3. If host = "" for the entry found in the db table, AND P with the privileges for the host in thehost table, i.e. remove all privileges that are not "Y" in both. (If host <> "", P is notaffected. In suchcases, host must have matched the connecting host’s name at least partially.Therefor it can be assumed that the privileges found in this row match the connecting host’sprofile.)4. OR (add) P with the privileges for the user from the user table, i.e. add all privileges that are"Y" in user.The connecting user gets the set of privileges P.Let’s show an example of the sorting and matching! Suppose that the user table contains this:+-----------+----------+-| Host | User | ...+-----------+----------+-| % | root | ...| % | jeffrey | ...| localhost | root | ...| localhost | | ...+-----------+----------+-Then the search order will be:localhost/rootlocalhost/anyany/jeffreyany/rootSo jeffrey attempting to connect on localhost will be matched by the localhost/any line, not bythe any/jeffrey line. The first match found is used!So if you have access problems, print out the user table, sort it by hand, and see where the matchisbeing made.Here follows an example to add a user ’custom’ that can connect from hosts ’localhost’,’server.domain’ and ’’. He wants to have password ’stupid’. The database’bankacount’ he only want to use from ’localhost’ and the ’customer’ database he wants to beable toreach from all three> mysql mysql.mysql> insert into users (host,user,password)values(’localhost’,’custom’,password(’stupid’));mysql> insert into users (host,user,password)values(’server.domain’,’custom’,password(’stupid’));mysql> insert into users (host,user,password)values(’’,’custom’,password(’stupid’));27mysql> insert into db(host,db,user,Select_priv,Insert_priv,Update_priv,Delete_priv,Create_priv,Drop_priv)values(’localhost’,’bankacount’,’custom’,’Y’,’Y’,’Y’,’Y’,’Y’,’Y’);mysql> insert into db(host,db,user,Select_priv,Insert_priv,Update_priv,Delete_priv,Create_priv,Drop_priv)values 5
  6. 6. (’%’,’customers’,’custom’,’Y’,’Y’,’Y’,’Y’,’Y’,’Y’);You can of course also use xmysqladmin, mysql_webadmin, mysqladmin and even xmysql toinsert/change and update values in the privilege tables. You can find these utilities in the Contribdirectory.Adding new user privileges to MySQLTo add privileges to the MySQL database:This assumes the current user has insert privileges for the mysql db table and reload privileges.Theserver (mysqld) has to be running. If it is not, start it with safe_mysqld --log &.> mysql mysqlinsert into user values(’%’,’monty’,password(’something’),’Y’,’Y’,’Y’,’Y’,’Y’,’Y’,’Y’,’Y’,’Y’,’Y’) ;insert into user (host,user,password) values(’localhost’,’dummy’,") ;insert into user values (’%’,’admin’,",’N’,’N’,’N’,’N’,’N’,’N’,’Y’,’N’,’Y’,’Y’) ;quit> mysqladmin reloadThis makes three new users:MontyFull superuser, but must use password when using MySQL.adminDoesn’t need a password but is only allowed to use mysqladmin reload, mysqladminrefresh and mysqladmin processlist. May be granted individual database privilegesthrough table db.dummyMust be granted individual database privileges through table db.Default privileges.The default privileges (set in ‘scripts/mysql_install_db’) is that root can do anything. Anyuser can do anything with any database whose name is ’test’ or starts with ’test_’. A normal usercan’tuse mysqladmin shutdown or mysqladmin processlist. See the script(‘scripts/mysql_install_db’) for an example on how to add other users.The privilege tables are read into mysqld with ’mysqladmin reload’. If the privilege tables areempty(or non-existent) full access are granted to everyone.28A example of permission setup.A common mistake is to try something like:INSERT INTO user VALUES(’%’,’jeffrey’,’bLa81m0’,’Y’,’Y’,’Y’,’N’,’N’,’N’,’N’,’N’, ’N’,’N’);Then (of course) a mysqladmin reload to make the authentication change take effect, then tryingto connect to the server:$ ./mysql -h sqlserver -u jeffrey -p bLa81m0 test Access deniedTry this instead:INSERT INTO user VALUES(’%’,’jeffrey’,password(’bLa81m0’),’Y’,’Y’,’Y’,’N’,’N’,’N’,’N’,’N’,’N’,’N’);And like before mysqladmin reload to make the authentication change take effect.Now things should work.Go to the first [p 5] , previous [p 24] , next [p 30] , last [p 87] section, table of contents [p 1] .29 6
  7. 7. Go to the first [p 5] , previous [p 26] , next [p 52] , last [p 87] section, table of contents [p 1] .MySQL language referenceLiterals. How do you write strings and numbers?STRINGSA string may have ’ or " around it. is a escape character. The following escape characters are recognised:0 A ascii 0 character.n A newline character.t A tab character.r A return character.b A backspace character.’ A ’ character." A " character. A character.% A % character. This is used in wild-card strings to search after %._ A _ character. This is used in wild-card strings to search after _.A ’ inside a string started with ’ may be written as ".A " inside a string started with " may be written as "".MySQL> select ’hello’, "’hello’", ’""hello""’, "’h"e"l"l"o"’, "hel""lo";1 rows in set (0.00 sec)+-------+---------+-----------+-------------+--------+| hello | ’hello’ | ""hello"" | ’h’e’l’l’o’ | hel"lo |+-------+---------+-----------+-------------+--------+| hello | ’hello’ | ""hello"" | ’h’e’l’l’o’ | hel"lo |+-------+---------+-----------+-------------+--------+mysql> select ’hello’, "hello", ’""hello""’, "’ello", ’e"l"lo’, ’’hello’;1 rows in set (0.00 sec)+-------+-------+-----------+-------+--------+--------+| hello | hello | ""hello"" | ’ello | e’l’lo | ’hello |+-------+-------+-----------+-------+--------+--------+| hello | hello | ""hello"" | ’ello | e’l’lo | ’hello |+-------+-------+-----------+-------+--------+--------+mysql> select "ThisnIsnFournlines";1 rows in set (0.00 sec)+--------------------+| ThisIsFourlines |+--------------------+| This30IsFourlines |+--------------------+If you want to insert binary data into a blob the following characters must be represented byescapesequences: 7
  8. 8. 0 Ascii 0. Should be replaced with "0" (A backslash and a 0 digit). Ascii 92, backslash’ Ascii 39, Single quote" Ascii 33, Double quoteNUMBERSIntegers are just a sequence of digits. Floats use . as a decimal separator.Examples of valid numbers are: 1221, 294.42, -32032.6809e+10.NULLWhen using the text file export formats, NULL may be represented by N. See section LOADDATAINFILE syntax [p 45]Column types.The following column types are supported:Name Description SizeTINYINT [(max display size)][UNSIGNED] [ZEROFILL]A very small integer. Signed range -128 - 127. Unsignedrange 0 - 255.1SMALLINT [(max displaysize)]. [UNSIGNED][ZEROFILL]A small integer. Signed range -32768 - 32767. Unsignedrange 0 - 65535.2MEDIUMINT [(max displaysize)] [UNSIGNED][ZEROFILL]A medium integer. Signed range -8388608-8388607.Unsigned range 0 - 16777215.3INT [(max display size)][UNSIGNED] [ZEROFILL]A normal integer. xSigned range -2147483648 -2147483647. Unsigned range 0 - 4294967295.4BIGINT [(max display size)][UNSIGNED] [ZEROFILL]A large integer. Signed range -9223372036854775808 -9223372036854775807. Unsigned Range 0 -18446744073709551615.831FLOAT(Precision)A small floating point number. Precision can be 4 or 8.FLOAT(4) is a single precision number and FLOAT(8) is adouble precision number (se the DOUBLE entry). Thissyntax is for ODBC compatibility. Range-3.402823466E+38F - -1.175494351E-38, 0, 8
  9. 9. -1.175494351E-38 - 3.402823466E+38F.4FLOAT [(max displaysize,number of decimals)]A small floating point number. Cannot be unsigned. Range-3.402823466E+38F - -1.175494351E-38, 0,-1.175494351E-38 - 3.402823466E+38F.4DOUBLE PRECISION [(maxdisplay size,number ofdecimals)]A normal floating point number. Cannot be unsigned.Range -1.7976931348623157E+308 --2.2250738585072014E-308, 0,2.2250738585072014E-308 - 1.7976931348623157E+308.8REAL [(length,decimals)] Same as DOUBLE 8DECIMAL [(max displaysize,number of decimals)]An unpacked floating point number. Cannot be unsigned.Currently the same range maximum range as a double.Behaves as a CHAR columnM+DNUMERIC [(length,decimals)] Same as DECIMAL M+DTIMESTAMP [(display size)] An automatic timestamp. 4DATEA type to store date information. Uses the"YYYY-MM-DD" syntax, but may be updated with anumber or a string. Understands at least the followingsyntaxes: ’YY-MM-DD’, ’YYYY-MM-DD’, ’YYMMDD’,’YYMM’, ’YY’. Range 0000-00-00 to 9999-12-31.4TIMEA type to store time information. Uses the "HH:MM:SS"syntax, but may be updated with a number or a string.Understands at least the following syntaxes: ’HH:MM:DD,’HHMMDD’, ’HHMM’, ’HH’.3DATETIMEA type to store date and time information. Format"YYYY-MM-DD HH:MM:SS". Takes 8 bytes. Range’0000-01-01 00:00:00’ - ’9999-12-31 23:59:59’.8CHAR(max_length) [binary]A fixed length string that is always filled up with spaces tothe specified length. Range 1 - 255 characters. All endspace are removed when retrieved. Is sorted and comparedcase insensitively unless the binary keyword is given.M 9
  10. 10. VARCHAR(max_length)[binary]A variable length string that is stored with its length.Maximum range 1 - 255 characters. Is sorted and comparedcase insensitively unless the binary keyword is given.L+1TINYTEXT and TINYBLOB A TEXT/BLOB with max length of 255 characters. L + 1TEXT and BLOB A TEXT/BLOB with max length of 65535 characters. L + 232MEDIUMTEXT andMEDIUMBLOBA TEXT/BLOB with max length of 1677216 characters. L + 3LONGTEXT and LONGBLOBA TEXT/BLOB with max length of 4294967295characters.L+4ENUM(’value’,’value2’,...)A string object that can have only one set of allowedvalues. See section More about data types [p 33] .1 or2SET(’value’,’value2’,....)A string object that can have one or many values of a set ofallowed values. See section More about data types [p 33] .1-8More about data typesDatabase size info.In the above table L means the actual length of a instance and M the maximum length. So L+1for"abcd" means 5 bytes in the database.If you use any data type with a L in the length field you will get a variable length record format.The numeric typesAll integer types can have a optional argument unsigned. This can be used when you only wanttoallow positive numbers in the column or you need a little bigger numerical range for the column.Also for all integer columns the optional argument ZEROFILL means that the column will bepaddedwith zeroes upto the maximum length.Max display size and decimals are for formating and calculation of max column width.TIMESTAMP typeHas a range of 1 Dec 1970 time 0.00 to sometime in the year 2106 and a resolution of onesecond.Will be automatically updated if not used in a statement that updates a row or if set to NULL.Can be a(part of) a key. Note that if you have many timestamp fields in a row, then only the firsttimestampfield will be automatically updated. Any timestamp field will be set to the current time if set toNULL. 10
