Secrets of Best MySQL Optimization Presented by – Sonali Minocha OSSCube

Who Am I?

Why Tune a Database?

Who Tunes?

What is Tuned?

How much tuning is enough?

Application Development (Optimizing Queries)

Index Optimizations

MySQL Cluster Tutorial, OSSPAC 09 Singapore, © OSSCube

EXPLAIN Types system The table has only one row const At the most one matching row, treated as a constant eq_ref One row per row from previous tables ref Several rows with matching index value ref_or_null Like ref, plus NULL values index_merge Several index searches are merged unique_subquery Same as ref for some subqueries index_subquery As above for non-unique indexes range A range index scan index The whole index is scanned ALL A full table scan

EXPLAIN Extra Using index The result is created straight from the index Using where Not all rows are used in the result Distinct Only a single row is read per row combination Not exists A LEFT JOIN missing rows optimization is used Using filesort An extra row sorting step is done Using temporary A temporary table is used Range checked for each record The read type is optimized individually for each combination of rows from the previous tables

Optimizer Hints STRAIGHT_JOIN Forces the optimizer to join the tables in the given order SQL_BIG_RESULTS Together with GROUP BY or DISTINCT tells the server to use disk-based temp tables SQL_BUFFER_RESULTS Tells the server to use a temp table, thus releasing locks early (for table-locks) USE INDEX Hints to the optimizer to use the given index FORCE INDEX Forces the optimizer to use the index (if possible) IGNORE INDEX Forces the optimizer not the use the index

Selecting Queries to Optimize The slow query log Logs all queries that take longer than long_query_time Can also log all queries that don’t use indexes with --log-queries-not-using-indexes To log slow administrative commands use --log-slow-admin-statements To analyze the contents of the slow log use mysqldumpslow

The slow query log

Logs all queries that take longer than long_query_time

Can also log all queries that don’t use indexes with --log-queries-not-using-indexes

To log slow administrative commands use --log-slow-admin-statements

To analyze the contents of the slow log use mysqldumpslow

The general query log can be use to analyze: Reads vs. writes Simple queries vs. complex queries etc

The general query log can be use to analyze:

Reads vs. writes

Simple queries vs. complex queries

etc

Database Designing (Optimizing Schemas)

Normalization

Table Optimizations

Choosing Best Suited Storage Engine Understanding benefits and drawbacks of each storage engine is very important while designing application. Different storage engine has different index capability ,application need should be kept in mind while choosing storage engine

Understanding benefits and drawbacks of each storage engine is very important while designing application.

Different storage engine has different index capability ,application need should be kept in mind while choosing storage engine

MyISAM-Specific Optimizations

InnoDB-Specific Optimizations InnoDB uses clustered indexes The length of the PRIMARY KEY is extremely important The rows are always dynamic Using VARCHAR instead of CHAR is almost always better Maintenance operations needed after Many UPDATE / DELETE operations The pages can become underfilled

InnoDB uses clustered indexes

The length of the PRIMARY KEY is extremely important

The rows are always dynamic

Using VARCHAR instead of CHAR is almost always better

Maintenance operations needed after

Many UPDATE / DELETE operations

The pages can become underfilled

Monitoring Threads in MySQL

MEMORY-Specific Optimizations

Optimizing the Server

Performance Monitoring

Tuning MySQL Parameters Some MySQL options can be changed online The dynamic options are either SESSION specific Changing the value will only affect the current connection GLOBAL Changing the value will affect the whole server Both When changing the value SESSION/GLOBAL should be specified

Some MySQL options can be changed online

The dynamic options are either

SESSION specific

Changing the value will only affect the current connection

GLOBAL

Changing the value will affect the whole server

Both

When changing the value SESSION/GLOBAL should be specified

Online changes are not persistant over a server restart The configuration files have to be changed as well The current values of all options can be found with SHOW SESSION/GLOBAL VARIABLES

Online changes are not persistant over a server restart

The configuration files have to be changed as well

The current values of all options can be found with SHOW SESSION/GLOBAL VARIABLES

Status Variables

SQL/Parser Model Client2 ClientN Client1 MySQL Server Connection Thread Pool Parser Query 101101 Optimizer Storage Engines InnoDB MyISAM MERGE MEMORY Federated ARCHIVE NDBCluster Query Cache

InnoDB

MyISAM

MERGE

MEMORY

Federated

ARCHIVE

NDBCluster

Query Cache Stores SELECT queries and their results Purpose: improve performance for frequently requested data The data in the query cache is invalidated as soon as a modification is done in the table Controlled with the query_cache_size variable

