MySQL优化的步骤详解

       在开发过程中,虽然感觉优化sql语句很重要,但是往往更加重视的是功能实现,为了使自己以后写Mysql语句效率更高,有必要对Mysql优化做一个小小归纳。

步骤一、通过show status 命令了解各种sql执行的效率

show [session|gobal] status

session级别表示统计当前连接的结果。

global级别表示统计自数据上次启动至今的结果。

如果不写级别,默认的是session级别

eg:SHOW GLOBAL STATUS;

Variable_name Value
Aborted_clients 6
Aborted_connects 0
Binlog_cache_disk_use 0
Binlog_cache_use 0
Binlog_stmt_cache_disk_use 0
Binlog_stmt_cache_use 0
Bytes_received 95645
Bytes_sent 1285066
Com_admin_commands 0
Com_assign_to_keycache 0
Com_alter_db 0
Com_alter_db_upgrade 0
Com_alter_event 0
Com_alter_function 0
Com_alter_procedure 0
Com_alter_server 0
Com_alter_table 6
Com_alter_tablespace 0
Com_alter_user 0
Com_analyze 0
Com_begin 0
Com_binlog 0
Com_call_procedure 0
Com_change_db 8
Com_change_master 0
Com_check 0
Com_checksum 0
Com_commit 0
Com_create_db 0
Com_create_event 0
Com_create_function 0
Com_create_index 0
Com_create_procedure 0
Com_create_server 0
Com_create_table 5
Com_create_trigger 0
Com_create_udf 0
Com_create_user 0
Com_create_view 6
Com_dealloc_sql 0
Com_delete 2
Com_delete_multi 0
Com_do 0
Com_drop_db 0
Com_drop_event 0
Com_drop_function 0
Com_drop_index 0
Com_drop_procedure 0
Com_drop_server 0
Com_drop_table 0
Com_drop_trigger 0
Com_drop_user 0
Com_drop_view 1
Com_empty_query 2
Com_execute_sql 0
Com_flush 0
Com_get_diagnostics 0
Com_grant 0
Com_ha_close 0
Com_ha_open 0
Com_ha_read 0
Com_help 0
Com_insert 15
Com_insert_select 0
Com_install_plugin 0
Com_kill 0
Com_load 0
Com_lock_tables 0
Com_optimize 0
Com_preload_keys 0
Com_prepare_sql 0
Com_purge 0
Com_purge_before_date 0
Com_release_savepoint 0
Com_rename_table 0
Com_rename_user 0
Com_repair 0
Com_replace 0
Com_replace_select 0
Com_reset 0
Com_resignal 0
Com_revoke 0
Com_revoke_all 0
Com_rollback 0
Com_rollback_to_savepoint 0
Com_savepoint 0
Com_select 414
Com_set_option 525
Com_signal 0
Com_show_binlog_events 0
Com_show_binlogs 0
Com_show_charsets 0
Com_show_collations 0
Com_show_create_db 0
Com_show_create_event 0
Com_show_create_func 0
Com_show_create_proc 0
Com_show_create_table 260
Com_show_create_trigger 0
Com_show_databases 8
Com_show_engine_logs 0
Com_show_engine_mutex 0
Com_show_engine_status 0
Com_show_events 0
Com_show_errors 0
Com_show_fields 102
Com_show_function_code 0
Com_show_function_status 0
Com_show_grants 0
Com_show_keys 86
Com_show_master_status 0
Com_show_open_tables 0
Com_show_plugins 0
Com_show_privileges 0
Com_show_procedure_code 0
Com_show_procedure_status 0
Com_show_processlist 1
Com_show_profile 0
Com_show_profiles 115
Com_show_relaylog_events 0
Com_show_slave_hosts 0
Com_show_slave_status 0
Com_show_status 247
Com_show_storage_engines 0
Com_show_table_status 1
Com_show_tables 14
Com_show_triggers 5
Com_show_variables 5
Com_show_warnings 0
Com_slave_start 0
Com_slave_stop 0
Com_stmt_close 0
Com_stmt_execute 0
Com_stmt_fetch 0
Com_stmt_prepare 0
Com_stmt_reprepare 0
Com_stmt_reset 0
Com_stmt_send_long_data 0
Com_truncate 0
Com_uninstall_plugin 0
Com_unlock_tables 0
Com_update 27
Com_update_multi 0
Com_xa_commit 0
Com_xa_end 0
Com_xa_prepare 0
Com_xa_recover 0
Com_xa_rollback 0
Com_xa_start 0
Compression ON
Connection_errors_accept 0
Connection_errors_internal 0
Connection_errors_max_connections 0
Connection_errors_peer_address 0
Connection_errors_select 0
Connection_errors_tcpwrap 0
Connections 10
Created_tmp_disk_tables 128
Created_tmp_files 5
Created_tmp_tables 910
Delayed_errors 0
Delayed_insert_threads 0
Delayed_writes 0
Flush_commands 1
Handler_commit 108
Handler_delete 2
Handler_discover 0
Handler_external_lock 782
Handler_mrr_init 0
Handler_prepare 0
Handler_read_first 73
Handler_read_key 2109
Handler_read_last 0
Handler_read_next 42
Handler_read_prev 0
Handler_read_rnd 1882
Handler_read_rnd_next 94791
Handler_rollback 0
Handler_savepoint 0
Handler_savepoint_rollback 0
Handler_update 195
Handler_write 93316
Innodb_buffer_pool_dump_status not started
Innodb_buffer_pool_load_status not started
Innodb_buffer_pool_pages_data 397
Innodb_buffer_pool_bytes_data 6504448
Innodb_buffer_pool_pages_dirty 0