  11. 11. Depending on the display size one gets one of the following formats: "YYYY-MM-DDHH:MM:SS","YY-MM-DD HH:MM:SS", "YYYY-MM-DD" or "YY-MM-DD".TEXT and BLOB typesThese are objects that can have a variable length without upper limit. All TEXT and BLOBobjects arestored with is length (saved in 1 to 4 bytes depending on the type of object). The maximumTEXT andBLOB length you can use is dependent on available memory and client buffers. The onlydifferencesbetween TEXT and BLOB is that TEXT is sorted and compared case insensitively while BLOBiscompared case insensitive (by character values). TEXT and BLOB objects CANNOT be keys.A BLOB is a binary large object which can hold any amount of data :) There are 4 kinds of blobsSeesection Column types. [p 31] . Normally one can regard a BLOB as a VARCHAR without aspecifiedlimit.33TEXT is a BLOB that is sorted and compared case insensitively.There are some constraints because of the message buffer used. The default size of the buffer is64Kfor the server and 512K for the clients. To change the buffer length for the server, use mysqld -Omax_allowed_packet=max_blob_length. This allows the message buffer to grow up to thislimit when needed.MyODBC defines BLOB:s as LONGVARBINARY and TEXT:s as LONGVARCHAR.Restrictions for BLOB and TEXT columns:1. A BLOB or TEXT cannot be a key or a part of a key2. When one sorts or groups a BLOB or TEXT only the first max_sort_length (default 1024)of the blob is used. This value can be changed by the -O option when starting the mysqld demon.One can group on an expression involving a BLOB/TEXT: SELECTid,SUBSTR(blob,1,100) GROUP BY 23. There is no end space truncation for BLOB and TEXT as there is for CHAR and VARCHAR.ENUM typeA string object that can have only one set of allowed values. The value to be stored may be givencaseindependently. If one tries to store a non existing value, "" is stored. If used in a number contextthisobject returns/stores the value index. If there is less than 255 possible values this object occupies1byte, else two bytes (with a maximum of 65535 different values). Note that if a integer is put intheENUM you get the corresponding string with the first counting as number zero. Sorting onENUM typesare done after the order of the strings in the enum.For example the column test ENUM("one","two", "three") can have any off these values:"one""two""three" 11
  12. 12. SET typeA string object that can have one or many values of a set of allowed values. Each value isseparated bya ’,’. If used in a number context this object returns/stores the bit positions of the used values.Thisobject occupies (number_of_different_values-1)/8+1 bytes, rounded up to 1,2,3,4 or 8. One can’thavemore than 64 different values. Note that if a integer is put in the SET you get the correspondingstringwith the first bit corresponding to the first string.Sorting on SET types are done numericaly.For example the column test SET("one","two") can have any off these values:"""one""two""one,two"34Column indexesYou can have indexes on all MysQL column types except BLOB and TEXT. Using indexes ontherelevant columns is the most important thing is getting you selects to run fast.For CHAR and VARCHAR columns you can have a index on a prefix. The example below showhowto create a index for the first 10 characters of a column. This is much faster and requires less diskspace than having a index on the whole field.CREATE TABLE test (name CHAR(200),KEY name (last_name(10));Multiple field indexesMySQL can have one index on parts of different columns.A multiple column key can be considered as a sorted array where the columns are concatenated.Suppose that you have a table:CREATE TABLE test (id INT,last_name CHAR(30),first_name CHAR(30),PRIMARY KEY (id),KEY name (last_name,first_name));Then the key ’name’ is a key over last_name and first_name.The ’name’ key will be used in the following queries:SELECT * FROM test WHERE last_name="Widenius";SELECT * FROM test WHERE last_name="Widenius" AND first_name="Michael";SELECT * FROM test WHERE last_name="Widenius" AND (first_name="Michael" ORfirst_name="Monty");SELECT * FROM test WHERE last_name="Widenius" and first_name >="M" and first_name <"N";The ’name’ key will NOT be used in the following queries:SELECT * FROM test WHERE first_name="Michael";SELECT * FROM test WHERE last_name="Widenius" or first_name="Michael";Type mapping (to ease moving tabel definitions between different 12
  13. 13. databases engines)To support easier use of code from different SQL vendors, MySQL does supports the followingmappings:35binary(num) char(num) binarychar varying varcharfloat4 floatfloat8 doubleint1 tinyintint2 smallintint3 mediumintint4 intint8 bigintlong varbinary bloblong varchar textmiddleint mediumintvarbinary(num) varchar(num) binaryCREATE TABLE syntax.CREATE TABLE table_name ( create_definition,... )create_definition:column_name type [DEFAULT default_value] [NOT NULL | NULL] [ PRIMARY KEY ][reference_definition]or PRIMARY KEY ( key_column_name,... )or KEY [key_name] KEY( key_column_name,...)or INDEX [key_name] ( key_column_name,...)or UNIQUE [key_name] ( key_column_name,...)or FOREIGN KEY key_name ( key_column_name,...) [reference_definition]or CHECK (expr)key_column_name:column_name [ (length) ]reference_definition:REFERENCES table_name [( key_coulmn_name,...)][ MATCH FULL | MATCH PARTIAL][ ON DELETE reference_option][ ON UPDATE reference_option]reference_option:RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULTThe FOREIGN KEY, CHECK and REFERENCE syntax are only for compatibility. They don’tactually do anything.If a column doesn’t have a DEFAULT value and is not declared as NOT NULL, the defaultvalue isNULL.36ZEROFILL means that number is pre-zeroed to maximal length. With INT(5) ZEROFILL avalue of 5 is retrieved as 00005.BINARY means that the column will be compared case sensitive. The default is that all stringsare compared case insensitive. BINARY is ’sticky’ which means that if a column marked’binary’ is used in a expression, the whole expression is compared ’binary’.INDEX is only a synonym for KEY. 13
  14. 14. If one doesn’t assign a name to a key, the key will get the same name as the first key_columnwith an optional _# to make it unique.Key columns and timestamp columns can’t be NULL. For these columns the NULL attribute issilently removed.With column_name(length) syntax one can specify a key which is only a part of a stringcolumn. This can make the index file much smaller.A number column may have the additional attribute AUTO_INCREMENT to automatically getthe largest value+1 for each insert where column value is NULL or 0. See section How can I getthe unique ID for the last row? [p 83]One can insert NULL for timestamp and auto_increment columns. This results in the currenttime/ the next number.Blob columns can’t be keys.When one groups on a blob only the first ’max_sort_length’ bytes are used See sectionLimitations of BLOB and TEXT types [p 21] .Deleted records are in a linked list and subsequent inserts will reuse old positions. To get smallerfiles one can use the isamchk utility to reorganise tables.Each null column takes 1 bit extra, rounded up to the nearest byte.The maximum record length can be calculated as follows: 1+ sum_of_column_lengths +null_columns/8 + number of variable length columns.In some cases an attribute may silently change after creation: VARCHAR columns with a lengthof1 or 2 are changed to CHAR. When using one VARCHAR column all CHAR columns longerthan 2are changed to VARCHAR’s.On INSERT/UPDATE all strings (CHAR and VARCHAR) are silently chopped/padded to themaximal length given by CREATE. All end spaces are also automatically removed. For exampleVARCHAR(10) means that the column can contain strings with a length up to 10 characters.Something/0 gives a NULL value.The regular expression function (REGEXP and RLIKE) uses ISO8859-1 (Latin1) when decidingthe type of a character.ALTER TABLE syntaxALTER [IGNORE] TABLE table_name alter_specification [, alter_specification ...]alter_specification:ADD [COLUMN] create_definitionor CHANGE [COLUMN] old_column_name create_definitionor ALTER [COLUMN] column_name { SET DEFAULT literal | DROP DEFAULT }or DROP [COLUMN] column_nameor DROP PRIMARY KEYDROP INDEX key_nameALTER TABLE works by creating a temporary table and copying all information to it and thenthe old table is deleted and the new one is renamed. This is done in such a way that all updatesare automatically redirect to the new table without any failed updates. While the ALTERTABLEis working, the old table is readable for other clients. Table updates/writes to the table are stalled37and only executed after the new table is ready.If IGNORE isn’t specified then the copy will be aborted and rolled back if there exists anyduplicated unique keys in the new table. This is a MySQL extension. 14