Stores SELECT queries and their results

Purpose: improve performance for frequently requested data

The data in the query cache is invalidated as soon as a modification is done in the table

Controlled with the query_cache_size variable

The Qcache_% status variables help monitoring the cache The utilisation ratio: Qcache_hits vs. Com_select The query cache can be emptied with RESET QUERY CACHE

The Qcache_% status variables help monitoring the cache

The utilisation ratio: Qcache_hits vs. Com_select

The query cache can be emptied with RESET QUERY CACHE

Some Thread Specific Options read_buffer_size (default 128Kb) and read_rnd_buffer_size (default 256Kb) Size of cache used for table scanning Not equivalent to block size The database is not divided into blocks but directly into records Increase if you do many sequential scans sort_buffer_size (default 2Mb) Size of the GROUP BY / ORDER BY cache If more memory is needed it will be taken from the disk tmp_table_size (default 32Mb) Limit after which temporary tables will not be MEMORYs anymore, but MyISAM tables

read_buffer_size (default 128Kb) and read_rnd_buffer_size (default 256Kb)

Size of cache used for table scanning

Not equivalent to block size

The database is not divided into blocks but directly into records

Increase if you do many sequential scans

sort_buffer_size (default 2Mb)

Size of the GROUP BY / ORDER BY cache

If more memory is needed it will be taken from the disk

tmp_table_size (default 32Mb)

Limit after which temporary tables will not be MEMORYs anymore, but MyISAM tables

Some Global Options table_cache (default 64) Cache for storing open table handlers Increase this if Opened_tables is high thread_cache (default 0) Number of threads to keep for reuse Increase if threads_created is high Not useful if the client uses connection pooling

table_cache (default 64)

Cache for storing open table handlers

Increase this if Opened_tables is high

thread_cache (default 0)

Number of threads to keep for reuse

Increase if threads_created is high

Not useful if the client uses connection pooling

max_connections (default 100) The maximum allowed number of simultaneous connections Very important for tuning thread specific memory areas Each connection uses at least thread_stack of memory

max_connections (default 100)

The maximum allowed number of simultaneous connections

Very important for tuning thread specific memory areas

Each connection uses at least thread_stack of memory

MyISAM Global Options key_buffer_size (default 8Mb) Cache for storing indices Increase this to get better index handling Miss ratio ( key_reads / key_read_requests ) should be very low, at least < 0.03 (often < 0.01 is desirable) Row caching is handled by the OS

key_buffer_size (default 8Mb)

Cache for storing indices

Increase this to get better index handling

Miss ratio ( key_reads / key_read_requests ) should be very low, at least < 0.03 (often < 0.01 is desirable)

Row caching is handled by the OS

MyISAM Thread-Specific Options myisam_sort_buffer_size (default 8Mb) Used when sorting indexes during REPAIR/ALTER TABLE myisam_repair_threads (default 1) Used for bulk import and repairing Allows for repairing indexes in multiple threads myisam_max_sort_file_size The max size of the file used while re-creating indexes

myisam_sort_buffer_size (default 8Mb)

Used when sorting indexes during REPAIR/ALTER TABLE

myisam_repair_threads (default 1)

Used for bulk import and repairing

Allows for repairing indexes in multiple threads

myisam_max_sort_file_size

The max size of the file used while re-creating indexes

InnoDB-Specific Optimization innodb_buffer_pool_size (default 8Mb) The memory buffer InnoDB uses to cache both data and indexes The bigger you set this the less disk i/o is needed Can be set very high (up to 80% on a dedicated system)

innodb_buffer_pool_size (default 8Mb)

The memory buffer InnoDB uses to cache both data and indexes

The bigger you set this the less disk i/o is needed

Can be set very high (up to 80% on a dedicated system)

innodb_flush_log_at_trx_commit (default 1) 0 writes and sync’s once per second (not ACID) 1 forces sync to disk after every commit 2 write to disk every commit but only sync’s about once per second

innodb_flush_log_at_trx_commit (default 1)

0 writes and sync’s once per second (not ACID)

1 forces sync to disk after every commit

2 write to disk every commit but only sync’s about once per second

InnoDB-Specific Optimization innodb_log_buffer_size (default 1Mb) Larger values allows for larger transactions to be logged in memory Sensible values range from 1M to 8M innodb_log_file_size (default 5Mb) Size of each InnoDB redo log file Can be set up to buffer_pool_size

innodb_log_buffer_size (default 1Mb)

Larger values allows for larger transactions to be logged in memory

Sensible values range from 1M to 8M

innodb_log_file_size (default 5Mb)

Size of each InnoDB redo log file

Can be set up to buffer_pool_size

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