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When things go wrong: how to find SQL error

Sveta SmirnovaPrinciple Technical Support Engineer, MySQL @ Oracle

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Part 2. Complex things

• Crashes

• Problems repeatable only in production/high load

• Sandbox

• Best practices

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Crashes

• Symptoms:– «Lost connection to MySQL server» returned to the application

– Message in the error log file

• Check error log file

• If message contains a query try it using MySQL command line client

• If crash is repeatable fix the query

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Crashes

• If there is no query try general query log: it will contain query unless this was crash during writing to general query log

• Check backtrace for a clue about what can cause the problem

• Create core file and analyze it using debugger if needed

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Backtrace help

• stack_bottom = 0x450890f0 thread_stack 0x40000• /5.1.39/bin/mysqld(my_print_stacktrace+0x2e)[0x8ac81e]• /5.1.39/bin/mysqld(handle_segfault+0x322)[0x5df502]• /lib64/libpthread.so.0[0x3429e0dd40]• /5.1.39/bin/mysqld(_ZN6String4copyERKS_+0x16)[0x5d9876]• /5.1.39/bin/mysqld(_ZN14Item_cache_str5storeEP4Item+0xc9)

[0x52ddd9]• /5.1.39/bin/

mysqld(_ZN26select_singlerow_subselect9send_dataER4ListI4ItemE+0x45)[0x5ca145]

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Backtrace help

• /5.1.39/bin/mysqld[0x6386d1]• ...• Trying to get some variables.• Some pointers may be invalid and cause the dump to abort...• thd->query at 0x6a39e60 = select 1 from `t1` where `c0` <>

(select geometrycollectionfromwkb(`c3`) from `t1`)• thd->thread_id=2• thd->killed=NOT_KILLED

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Crashes

• If problem is not particular query

• Analyze other messages in the log

• If there is a clue fix the problem

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Too greedy buffers

• key_buffer_size=235929600• read_buffer_size=4190208• max_used_connections=17• max_connections=2048• threads_connected=13• It is possible that mysqld could use up to• key_buffer_size + (read_buffer_size +

sort_buffer_size)*max_connections = 21193712 K• -----• 21193712K ~= 20G

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Crashes

• Check if you allocated reasonable values for buffers

• Set realistic value of max_connections based on your operating system resources

• Use monitoring tools of your operating system to find which application use enormous amount of resources which lead to crashes of MySQL server.

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Production applications

• Be sure you have error logging turned on

• In case of a problem check error log first

• Turn InnoDB Monitoring to on to have information about transactions in error log file

• Use slow query log to have information about all queries which run slowly

• Tune time of query considered slow

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Production applications

• Once problem query is found try to run it isolated. If needed in test environment.

• If problem is not repeatable analyze if this is effect of options or concurrency.

• If problem is options adjust them in such a way so problem is not repeatable

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Concurrency

• Analyze if this is effect of locks:– SHOW PROCESSLIST

– ENGINE INNODB STATUS

– PERFORMANCE_SCHEMA.*_instances tables

• If yes rewrite application so queries which lock each other don't run concurrently

• If not try you can enable core files and use kill to create a core or attach debugger to the running process

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Replication errors

• IO errors– Check error log first

– Check connection and network interface

– Try to connect to master using replication credentials

• All replication SQL errors can be checked using command line client too– They are single threaded, therefore it is usually easy to identify

what caused error

– SHOW SLAVE STATUS usually contains message about SQL error

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Replication SQL errors

• Check error log for a query which caused the problem

• Check if table definitions are same on both master and slave

• Check if table on master and slave are synchronised

• Compare master and slave options. If there are different try these you have on master

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Replication SQL errors

• Get query as slave gets it from master binary log

• mysqlbinlog binlog_name.00000X

• Use option –verbose to decode row events into SQL statements

• Analyze how real query would be executed on master

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Sandbox

• Create environment same as production server, but which you can freely change and even destroy– Same server version

– Same options

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Sandbox

• MySQL Sandbox

• https://launchpad.net/mysql-sandbox

• Easiest method to deploy copy of your production environment

• You just need package of MySQL-version-you-use-***.tar.gz

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Sandbox

• $make_sandbox mysql-5.4.2-beta-linux-x86_64-glibc23.tar.gz• unpacking /mysql-5.4.2-beta-linux-x86_64-glibc23.tar.gz• ...• The MySQL Sandbox, version 3.0.05• (C) 2006,2007,2008,2009 Giuseppe Maxia• installing with the following parameters:• upper_directory = /users/ssmirnova/sandboxes

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Sandbox

• ...• ........ sandbox server started• Your sandbox server was installed in

$HOME/sandboxes/msb_5_4_2

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Sandbox

• Load dump of your production database into your sandbox

• Now you have working copy you can experiment with

• To dump only part of data:– CREATE TABLE test_problem LIKE problem;

– INSERT INTO test_problem SELECT FROM problem WHERE [condition which exists in the original query, but executed properly]

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Sandbox

• You also can create «sandbox» database in production server:– CREATE DATABASE sandbox

– USE sandbox

– CREATE TABLE problem LIKE production.problem

– INSERT INTO problem SELECT * FROM production.problem [WHERE ...]

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Sandbox

• Make sure you don't kill production server if testing in «sandbox» database

• For scenarios which can kill server use separate MySQL server or, better, physical server

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Links

• http://sql-error.microbecal.com/en/

• http://forge.mysql.com/wiki/MySQL_Proxy

• https://launchpad.net/mysql-sandbox

• http://dev.mysql.com/

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Thank you!

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