  15. 15. The CHANGE column_name, DROP column_name and DROP INDEX are MySQLextensions to ANSI SQL92.The optional word COLUMN is a pure noise word and can be omitted.The ADD and CHANGE takes the same create_definition as CREATE TABLE. See sectionCREATE TABLE syntax. [p 36] .ALTER COLUMN sets a new default value or removes the old default value for a column.DROP INDEX removes an index. This is an MySQL extension.The FOREIGN KEY syntax in MySQL exists only for compatibility with other SQL vendorsCREATE TABLE commands: It doesn’t do anything. See section How standards compatible areMySQL [p 19]If one drops a column_name which is part of some key, this key part is removed. If all key partsare removed then the key is removed.DROP PRIMARY KEY drops the first UNIQUE key in the table.CHANGE tries to convert data to the new format as good as possible.With mysql_info(MYSQL*) one can retrieve how many records were copied and how manyrecords were deleted because of multiple keys.To use ALTER TABLE one has to have select, insert, delete, update, create and drop privilegeson the table.DROP TABLE syntax.DROP TABLE table_name [, table_name....]Removes one or more tables. All the data and the definition is removed so take it easy with thiscommand!DELETE syntax.DELETE FROM table_name WHERE where_definitionReturns records affected.If one does a delete without a where clause then the table is recreated which is much faster thandoinga delete for each row. In these cases, the SQL command returns zero as affected records.Using DELETE FROM table_name will work even if the data files do not exist as onlyinformation from the table definition file, table_name.frm, is used.All string comparisons are case independent with case according to ISO-8859-1 Latin1.LIKE is allowed on numerical columns.Compare with explicit NULL (column == NULL) is the same as if IS NULL was used (columnIS NULL). This is is done to be consistent with mSQL.38SELECT syntaxSELECT [STRAIGHT_JOIN] [DISTINCT | ALL] select_expression,... [INTOOUTFILE ’file_name’ ...] [ FROM tables... [WHERE where_definition ][GROUP BY column,...] [ ORDER BY column [ASC | DESC] ,..] HAVINGwhere_definition [LIMIT [offset,] rows] [PROCEDURE procedure_name]]Strings are automatically converted to numbers and numbers to strings when needed (ala perl). Ifin acompare operation ((=, <>, <= ,<, >=, >)) either if the arguments are numerical thearguments are compared as numbers, else the arguments are compared as strings. All stringcomparisons are by default done case-independent by ISO8859-1 (The Scandinavian letter setwhichalso works excellent with English).select 1 > "6x"; -> 0select 7 > "6x"; -> 1 15
  16. 16. select 0 > "x6"; -> 0select 0 = "x6"; -> 1A column name does not have to have a table prefix if the given column name is unique.A select expression may be given an alias which will be column name and can be used whensorting and grouping or in the HAVING concat(last_name,’ ’,first_name) as name from table order by nameIn LIKE expressions % and _ may be preceded with ’’ to skip the wild-card meaning.A DATE is a string with one of the following syntaxes:YYMMDD (Year is assumed to be 2000 if YY < 70.)YYYYMMDDYY.MM.DD Where ’.’ may be any non numerical separator.YYYY.MM.DD Where ’.’ may be any non numerical separator.IFNULL() and IF() returns number or string value according to use.ORDER and GROUP columns may be given as column names, column alias or column numbersinSELECT clauses.The HAVING clause can take any columns or alias in the select_expressions. It is applied last,just before items are sent to the client, without any optimisation. Don’t use it for items thatshould be in the WHERE clause. You can’t write (yet):SELECT user,MAX(salary) FROM users GROUP BY users HAVING max(salary)>10Change it to:SELECT user,MAX(salary) AS sum FROM users GROUP BY users HAVING sum > 10STRAIGHT_JOIN forces the optimiser to join the tables in the same order that the tables aregiven in the FROM clause. One can use this to get a query to be done more quickly if theoptimiser joins the tables in not optimal order. See section EXPLAIN syntax. Get informationabout a SELECT. [p 47]LIMIT takes one or two numerical arguments.If one argument, the argument indicates the maximum number of rows in a result.If two arguments, the first argument says the offset to the first row to return, the second is39the maximum number of rows.INTO OUTFILE ’filename’ writes the given set to a file. The file can not exist from before. Seesection LOAD DATA INFILE syntax [p 45] .FunctionsA select_expression or where_definition can consist of any expression using thefollowing functions:Group functions.() ParenthesisNormal mathematical operations.+-*/ A division by zero results in a NULL.Bit functions. These have a range of maximum 64 bits because MySQL uses longlong arithmetic.|&BIT_COUNT()Number of set bits in an argument. 16
  17. 17. Normal logical. Returns TRUE (1) or FALSE (0).NOT!ORANDComparison operators. Returns TRUE (1) or FALSE (0). These functions work for both numbersandstrings.= Equal<> Not equal. Synonym: !=<= Smaller or equal.< Smaller.>= Bigger or equal> BiggerISNULL(A)Returns 1 if A is NULL else 0. Same as ’( A == NULL ’).A BETWEEN B AND CA is bigger or equal as B and A is smaller or equal to C. Is the same as (A >= B AND A <= C).40String comparison functions.expr IN (value,...)Returns 1 if expr is any of the values in the IN list, else it returns 0. If all values are constants,then all values are evaluated according to the type of expr and sorted. The search for them item isthen done by using a binary search. This means IN is very quick when used with constants in theIN part.expr LIKE exprSQL simple regular expression comparison. Returns TRUE (1) or FALSE (0). With LIKE youhave two wild characters. ’%’ stands for any number of characters, even zero characters. ’_’stands exactly one character. If you want to search after a ’%’ or a ’_’ you must precede it witha ’’ For example the string "Monty was here" is matched by "Monty%", "%Monty ___ h%"and "%was%".expr NOT LIKE exprReturns TRUE (1) or FALSE (0).expr REGEXP exprChecks string against extended regular expr.expr NOT REGEXP exprChecks string against extended regular expr.STRCMP()Returns 0 if the strings are the same. Otherwise return -1 if first argument is smaller according tosort-order, otherwise it returns 1.Control flow functions.IFNULL(A,B)If A is not null it returns A, else B.IF(A,B,C)If A is true (<> 0 and <> NULL) then return B, else return C.Mathematical functions. These returns NULL in the case of a error.- SignABS()SIGN() 17
  18. 18. Sign of argument. Returns -1, 0 or 1.MOD()% Module (like in C). Equivalent with MOD().FLOOR()CEILING()ROUND()EXP()LOG()LOG10()POW()SQRT()PI()COS()SIN()41TAN()TAN2()ACOS()ASIN()ATAN()RAND([integer_expr])Returns a random float, 0 <= x <= 1.0, using integer_expr as the option seed value.MIN()Min value of arguments. Must have 2 or more arguments, else these are GROUP BY functions.The arguments are compared as numbers.MAX()Max value of arguments. Must have 2 or more arguments, else these are GROUP BY functions.The arguments are compared as numbers.String functions.ASCII(string_exp)Returns the ASCII code value of the leftmost character of string_exp. If string_exp is NULL, itthen returns NULL.CHAR(num,...)Returns a string that consist of the characters given by the ASCII code values of the arguments.NULLS are skipped.CONCAT()Concatenates strings. May have more than 2 arguments.LENGTH()Length of string. Synomyes: CHAR_LENGTH(), OCTET_LENGTH()LOCATE(A,B)Returns position of B substring in A. The first position is 1. Returns 9 if b is not in A.LOCATE(A,B,C)Returns position of B substring in A starting at C. Synonym: POSITION(B IN A)LEFT(str,length)Gets length characters from beginning of string.RIGHT(str,right)Gets length characters from end of string. Synonym: SUBSTRING(A FROM B)LTRIM(str)Removes space characters from the beginning of string. 18
  19. 19. RTRIM(str)Removes space characters from the end of string.TRIM([[ BOTH | LEADING | TRAILING] [ A ] FROM ] B)Returns a character string with all A prefixes and/or suffixes removed from B. If BOTH,LEADING and TRAILING isn’t used BOTH are assumed. If A is not given, then spaces areremoved.SOUNDEX(str)Gets a soundex string from str. Two strings that sound ’about the same’ should have identicalsoundex strings. A ’standard’ soundex string is 4 long, but this function returns an arbitrary longstring. One can use SUBSTRING on the result to get a ’standard’ soundex string. All non alphacharacters are ignored in the given string. All characters outside the A-Z range are treated asvocals.SUBSTRING(A,B,C)Returns substring from A starting at B with C chars.42SUBSTRING(A FROM B FOR C)Same as SUBSTRING(A,B,C). This is ANSI SQL 92.SUBSTRING(A FROM B)Same as RIGHT(A,B). This is ANSI SQL 92.SUBSTRING_INDEX(string,delimiter,count)Returns the substring from ’string’ after ’count’ delimiters. If count is positive the strings aresearched from left else if count is negative the substrings are searched and returned from right.substring("",".",2) would return "www.tcx" andsubstring("",".",-2) would return "".REPLACE(A,B,C)Replaces all occurrences of B in A with C.REPEAT(string,count)Repeats string count times. If count <= 0 returns a empty string. If string or count is NULL orLENGTH(string)*count > max_allowed_size returns NULL.INSERT(org,start,length,new)Replaces substring org[start...length] with new. First position in string=1.LCASE(A)Changes A to lower case. Synonym: LOWER()UCASE(A)Changes a to upper case. Synonym: UPPER()ENUM(A,a,b,c,d)Returns 1 if A == a, 2 if A == b. If no match 0 is returned. A,a,b,c,d... are strings.ELT(N,a,b,c,d,...)Returns a if N == 1, b if N == 2. a,b,c,d are strings.FIELD(A,a,b,c,d,...)Returns index of A in the a,b,c... list. The complement of ELT().Date and time functions.PERIOD_ADD(P:N)Adds N months to period P (of type YYMM or YYYYMM). Returns YYYYMM.PERIOD_DIFF(A,B)Returns months between periods A,B. A and B should be of format YYMM or YYYYMM.Example: PERIOD_DIFF(9612,199712).TO_DAYS(DATE) 19