Innodb_buffer_pool_bytes_dirty 0
Innodb_buffer_pool_pages_flushed 193
Innodb_buffer_pool_pages_free 7795
Innodb_buffer_pool_pages_misc 0
Innodb_buffer_pool_pages_total 8192
Innodb_buffer_pool_read_ahead_rnd 0
Innodb_buffer_pool_read_ahead 0
Innodb_buffer_pool_read_ahead_evicted 0
Innodb_buffer_pool_read_requests 4642
Innodb_buffer_pool_reads 364
Innodb_buffer_pool_wait_free 0
Innodb_buffer_pool_write_requests 872
Innodb_data_fsyncs 129
Innodb_data_pending_fsyncs 0
Innodb_data_pending_reads 0
Innodb_data_pending_writes 0
Innodb_data_read 6033408
Innodb_data_reads 402
Innodb_data_writes 281
Innodb_data_written 6534656
Innodb_dblwr_pages_written 193
Innodb_dblwr_writes 14
Innodb_have_atomic_builtins ON
Innodb_log_waits 0
Innodb_log_write_requests 574
Innodb_log_writes 46
Innodb_os_log_fsyncs 61
Innodb_os_log_pending_fsyncs 0
Innodb_os_log_pending_writes 0
Innodb_os_log_written 202752
Innodb_page_size 16384
Innodb_pages_created 34
Innodb_pages_read 363
Innodb_pages_written 193
Innodb_row_lock_current_waits 0
Innodb_row_lock_time 0
Innodb_row_lock_time_avg 0
Innodb_row_lock_time_max 0
Innodb_row_lock_waits 0
Innodb_rows_deleted 0
Innodb_rows_inserted 3
Innodb_rows_read 406
Innodb_rows_updated 2
Innodb_num_open_files 32
Innodb_truncated_status_writes 0
Innodb_available_undo_logs 128
Key_blocks_not_flushed 0
Key_blocks_unused 14344
Key_blocks_used 3
Key_read_requests 381
Key_reads 1
Key_write_requests 117
Key_writes 50
Last_query_cost 0.000000
Last_query_partial_plans 0
Max_used_connections 3
Not_flushed_delayed_rows 0
Open_files 70
Open_streams 0
Open_table_definitions 120
Open_tables 117
Opened_files 1042
Opened_table_definitions 144
Opened_tables 147
Performance_schema_accounts_lost 0
Performance_schema_cond_classes_lost 0
Performance_schema_cond_instances_lost 0
Performance_schema_digest_lost 0
Performance_schema_file_classes_lost 0
Performance_schema_file_handles_lost 0
Performance_schema_file_instances_lost 0
Performance_schema_hosts_lost 0
Performance_schema_locker_lost 0
Performance_schema_mutex_classes_lost 0
Performance_schema_mutex_instances_lost 0
Performance_schema_rwlock_classes_lost 0
Performance_schema_rwlock_instances_lost 0
Performance_schema_session_connect_attrs_lost 0
Performance_schema_socket_classes_lost 0
Performance_schema_socket_instances_lost 0
Performance_schema_stage_classes_lost 0
Performance_schema_statement_classes_lost 0
Performance_schema_table_handles_lost 0
Performance_schema_table_instances_lost 0
Performance_schema_thread_classes_lost 0
Performance_schema_thread_instances_lost 0
Performance_schema_users_lost 0
Prepared_stmt_count 0
Qcache_free_blocks 1
Qcache_free_memory 1039896
Qcache_hits 0
Qcache_inserts 0
Qcache_lowmem_prunes 0
Qcache_not_cached 404
Qcache_queries_in_cache 0
Qcache_total_blocks 1
Queries 1888
Questions 1887
Select_full_join 1
Select_full_range_join 0
Select_range 23
Select_range_check 0
Select_scan 727
Slave_heartbeat_period 0.000
Slave_last_heartbeat  
Slave_open_temp_tables 0
Slave_received_heartbeats 0
Slave_retried_transactions 0
Slave_running OFF
Slow_launch_threads 0
Slow_queries 0
Sort_merge_passes 0
Sort_range 0
Sort_rows 1964
Sort_scan 151
Ssl_accept_renegotiates 0
Ssl_accepts 0
Ssl_callback_cache_hits 0
Ssl_cipher  
Ssl_cipher_list  
Ssl_client_connects 0
Ssl_connect_renegotiates 0
Ssl_ctx_verify_depth 0
Ssl_ctx_verify_mode 0
Ssl_default_timeout 0
Ssl_finished_accepts 0
Ssl_finished_connects 0
Ssl_server_not_after  
Ssl_server_not_before  
Ssl_session_cache_hits 0
Ssl_session_cache_misses 0
Ssl_session_cache_mode NONE
Ssl_session_cache_overflows 0
Ssl_session_cache_size 0
Ssl_session_cache_timeouts 0
Ssl_sessions_reused 0
Ssl_used_session_cache_entries 0
Ssl_verify_depth 0
Ssl_verify_mode 0
Ssl_version  
Table_locks_immediate 386
Table_locks_waited 0
Table_open_cache_hits 656
Table_open_cache_misses 130
Table_open_cache_overflows 0
Tc_log_max_pages_used 0
Tc_log_page_size 0
Tc_log_page_waits 0
Threads_cached 1
Threads_connected 2
Threads_created 3
Threads_running 1
Uptime 286258
Uptime_since_flush_status 286258

