Secrets of Best MySQL Optimization
presented by
Sonali MinochaOSSCube
Who Am I?
Why Tune a Database?
Who Tunes?
What is Tuned?
How much tuning is enough?
Application Development(Optimizing Queries)
Index Optimizations
EXPLAIN Typessystem 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 ExtraUsing 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 HintsSTRAIGHT_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 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
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
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
• 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
Connection Thread Pool
Parser Query 101101
Client1
Optimizer
Storage Engines InnoDB MyISAM MERGE MEMORY Federated ARCHIVE NDBCluster
Query Cache
MySQL Server
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
• 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
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
• 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
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
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_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
Q n A
Thank you for your time and attention
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