  20. 20. Changes a DATE to a daynumber. DATE may be a date string, a datetime string, a timestamp([6|8 | 14]) or a number of format YYMMDD or YYYYMMDD.FROM_DAYS()Changes a daynumber to a DATE.WEEKDAY(DATE)Gets weekday for date (0 = Monday, 1 = Tuesday). DATE may be a date string, a datetimestring,a timestamp([6 | 8 | 14]) or a number of format YYMMDD or YYYYMMDD.CURDATE()Returns date of today. In form YYYYMMDD or "YYYY-MM-DD" depending on whetherCURDATE() is used in a number or string context. Synonymes: CURRENT_DATE().CURTIME()Returns current time in form HHMMSS or "HH:MM:SS", depending on whether CURTIME() isused in a number or string context. Synonymes: CURRENT_TIME().43NOW()Returns current time. In format YYYYMMDDHHMMSS or "YYYY-MM-DD HH:MM:SS"depending on whether NOW() is used in a number or string context. Synonymes:CURENT_TIMESTAMP(), SYSDATE().UNIX_TIMESTAMP([DATE])If called without any arguments a unix timestamp (seconds in GMT since 1970.01.01 00:00:00)is returned. Normally it is called with a timestamp column as an argument in which case itreturns the timestamp. DATE may also be a date string, a datetime string, or a number of formatYYMMDD or YYYMMDD in local time.FROM_UNIXTIME(unix_timestamp)Returns a string of the timestamp in YYYY-MM-DD HH:MM:SS or YYYYMMDDHHMMSSformat depending on context.SEC_TO_TIME(seconds)Returns the hours, minutes and seconds of the argument in H:MM:SS or HMMSS formatdepending on context.Miscellaneous functions.DATABASE()Returns current database name.USER()Returns current user name. Synonymes: SESSION_USER() and SYSTEM_USER().PASSWORD()Calculates a password string. This must be used to store a password in the ’user’ grant table.ENCRYPT(string[,salt])Crypt messsage with the unix crypt() command. The salt should be a string with 2 characters.LAST_INSERT_ID()Returns the last automaticly generated value that was set in a auto_increment column. SeesectionHow can I get the unique ID for the last row? [p 83]FORMAT(nr,NUM)Formats number to a format like ’#,###,###.##’ with NUM decimals.Functions for GROUP BY clause.count(expr)Number of rows. count(*) is optimised to return very quickly if no other column is used in the 20
  21. 21. SELECT.avg(expr)Average value of expr.min(expr)max(expr)Minimum/Maximum value of expr. min() and max() may take a string argument and will thenreturn the minimum/maximum string value.sum(expr)Sum of expr.std(expr)Standard derivation of expression. This is a extension to ANSI SQL.bit_or(expr)The logical OR of all bits in expr. Caclulation done with 64 bits precision.44bit_and(expr)The logical AND of all bits in expr. Caclulation done with 64 bits precision.MySQL has extended the use of GROUP BY. You can use columns or calculations in theSELECTexpressions which doesn’t appear in the GROUP BY part. This stands for ’any possible value forthisgroup’. By using this, one can get a higher performance by avoiding sorting and grouping onunnecessary items. For example, in the following query one doesn’t need to sort on,,COUNT(*) from a,b WHERE GROUP BY a.idINSERT syntaxINSERT INTO table [ (column_name,...) ] VALUES (expression,...)or INSERT INTO table [ (column_name,...) ] SELECT ....An expression may use any previous column in column_name list (or table if no column namelist isgiven).The following holds for a multi-row INSERT statement:The query cannot contain an ORDER BY clause.The target table of the INSERT statement cannot appear in the FROM clause of the query.If one uses INSERT INTO ... SELECT ... then one can get the following info string withthe C API function mysql_info(). @result{Records: 100 Duplicates: 0 Warnings: 0}Duplicates are number of rows which couldn’t be written because some key would be duplicated.Warnings are columns which were set to NULL, but have been declared NOT NULL. These willbe set to their default value.If one sets a time stamp value to anything other than NULL, the time stamp value will be copiedto the result table.Auto increment fields works as usual.LOAD DATA INFILE syntaxLOAD DATA INFILE ’table_interval.text’ [REPLACE | IGNORE] INTO TABLEtable_interval [FIELDS [TERMINATED BY ’,’ [OPTIONALLY] ENCLOSED BY’"’ ESCAPED BY ’’ ]] [LINES TERMINATED BY ’n’] [(field list)]This is used to read rows from a text file, located on the server, at a very high speed. The server-clientprotocol doesn’t yet support files over a connection. If you only have the file on the client, usercp orftp to copy it, possibly compressed, to the server before using LOAD DATA INFILE. 21
  22. 22. To write data to a text file, use the SELECT ... INTO OUTFILE ’interval.txt’ fieldsterminated by ’,’ enclosed by ’"’ escaped by ’’ lines terminatedby ’n’ FROM .... syntax.Normally you don’t have to specify any of the text file type options. The default is a compacttext filewith columns separated with tab characters and all rows end with a newline. Tabs, newlines and inside fields are prefixed with a . NULL’s are read and written as N.45FIELDS TERMINATED BY has the default value of t.FIELDS [OPTIONALLY] ENCLOSED BY has the default value of ".FIELDS ESCAPED BY has the default value of ’’.LINES TERMINATED BY has the default value of ’n’.FIELDS TERMINATED BY and LINES TERMINATED BY may be more than 1 character.If LINES TERMINATED BY is an empty string and FIELDS TERMINATED BY is not emptythenlines are also terminated with FIELDS TERMINATED BY.If FIELDS TERMINATED BY and FIELDS ENCLOSED BY both are empty strings (") thenthisgives a fixed row format ("not delimited" import format). With a fixed row size NULL valuesareoutputed as a blank string. If you specify OPTIONALLY in ENCLOSED BY, then only stringsareenclosed in ENCLOSED BY by the SELECT ... INTO statement.Duplicated ENCLOSED BY chars are removed from strings that start with ENCLOSED BY. Forexample: With ENCLOSED BY ’"’:"The ""BIG"" boss" -> The "BIG" bossThe "BIG" boss -> The "BIG" bossIf ESCAPED BY is not empty then the following characters will be prefixed with the escapecharacter: ESCAPED BY, ASCII 0, and the first character in any of FIELDS TERMINATEDBY,FIELDS ENCLOSED BY and LINES TERMINATED BY.If FIELDS ENCLOSED BY is not empty then NULL is read as a NULL value. If FIELDSESCAPEDBY is not empty then N is also read as a NULL value.If REPLACE is used, then the new row will replace all rows which have the same unique key. IfIGNORE is used, the row will then be skipped if there already exist a record with an identicaluniquekey. If none of the above options are used an error will be issued. The rest of the text file will beignored if one gets a duplicate key error.Some possible cases that’s not supported by LOAD DATA:Fixed size rows (FIELDS TERMINATED BY and FIELDS ENCLOSED BY both are empty)and BLOB fields.If some of the separators are a prefix of another.FIELDS ESCAPED BY is empty and the data contains LINES TERMINATED BY or FIELDSENCLOSED BY followed by FIELDS TERMINATED BY.All rows are read into the table. If a row has too few fields the rest of the fields are set to defaultvalues.For security reasons the text file must either reside in the database directory or be readable by all. 22
  23. 23. For more information about the escaped syntax See section Literals. How do you write stringsandnumbers? [p 30] .46When the LOAD DATA query is done, one can get the following info string with the C APIfunctionmysql_info().@result{Records: 1 Deleted: 0 Skipped: 0 Warnings: 0}Warnings are incremented for each column which can’t be stored without loss of precision, foreachcolumn which didn’t get a value from the read text line (happens if the line is too short) and foreachline which has more data than can fit into the given columns.Example that loads all fields;LOAD DATA INFILE ’persondata.text’ INTO TABLE persondata;See section How should I arrange my table to be as fast/small as possible? [p 59]UPDATE syntaxUPDATE table SET column=expression,... WHERE where_definitionAll updates are done from left to right. If one accesses a column in the expression update willthen usethe current value (a given value or the default value) of the column.UPDATE persondata SET count=count+1SHOW syntax. Get information about names of columns.SHOW DATABASES [LIKE wild]or SHOW TABLES [FROM database] [LIKE wild]or SHOW COLUMNS FROM table [FROM database] [LIKE wild]Gives information about databases, tables or columns. The wild is a LIKE string. FIELDS maybeused as an alias for COLUMNS.EXPLAIN syntax. Get information about a SELECT.EXPLAIN SELECT select_optionsGives information about how and in which order tables are joined. With the help of EXPLAINone cansee when one has to add more keys to tables to get a faster select that uses keys to find therecords.You can also see if the optimiser joins the tables in an optimal order. One can force the optimisertouse a specific join order with the STRAIGHT_JOIN option to select.The different join types are:systemThe table has only 1 record (= system table)constThe table has at most 1 matching record which will be read at the start of the query. All columnsin this table will be regarded as constants by the rest of the optimiser.eq_refOne record will be read from this table for each combination of the previous tables.47refAll records with matching keys will be read from this table for each combination of the previous 23