主要参数描述

Connections:视图连接mysql服务器的次数

Uptime:服务器工作时间

Slow_queries:慢查询的次数

Com_xxx表示每个xxx语句执行的次数

Com_select  执行select次数

Com_insert  执行insert次数,批量插入时候,只累加1次

Com_update 执行Update操作次数

Com_delete 执行删除操作次数

innodb_rows_xxx类型的参数只对innodb存储引擎有效

解析:通过上面一些参数,可以容易了解当前数据库的应用时插入更新为主还是查询为主,以及执行比例。对应更新操作的计数,是对执行次数的计数,不论提交还是回滚都会累加。

步骤二:定位执行效率较低的sql语句

通过慢查询日志定位哪些是执行效率的sql语句。用--log-slow-queries[=file_name]选项启动,mysqld写一个包含所有执行超过long_query_time秒的sql语句的日志文件。

步骤三:通过EXPLAIN分析低效率SQL的执行计划

在步骤二中,我们可以查询到低效率的sql语句,在此我们通过explain或desc可以获取mysql执行select信息。

eg:

EXPLAIN 
SELECT 
  t0.* 
FROM
  t3 AS t0
  LEFT JOIN `t2` AS t1 
    ON t0.`id1` = t1.`id1` 
WHERE t0.id1 = 5 ;

执行结果如下:


解析:select_type:表示select类型。常见的取值有SIMPLE(简单表,即不使用连接或者子查询)、PRIMARY(主查询,即外层的查询)、UNION(union中的第二个或者后面的查询语句)、SUBQUERY(子查询中的第一个SELECT)等。
talbe:输出结果集的表。

type:表的连接类型。性能由高到底:system(表中仅有一行)、const(表中最多有一个匹配行)、eq_ref、ref、ref_null、index_merge、unique_subquery、index_subquery、range、idnex等

possible_keys:查询时,可能使用的索引

key:实际使用的索引

key_len:索引字段的长度

rows:扫描行的数量

Extra:执行情况的说明和描述

步骤四:确定问题并采取相应的优化措施

经过上面的步骤,可以确定问题出现的原因,此时我们可以根据情况,采取相应的措施。常见的措施有1.建立相应的索引2.优化sql语句3.分表等。

备注:如果索引正在工作,handler_read_key的值将很高,这个值代表了一个行被索引值读的次数,很低的值表名增加索引得到的性能改善不高,因为索引并不经常使用。handler_read_rnd_next值高则意味着查询运行低效,并且应该建立索引补救。如果正在进行大量的表扫描,handler_read_rnd_next值较高,则通常说明表索引不正确或写入的查询没有利用索引,如下图。

MySQL优化的步骤详解_第1张图片

我们应当定期分析表盒检查表

检查表使用如下命令(检查t3表)

ANALYZE TABLE t3;
CHECK TABLE t3;

定期优化表使用命令如下

optimize table 表名

常见语句优化

1.优化Insert语句

(1)如果从同一个客户端插入数据,尽力使用多个字表的insert语句和多行插入,减少单行插入,这种方式大大减少客户端与数据库直接的连接、关闭等消耗。eg:

INSERT INTO t3 VALUES(1,2),(8,5),(6,5),(4,3)

(2)如果从不同的客户插入很多行,能通过使用inset delayed语句得到更高的速度。

(3)如果进行批量插入,可以增加bulk_insert_buffer_size变量方法,提高速度。

2.优化group by语句

默认情况下使用group by col1,col2....会对查询进行相应的排序,如果用户想要避免排序结果的消耗,可以指定order by null 禁止排序。通过查询结果中extra字段可以看出:


3.优化order by语句

在某些情况中,mysql可以使用一个索引来满足order by子句,而不需要额外的排序,where条件和order by使用相同的索引。

4.优化含有or语句

对于含有or的查询子句,如果要使用索引,则or之间的每个条件列必须用到索引;否则,应该考虑添加索引。

5.使用sql提示

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