  24. 24. tables.allA full table scan will be done for each combination of the previous tables.DESCRIBE syntax. Get information about columns.(DESCRIBE | DESC) table [column]Gives information about columns. This command is for Oracle compatibility. See section SHOWsyntax. Get information about names of columns. [p 47] Column may be a column name or astring.Strings may contain wild cards.Lock tables syntaxLOCK TABLES table_name [AS alias] READ|WRITE [, table_name READ|WRITE ] ...UNLOCK TABLESLocks tables for this thread. Many threads may have a READ lock on the same table, but one canusea table with a WRITE lock. One can’t update a table on which one has a read LOCK. When oneusesLOCK TABLES one must lock all tables one is going to use!LOCK TABLES t READ, t as t2 READSELECT * from t,t2;All tables are automaticly unlocked when one issues another LOCK TABLES or if theconnection tothe server is closed.SET OPTION syntax.SET OPTION SQL_VALUE_OPTION=value, ...The used options remain in effect for the whole current session.The different options are:SQL_SELECT_LIMIT=valueThe maximum number of records to return in any select. If a select has a limit clause it overridesthis statement. The default value for a new connection is ’unlimited’.SQL_BIG_TABLES= 0 | 1If set to 1 then all temporary tables are stored on disk based. This will be a little slower, but onewill not get the error The table ### is full anymore for big selects that require bigtemporary tables. The default value for a new connection is 0 (use in memory temporary tables).SQL_BIG_SELECTS= 0 | 1If set to 1 then MySQL will aborts if one will try to make a select with probably will take a verylong time. This is nice to SELECTS with a wrong WHERE statement. A big query is defined asaSELECT that will probably have to examine more than max_join_size rows. The defaultvalue for a new connection is 0 (allow all SELECT’s).48GRANT syntax. (Compatibility function).GRANT (ALL PRIVILEGES | (SELECT, INSERT, UPDATE, DELETE,REFERENCES (column list), USAGE))ON table TO user,... [WITH GRANT OPTION]This command doesn’t do anything. It is only in MySQL for compatibility reasons. Privileges inMySQL is handled with the mysql grant tables. See section How does the privilege system work?[p26] 24
  25. 25. CREATE INDEX syntax (Compatibility function).CREATE [UNIQUE] INDEX index_name ON table_name ( column_name,... )This function doesn’t do anything. It is only in MySQL for compatibility reasons. You can createnewindex with ALTER TABLE. See section ALTER TABLE syntax [p 37]DROP INDEX syntax (Compatibility function).DROP INDEX index_nameThis always succeeds. You can drop an index with ALTER TABLE. See section ALTERTABLEsyntax [p 37]Is MySQL picky about reserved words?It seems that I can’t create a table with column names timestamp or group.Functions don’t clash with table or column names. (For example ABS is an allowed columnname).The only restriction is that space is not allowed between a function name and the ’(’ when usingfunctions.The following are reserved words in MySQL. Most of them (for example) group, are forbiddenbyANSI SQL92 as column and/or table names. A few are because MySQL needs them and is(currently)using a Yacc parser:49action add all alter andas asc auto_increment between bigintbit binary blob both bycascade char character change checkcolumn columns create data databasesdate datetime dec decimal defaultdelete desc describe distinct doubledrop escaped enclosed enum explainfields float float4 float8 foreignfrom for full grant grouphaving ignore in index infileinsert int integer interval int1int2 int3 int4 int8 intois key keys leading likelines limit lock load longlongblob longtext match mediumblob mediumtextmediumint middleint numeric no notnull on option optionally ororder outfile partial precision primaryprocedure privileges read real referencesregexp repeat replace restrict rlikeselect set show smallint sql_big_tablessql_big_selects sql_select_limit straight_join table tablesterminated text time timestamp tinyblobtinytext tinyint trailing to uniqueunlock unsigned update usage valuesvarchar varying with write where 25
  26. 26. zerofillThe following symbols (from the table above) are disallowed by ANSI SQL but allowed byMySQLas column/table names. This is because some of theese names are very natural names so a lot ofpeople has already used them.ACTIONBITDATE50ENUMNOTEXTTIMETIMESTAMPGo to the first [p 5] , previous [p 26] , next [p 52] , last [p 87] section, table of contents [p 1] .51Go to the first [p 5] , previous [p 30] , next [p 57] , last [p 87] section, table of contents [p 1] .How safe/stable is MySQLHow stable is MySQL?At TcX, MySQL has worked without any problems in our projects since nid 1996. Whenreleased to awider public we noticed that there where some parts of ’untested code’ in MySQL that wasquicklyfound by the new user group that don’t do queries exactly like we do at TcX. Each new releasehavehad fewer non portability problems than the previous one, even though they have had a lot ofnewfeatures, and we hope that one of the next releases will be possible to be labelled ’stable’.Each release of MySQL has been usable and there has only been problems when users start tousecode from ’the gray zones’. Naturally, outdoor users can’t know what the gray zones are and Ihopethis section will clarify those currently known.We will here try to answer some of the more important questions that seems to concern a lot ofpeopleand clarify some issues. This section has been put together from the information that has comeforward in the mailing list which is very active in reporting bugs.How stable is MySQL? Can I depend on MySQL in this project!This is about the 3.21.x version of MySQL. All known and reported bugs are fixed in the latestversion with the exception of the bugs listed in the BUGS file which are things that are ’design’related.MySQL is written in multiple layers and different independent modules. Here is a list of thedifferentmodules and how tested each of them are.The ISAM table handler. Stable(ISAM) where all data is stored. In all MySQL releases there hasn’t been a single (reported) bugin this code. The only known way to get a corrupted table is to kill the server in the middle of aupdate and because all data is flushed to disk between each query even this is unlikely to destroy 26
  27. 27. any data beyond rescue. There hasn’t been a single bug report about lost data because of bugs inMySQL either.The parser and lexical analyser. StableThere hasn’t been a single reported bug in this system for a couple of months.The C client code. StableNo known problems. In early 3.20 releases there were some limitations in the send/receive buffersize. In 3.21.x the send/receive buffer is now dynamic up to a default of 512K.mysql, mysqladmin and mysqlshow. StableThe command line clients have had very few bugs.mysqldump and mysqlimport. BetaRewritten for 3.21.Basic SQL. StableThe basic SQL function system and string classes and dynamic memory handling. Not a singlereported bug on this system.52Query optimiser. GammaSome changes in 3.21.Range optimiser. AlphaTotally rewritten for 3.21.xJoin optimiser. GammaSmall changes for 3.21.GROUP BY, ORDER BY and related function COUNT(). BetaRewritten for 3.21 and throughly tested.Locking. GammaThis is very system dependent. One some system there is big problems to use standard OSlocking (fcntl). In these case one should run the MySQL demon with the --skip-locking flag.Known problems are some Linux systems and SUNOS when using NFS mounted file systems.Linux threads. GammaThe only found problem is with the fcntl() call, which is fixed by using --skip-locking. Somepeople have reported lockup problems with the 0.5 release.Solaris 2.5+ pthreads. StableWe use this for all our production work.MIT threads (Other systems). BetaNo reported bugs since 3.20.15 and no known bugs since 3.20.16. On some system there is’misfeature’ where some operations are quote slow (a 1/20 second sleep is done between eachquery).Other thread implementions. AlphaThe ports to other systems is very new and may still have bugs, either in MySQL or most oftenin the thread implementation itself.LOAD DATA..., INSERT ... SELECT. StableSome people have thought they have found bugs in this but has turned up beingmisunderstandings. So check the manual before reporting bugs!ALTER TABLE. GammaPartly rewritten for 3.21.mysqlperl. StableNo bugs reported except a lot of compiling and linking problems.DBD. BetaNow maintained by Jochen Wiedmann . Thanks! 27
  28. 28. mysqlaccess. BetaWritten and maintained by Thanks!The Technical Documentation. BetaIt is improving.MyODBC (uses ODBC SDK 2.5). BetaIt seams to work well with some programs. There is some known problems with Office97.PowerBuilder doesn’t work yet. Error messages aren’t returned right when using a MicrosoftNew ODBC 3.0 driver but this may well be a bug in the Microsoft ODBC 3.0 driver. It isn’t fullyup to ODBC level 2 yet in every regard. (My personal opinion is that the ODBC levels aredefined in such a way that it’s almost impossible to make a simple ODBC driver that works withall products). Sometimes the MyODBC code must be ported to ODBC 3.0.TcX provides email support for paying customers, but the MySQL mailing list usually providesanswers to all common questions. Bugs are usually fixed right away with a patch that usuallyworksand for serious bugs there is almost always a new release.53Why are there is so many release of MySQL? Is it becausethere are so many bugs?Well, MySQL is evolving quite rapidly here at TcX and we want to share this with other MySQLusers. We try to make a release when we have a very useful feature that others seem to have aneedfor.We also try to help out users who request features that are easy to implement. We also take noteonwhat our licensed users want to have and we especially take notes on what our extended emailsupported customers want and try to help them out.No one has to download a new release. The NEWS file will tell you if the new release hassomethingyou really want.If there is, by any chance, a fatal bug in a release we will make a new release as soon as possible,thatfixes the problem. We would like other companies to do this too :)The 3.21.x version has had many big changes for portability to a lot of different systems. Whenthe3.21 release is stable we will remove the alpha/beta suffix and move active development to the3.22.Bugs will still be fixed in the stable version. We don’t believe in a complete freeze, as this alsoleavesout bug fixes and things that ’must be done’. ’Somewhat freezed’ means that we will maybe addsmallthings that ’almost surely will not affect anything thats already working’.Checking a table for errors.If MySQL crashed (for example if the computer is turned off) when all data is not written to diskthetables may have become corrupted. To check a table use:isamchk table_nameThis finds 99.99 % of all errors. What it can’t find is when only the data file has been corrupted.isamchk -e table_name 28
  29. 29. This goes through all data and does a complete check.isamchk -ei table_nameAs the above but it also prints some statistics.We at TcX run a cron job on all our important tables once a week.35 0 * * 0 /path/to/isamchk -s /path/to/dbs/*/*.ISMThis prints out any crashed tables so we can go and examine and repair them when needed.As we haven’t had any unexpected crashed (without hardware trouble) tables for a couple ofyearsnow (this is really true), once a week is more than enough of us.Of course, whenever the machine has done a reboot in the middle of a update one usually has tocheckall the tables that could have been affected. (This is a ’expected crashed table’)We recommend that to start with one should do a isamchk -s on all updated tables each nightuntilone gets to trust MySQL as much as we trust it.54Naturally, one could add a check to safe_mysql that, if there is an old pid file left after a reboot,itshould check all tables that have been modified the last 24 hours.How to repair tables.The file format that MySQL uses to store data has been extensively tested, but there is alwaysinstances (like a hard kill on the mysqld process in the middle of a write, a hardware error or aunexpected shutdown of the computer) when some tables may be corrupted.The sign of a corrupted table is usually when queries abort unexpectedly and one gets errors like:table.frm is locked against change.Can’t find file ’table.ISM’ (Errcode: ###)Got error ### from table handler. (Error 135 is an exception in this case)Unexpected end of file.Record file is crashed.In thess cases you have to repair your tables. The isamchk extern utility can usually detect andfixmost things that can go wrong. See section The MySQL table check, optimize and repairprogram [p66] .If you are going to use isamchk on very large files, you should first decide how much memoryyouwant to give to isamchk. More memory gives more speed. For example, if you have more than32Mram, try:isamchk -O sortbuffer=16M -O keybuffer=16M -O readbuffer=1M-O writebuffer=1M .....Part 1; CheckingCheck the permissions of the table files. Make them readable for the user running to the database directory.Run ’isamchk *.ISM’ or (’isamchk -e *.ISM’ if you have more time).You only have to repair those tables that your isamchk gives an error. Use option -s to avoidunnecessary information.Part 2; Easy safe repair.If you get weird errors when checking or repairing, like out of memory or if isamchk 29
  30. 30. crashes, go to part 3.Try first ’isamchk -rq table’. This will try to repair the .ISM file without touching theimportant data table (.ISD). If the data file (.ISD) contains everything and the delete linkspoint at the right places in the data file, this should work and the table is fixed. Startrepairing next table.Make a backup of the data file (table.ISD) before continuing.Use ’isamchk -r table’. This will remove wrong records and deleted records from the datafile and reconstruct the index (.ISM) file.If the above fails, use ’isamchk -ro table’. This is a little slower but a crash-safer version ofthe above.Part 3; Hard repair.This should only happen if the first 16K block in the .ISM file is destroyed, contains wronginformation or if the .ISM is missing.In this case we have to create a new .ISM file. Do as follows:55Move the data file .ISD file to some safe place.Recreate the .ISM file from the .frm file:shell> mysql databasemysql> delete from table_name;mysql> quiteCopy (don’t move) the data file (.ISD) back on the newly created .ISD fileGo back to Part 2. (This shouldn’t be a endless loop). isamchk -rq should now work.Part 4; Very hard repair.This can only happen if the descriptor file (.frm) also has crashed. This should never happen,because the .frm file isn’t written to after the table is created.Restore the .frm file from a backup and go back to Part 3. You can also restore the .ISM fileand go back to Part 2. In the latter case you should start with ’isamchk -r’.If you don’t have a backup but know exactly how the table was created, create a copy of thetable in another database and copy the .frm and .ISM file from there to your crasheddatabase and go back to Part 2;Is there anything special to do when upgrading/downgradingMySQL?One can always move the MySQL form and data files between different versions on the samearchitecture as long as one has the same base version of MySQL. The current base version is ofcourse3. If one changes the sort order by recompiling MySQL one has to do a isamchk -rq on the tables.Go to the first [p 5] , previous [p 30] , next [p 57] , last [p 87] section, table of contents [p 1] .56Go to the first [p 5] , previous [p 52] , next [p 65] , last [p 87] section, table of contents [p 1] .How to get maximum performance out of MySQLHow does MySQL use memory ?You can get the currently used buffer sizes with:> ./mysqld --helpThis should result in a list of all mysqld options and configurable variables like the following.Seesection How can I change the buffer sizes of mysqld ? [p 61] .Possibly variables to option --set-variable (-O) are:back_log current value: 5join_buffer current value: 131072 30
  31. 31. key_buffer current value: 1048568max_allowed_packet current value: 65536max_connections current value: 90max_join_size current value: 4294967295max_sort_length current value: 1024net_buffer current value: 8192record_buffer current value: 131072table_cache current value: 64tmp_table_size current value: 131072sort_buffer current value: 2097144All threads share the same base memory.Nothing is memmaped yet (except compressed tables but that’s another story). This is because32bit memory space of 4GB is not large enough for most large tables. When we get a systemwith 64 bit address space we may add general support for memmaping.When starting mysqld one can specify a key buffer. This will buffer all keys in all tables atFIFO basis (variable keybuffer).Each connection uses some thread space, a stack and a connection buffer (variablenet_buffer_length).Each request doing a sequential scan over a table allocates a read buffer (variable record_buffer).Each request doing a sort, allocates a sortbuffer and one or two temporary files. The maximumextra disk-space needed is @math{(sort_key_length +sizeof(long))*2}.All joins are done in one pass and most temporary tables are memory based (HEAP) tables.Temporary tables with a big recordlength (= sum of all column length) or that contains BLOB’sare stored on disk. One current problem is that if the HEAP table exceeds the size oftmp_table_size, one will get the error: ’The table ### is full’. In the future we will fix thisby automatically changing the in memory (HEAP) table to a disk based (NISAM) table ifneeded.To go around this problem one can increase the -O tmp_table_size=# option to mysqld oruse the SQL option SQL_BIG_TABLES. See section SET OPTION syntax. [p 48] . In MySQL3.20 the maximum size of the temporary table was recordbuffer*16, so if you are using thisversion you have to increase recordbuffer. There also exists a patch to always storetemporary tables on disk, but this will affect the speed of all complicated queries.Almost all memory used when parsing and calculating is done on a local memory store. Nomemory overhead is needed for small items and the normal slow memory allocation/freeing isavoided. Memory is only allocated for unexpectedly large strings (this is done with malloc/free).57Each index file is opened once and the data file is opened once for each concurrently runningthread. For each concurrent thread a table structure, column structures for each column and abuffer that has the size of 3 * (maximum row length not counting BLOBS) is allocated. A BLOBuses 5 to 8 bytes +length of blob data.For each table having BLOBS, a buffer is enlarged dynamically to read in larger BLOB’s. If onescans a table there will be a allocated buffer as large as the largest BLOB.All used tables are saved in a cache and used by FIFO. Normally the cache is 64 tables. If a tablehas been used by 2 running threads at the same time, there will be 2 entries of the table in thecache.A MySQLadmin refresh closes all tables that are not used and marks all used tables to beclosed when the running thread finishes. This will effectively free most used memory.When running mysqld ps, other programs may report that is takes a lot of memory. This may be 31
  32. 32. caused by thread-stacks on different memory addresses. For example, the Solaris ps calculatestheunused memory between stacks as used memory. You can verify this by checking available swapwith’swap -s’. We have tested mysqld with commercial memory-leakage detectors so there shouldnot beany memory leaks.How does MySQL use keys?All keys, PRIMARY, UNIQUE and INDEX(), are stored in B trees. Strings are automaticallyprefixandend-space compressed.INDEX(col1, col2) creates a multiple index over the two columns. The key can be seen like aconcatenation of the given columns. If you use INDEX(col1), INDEX(col2) instead ofINDEX(col1,col2) you get two separate keys instead.SELECT * FROM table WHERE col1=# AND col2=#In a case of INDEX(col1,col2) the right row(s) can be fetched directly. In a case ofINDEX(col1),INDEX(col2) the optimiser decides which index will find fewer rows and this index will be usedtofetch the rows.If the table has an index INDEX(col1,col2,col3....) the prefix of this can be used by the optimisertofind the rows. This means that the above gives you search capabilities on: INDEX(col1) andINDEX(col1,col2) and INDEX(col1,col2,col3)...MySQL can’t use a sub part of a key to locate rows through a key.With the definition INDEX(col1,col2,col3):SELECT * FROM table WHERE col1=#SELECT * FROM table WHERE col2=#SELECT * FROM table WHERE col2=# and col3=#only the first query will use keys.58How does MySQL open & close tables?The cache of open tables can grow to a maximum of table-cache (default 64, changeable with -Otable_cache=#). A table is never closed, except when the cache is full and another thread tries toopena table or if one uses ’mysqladmin refresh’.When the limit is reached, MySQL closes as many tables as possible, until the cache size hasbeenreached or there are no more unused tables. This means that if all tables are in use by somethreads,there will be more open tables than the cache limit, but the extra tables will be closed eventually.Tables are closed according to last-used order.A table is opened (again) for each concurrent access. This means that if one has two threadsrunningon the same table or access the table twice in the same query (with AS) the table needs to beopenedtwice. The first open of any table takes 2 file descriptors, each following use of the table takesonlyone file descriptor. 32
  33. 33. How should I arrange my table to be as fast/small as possible?Use NOT NULL if possible. It makes everything faster and you save 1 bit per column.All columns have default values. Only insert when the default value isn’t acceptable. You don’thave to insert the columns of a timestamp or an autoincremented key in the insert statement. Seesection How can I get the unique ID for the last row? [p 83]Use the smaller INT types if possible to get smaller tables. For example, MEDIUMINT is oftenbetter than INT.If you don’t have any VARCHAR columns, a fixed size record format will be used. This is muchfaster but may unfortunately waste some space. See section What are the different row formats?Or when to use VARCHAR/CHAR? [p 63] .To make MySQL optimise queries better, run isamchk -a on the table once it is loaded withrelevant data. This updates a value for each index that tells how many rows that have the samevalue for this key in average. Of course, this is always 1 for unique indexes.To sort an index and data according to an index use isamchk -Sir1 (if you want so sort onindex 1). If you have a unique key that you want to read all records from in numeric order this isa good way to make that faster.When loading a table with data use LOAD DATA FROM INFILE. This is usually 20 timesfaster than using a lot of INSERTs. If the text file isn’t on the server, rcp it to the server first. Seesection LOAD DATA INFILE syntax [p 45] . You can even get more speed when loading data totables with many keys by doing:Create the table in mysql or perl with CREATE TABLE....Do a mysqladmin refresh.Use isamchk -k0 database/table_name. This will remove all use of keys fromthe table.Insert data into the table with LOAD DATA INFILE....If you have pack_isam and want to compress the table, run pack_isam on it.Recreate the keys with isamchk -rq database/table_name.Do mysqladmin refresh.The other possibility to get some more speed to both LOAD DATA FROM INFILE andINSERT is to enlarge the key buffer. This can be done with the -O key_buffer=# option to(safe)mysqld. For example 16M should be a good value if you have much RAM :)When dumping data as textfiles to other programs, use SELECT ... INTO OUTFILE. See59section LOAD DATA INFILE syntax [p 45] .When doing many inserts/updates in a row, you can get more speed by using LOCK TABLES onthe tables. ...FROM INFILE... and ...INTO OUTFILE... are atomic so you don’t haveto use LOCK TABLES when using these. See section Lock tables syntax [p 48] .To check how you are doing, run isamchk -evi on the .ISM file.What affects the speed of the INSERT statement?The time to insert a record consists of:Connect: (3)Sending query to server: (2)Parsing query: (2)Inserting record: (1 x size of record)Inserting keys: (1 x keys)Close: (1)Where (number) is proportional time. This does not take into calculation the initial overhead toopentables (which is done once for each simultaneous running query). 33
  34. 34. The size of the table slows down the insert of keys with N log N (B-trees).What affects the speed of DELETE statement?The delete time of a record is exactly proportional to the number of keys. To increase the seed ofdeletes you can increase the size of the key cache. The default key cache is 1M and to get fasterdeletes it should be increased a couple of times (try 16M if you have enough memory).What kind of optimisation is done on the WHERE clause?(Incomplete, MySQL does a lot of optimisations.)Brace removal (all unnecessary braces are removed). ((a AND b) AND c OR (((a ANDb) AND (c AND d)))) -> (a AND b) OR (a AND b AND c AND d)Constant folding. (a<b AND b=c) AND a=5 -> b>5 AND b=c A=5Constant condition removal (needed because of constant folding). (b>=5 AND b=5) OR(b=6 and 5=5) or (B=7 and 5=6) -> B=5 or B=6All constant expressions used by keys are evaluated only once.Return zero rows if impossible select.HAVING is merged with WHERE if one doesn’t use GROUP BY or group functions.For each sub join a simpler WHERE is constructed to get a fast WHERE evaluation for each subjoin and also to skip records as soon as possible.Find all keys that may be used. Use the key which finds least records. A key is used for thefollowing cases: =, >, >=, <, <=, BETWEEN and a LIKE with a character prefix like’something%’.Remove keys that don’t span all AND levels.have all preceding key_parts specified.key = 1 or A = 10 -> NULL (can’t use key.)60key = 1 or A = 10 and key=2 -> key = 1 OR key = 2key_part_1 = const and key_part_3 = const -> key_part_1 = constRead all constant tables. A constant table is:1. A table with 0 or 1 record.2. A table which uses only other const tables and constants on a full unique key.const_table.key = constant const_table.key_part_1 =const_table2.column and const_table.key_part_2 = constantFind the best join combination to join the tables, by trying all possibilities :(. If all columns inORDER BY and in GROUP comes from the same table, then this table is preferred first whenjoining.If there is a order clause and a different group clause or if the order or group contains columnsfrom other tables than the first table in the join cue, a temporary table is created.For each table use a ranged key, if possible, to read records. Each table index is queried and ifthere exists a key range that spans < 30% of the records then a key is used. If no such key can befound a quick table scan is used.Before each record is output-ed, skip those that match the HAVING clause.How can I change the buffer sizes of mysqld ?With the mysqld -O variable=size command. Example run:> mysqld --help (or > mysqld -?)>mysqld --help/my/monty/sql/mysqld Ver 3.21.0-alpa for SOLARIS 2.5 (SPARCstation)TcX Datakonsult AB, by Monty. This is FPL, not free softwareThis software comes with NO WARRANTY: see the file PUBLIC for details.Starts the mysql serverUsage: /my/monty/sql/mysqld [OPTIONS] 34
  35. 35. -b, --basedir=path path to installation directory-h, --datadir=path path to the database root-#, --debug=... output debug log. Often this is ’d:t:o,filename‘-T, --debug-info print some debug info at exit-?, --help display this help and exit-L, --language=... client error messages in given language-l, --log[=filename] log connections and queries to file--log-update[=filename]log updates to filename.# where # is a unique numberif not given.--log-isam[=filename]log all isam changes to file-P, --port=... Port number to use for connection-O, --set-variable var=optiongive a variable an value. --help lists variables--skip-new-routines don’t use new possible wrong routines.--skip-grant-tables start without grant tables. This gives anyone FULLACCESS to all tables!--skip-locking don’t use system locking. To use isamchk one hasto shut down the server.--skip-name-resolve Don’t resolve hostnames.All hostnames are IP’s or ’localhost’--skip-new-routines don’t use new possible wrong routines.--skip-unsafe-select skip unsafe select optimisations.61--socket=... Socket file to use for connection-V, --version output version information and exitCurrent base_dir: /my/montyCurrent data_dir: data/Current language: english/Possibly variables to option --set-variable (-O) are:back_log current value: 5join_buffer current value: 131072key_buffer current value: 1048568max_allowed_packet current value: 65536max_connections current value: 90max_join_size current value: 4294967295max_sort_length current value: 1024net_buffer current value: 8192record_buffer current value: 131072table_cache current value: 64tmp_table_size current value: 131072sort_buffer current value: 2097144> safe_mysqld -O key_buffer=512k -O sort_buffer=100k -O record_buffer=100k &The key_buffer is shared by all threads, the rest are allocated when needed.mysqld demon starts with a cd to ’mysql-data-dir’. After this, mysqld-data-dir is changed to ’./’(current dir). All paths (databases, pid file, and log file) are prefixed with ’./’.What options to use to get MySQL to run at full speed?More memory gives more speed. You can try something like: 35
  36. 36. mysqld -O key_buffer=16m -O sort_buffer=1m -O record_buffer=512k--skip-locking--skip-locking disables file locking between SQL requests. If this is used then the following canhappen:If mysqld or the machine crashes a table has a higher risk of being corrupted. Tables should atleast be checked with isamchk *.ISM after a crash.One MUST flush all tables with mysqladmin refresh before one tries to check/repair tableswith isamchk. (isamchk -d table_name is always allowed).The --skip-locking is default when compiling with MIT threads. This is because flock() isn’tfully support by MIT threads on all platforms.How to get MySQL to run as fast as possible with littlememory?If there are very many connections, ’swapping problems’ may occur, unless mysqld hasn’t beenconfigured to use very little memory for each connection.62For example, for 200 open connections one should have a table cache of at least 200 *(max_numberof tables in join).Of course MIT threads may slow down everything a bit, but for key based selects a select isusuallydown in one time frame so there shouldn’t be a mutex locking/thread juggling.If updates are a problem, updates can be delayed and then do many updates in a row later. Manyupdates done in a row are much quicker than one at a time.If the problem is with MIT threads and one is using FreeBSD x.x. upgrading to FreeBSD 3.0.(orhigher:) should help. This gives a possibility to use sockets (quicker than the current TCP/IP withMITthreads) and the thread package is much more integrated.What are the different row formats? Or when to useVARCHAR/CHAR?Actually using no VARCHAR or BLOB types results in a fixed row size. Otherwise CHAR andVARCHAR are the same.You can check the format used in a table with isamchk -d.MySQL has three different table formats:1. Fixed length tables;This is the default format.All non packed columns are space filled.Very quick.Easy to cash.Easy to reconstruct if crashed, of course, this only theoretical :-) as records are in fixedpositions.Don’t have to be reorganised unless a huge number of records are deleted.2. Dynamic tablesIs used if there exists any VARCHAR, TEXT or BLOB columns in a table.All strings are dynamic (except if length < 3).Each record is preceded with a bitmap for which columns are not empty (this isn’t the sameas null columns).Each string is saved with a length byte + string. If string is zero length or a number is zero ittakes no extra space, just the zero length bit for each column.Each record is uses the exact record space required. If a record becomes larger it is split into 36
  37. 37. as many pieces as required.Takes little disk space.If records are changed a lot, isamchk -r should be run now and then to reorganise thetable. This is to get a better layout. Use isamchk -ei table_name for some statistics.Not as easy to reconstruct because a record may be in many pieces and a link may bemissing.The expected row length for dynamic sized records is: 3 + (number_of_columns + 7) / 8 +(number of char columns) + packed_size_of_number_columns + length_of_strings +(null_columns + 7) / 8. There will be a penalty of 6 bytes for each link. A dynamic recordwill be linked whenever a update causes a enlargement of the record. Each new link will be63at least 20 bytes, so the next enlargement will probably go in the same link. If not, there willbe another link. You may check how many links there are with isamchk -ed. All linksmay be removed with isamchk -r.3. Compressed tables (this is only with UNIREG/pack_isam)Read only tables.Takes very little disk space. Minimises disk usage.Each record is compressed separately (very little access overhead).Can handle fixed or dynamic length records, but no BLOBs.Can be uncompressed with isamchk.Why so many open tables?When you run mysqladmin status you get something like:Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12This can be somewhat perplexing if you only have 6 tables.As MySQL is multithreaded it may have many queries on the same table at once. To minimisetheproblem with two threads having different states on the same file, I open the table again for eachconcurrent thread. This takes some memory and one extra file descriptor for the data file. Theindexfile descriptor is shared all threads.Go to the first [p 5] , previous [p 52] , next [p 65] , last [p 87] section, table of contents [p 1] .64Go to the first [p 5] , previous [p 57] , next [p 75] , last [p 87] section, table of contents [p 1] .MySQL UtilitesOverview of the different MySQL programsAll MySQL client that communicates with the server uses the following environment variables:Name DescriptionMYSQL_UNIX_PORT The default socket. Used with ’localhost’.MYSQL_TCP_PORT The default TCP port.MYSQL_PWD The default password.mysqlA SQL shell (with gnu readline). Supports interactive use or as a non interactive query tool.When used interactively result is given in a ascii-table format, but when used as a filter the resultis a tab-separated output.mysqlaccessScript to check the privileges for a host, user and database combination.mysqladminAdministration utility. Create/Drop of databases, reload (read new users) and refresh (flushtables 37
  38. 38. to disk, reopen log files). Also gives version and process info.mysqldThe SQL daemon. This should always be running.mysqldumpDump a MySQL database into a file with SQL statements or tab separated text files. Enchantedfreeware originally by Igor Romanenko.mysqlimportImports text-file(s) into respective table(s). Can use all formats supported by LOAD DATAINFILE. See section LOAD DATA INFILE syntax [p 45]mysqlshowShows information about database, tables, fields and keys.mysqlbugThis script should always be used when filing a bug report to the MySQL list.mysql_install_dbCreates the MySQL grant tables with default privileges. This is usually only executed wheninstalling the first MySQL release on a new system.isamchkCheck, optimise and repair MySQL tables.make_binary_releaseMakes a binary release of a compiled MySQL. This could be sent by ftp for the convenience of other MySQL users.msql2mysqlA shell script to convert a mSQL program to MySQL. Doesn’t handle all cases but gives a goodstart when converting.65replaceBinary used for msql2mysql. Utility program to change strings in place in files or on stdin. Usesa finite state machine to match longer strings first. Can be used to swap strings, for example’replace a b b a -- files’ swaps ’a’ and ’b’ in the given files.safe_mysqldStarts the mysqld demon with some safety features. Restarts on error and has logging of runtimeinformation to a log file.The MySQL table check, optimize and repair programFor infromation about how to use isamchk to repair a crached table: See section How to repairtables.[p 55] .Getting low level table informationTo get a description/statistics from a table use the forms below. We will explain some of theinformation in more detail later.isamchk -d table_nameisamchk in ’describe mode’. If one uses ’--skip-locking’ isamchk may report an error for a tablethat is updated while isamchk runs, but there isn’t any risk of destroying data. A short form.ISAM file: company.ISMData records: 1403698 Deleted blocks: 0Recordlength: 226Record format: Fixed lengthtable description:Key Start Len Index Type 38
  39. 39. 1 2 8 unique double2 15 10 multip. text packed stripped3 219 8 multip. double4 63 10 multip. text packed stripped5 167 2 multip. unsigned short6 177 4 multip. unsigned long7 155 4 multip. text8 138 4 multip. unsigned long9 177 4 multip. unsigned long193 1 textFor explanations see below.isamchk -d -v table_nameA little more verbose.ISAM file: company.ISMIsam-version: 2Creation time: 1996-08-28 11:44:22Recover time: 1997-01-12 18:35:29Data records: 1403698 Deleted blocks: 0Datafile: Parts: 1403698 Deleted data: 0Datafilepointer (bytes): 3 Keyfile pointer (bytes): 3Max datafile length: 3791650815 Max keyfile length: 4294967294Recordlength: 226Record format: Fixed lengthrtable description:66Key Start Len Index Type Root Blocksize Rec/key1 2 8 unique double 15845376 1024 12 15 10 multip. text packed stripped 25062400 1024 23 219 8 multip. double 40907776 1024 734 63 10 multip. text packed stripped 48097280 1024 55 167 2 multip. unsigned short 55200768 1024 48406 177 4 multip. unsigned long 65145856 1024 13467 155 4 multip. text 75090944 1024 49958 138 4 multip. unsigned long 85036032 1024 879 177 4 multip. unsigned long 96481280 1024 178193 1 textISAM fileName of isam file.Isam-versionVersion of isam format. Currently always 2.Creation timeWhen was the data file created.Recover timeWhen was the index/data file last reconstructed.Data recordsHow many records/rows.Deleted blocksHow many deleted blocks still have reserved space. See section How to repair tables. [p 55] 39
  40. 40. .Datafile: PartsFor dynamic record format this shows how many data blocks there are. For a optimised tablewithout splits this is the same as Data records.Deleted dataHow many bytes of non reclaimed deleted data.Datafile pointerHow many bytes the datafile pointer has. It is usually 2, 3 or 4 bytes. Most tables managewith 2 bytes but this cannot be controlled from MySQL yet. For fixed tables this is a recordaddress. For dynamic tables this is a byte address.Keyfile pointerHow many bytes has the datafile pointer. It is usually 1, 2 or 3 bytes. Most tables managewith 2 bytes but this is calculated automatically by MySQL. It is always a block address.Max datafile lengthHow long (in bytes) can the table’s data file (.ISD) get.Max keyfile lengthHow long (in bytes) can the table’s key file (.ISM) get.RecordlengthHow much space does each record/row take.Record formatWhich format does each record/row have. This example uses Fixed length.table descriptionA list of all keys in the table. For each key some low level information is presented.KeyThis key’s number.StartWhere in the record/row does this index-part start.67LenHow long is this index-part. For packed numbers this should always be the full lengthof the field. For strings it may be shorter than the full length.Indexunique or multip.. If one value can exist multiple times in this index.TypeWhat data-type does this index part have. This is a C data-type with the options packed,stripped or empty.RootAddress of the root index block.BlocksizeThe size of each index block. This is by default 1024 but may be changed a compiletime.Rec/keyThis is a statistical value used by the optimiser. It tells how many records there are pervalue for this key. A unique key always has a value of 1. This may be updated after atable is loaded (or greatly changed) with isamchk -a. If this is not updated at all, adefault value of 30 is given.The 9th key is a multiple part key with two parts.isamchk -eis table_nameShows only the most important information from a table. Slow since it must read the whole table. 40