MYSQL SQL优化思路和方法
一、优化SQL的一般步骤
1.1 了解各种SQL执行频率
mysql> show status like '%Com_%';
+-----------------------------+-------+
| Variable_name | Value |
+-----------------------------+-------+
| 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_instance | 0 |
| Com_alter_procedure | 0 |
| Com_alter_server | 0 |
| Com_alter_table | 0 |
| Com_alter_tablespace | 0 |
| Com_alter_user | 0 |
| Com_analyze | 0 |
| Com_begin | 0 |
| Com_binlog | 0 |
| Com_call_procedure | 0 |
| Com_change_db | 0 |
| Com_change_master | 0 |
| Com_change_repl_filter | 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 | 0 |
| Com_create_trigger | 0 |
| Com_create_udf | 0 |
| Com_create_user | 0 |
| Com_create_view | 0 |
| Com_dealloc_sql | 0 |
| Com_delete | 0 |
| 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 | 0 |
| Com_empty_query | 0 |
| Com_execute_sql | 0 |
| Com_explain_other | 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 | 0 |
| 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 | 0 |
| Com_set_option | 0 |
| 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 | 0 |
| Com_show_create_trigger | 0 |
| Com_show_databases | 0 |
| 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 | 0 |
| Com_show_function_code | 0 |
| Com_show_function_status | 0 |
| Com_show_grants | 0 |
| Com_show_keys | 0 |
| 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 | 0 |
| Com_show_profile | 0 |
| Com_show_profiles | 0 |
| Com_show_relaylog_events | 0 |
| Com_show_slave_hosts | 0 |
| Com_show_slave_status | 0 |
| Com_show_status | 3 |
| Com_show_storage_engines | 0 |
| Com_show_table_status | 0 |
| Com_show_tables | 0 |
| Com_show_triggers | 0 |
| Com_show_variables | 0 |
| Com_show_warnings | 0 |
| Com_show_create_user | 0 |
| Com_shutdown | 0 |
| Com_slave_start | 0 |
| Com_slave_stop | 0 |
| Com_group_replication_start | 0 |
| Com_group_replication_stop | 0 |
| Com_stmt_execute | 0 |
| Com_stmt_close | 0 |
| Com_stmt_fetch | 0 |
| Com_stmt_prepare | 0 |
| Com_stmt_reset | 0 |
| Com_stmt_send_long_data | 0 |
| Com_truncate | 0 |
| Com_uninstall_plugin | 0 |
| Com_unlock_tables | 0 |
| Com_update | 0 |
| 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 |
| Com_stmt_reprepare | 0 |
| Compression | OFF |
| Flush_commands | 1 |
| Handler_commit | 0 |
+-----------------------------+-------+
151 rows in set (0.00 sec)
针对的是所有存储引擎的表：
Com_select：执行select操作的次数，1次查询累计加1；
Com_insert：执行insert操作的次数，批量insert操作，只累加1；
Com_update：执行update操作的次数；
Com_delete：执行delete操作的次数。
针对Innodb存储引擎的表：
innodb_rows_read：select查询返回的行数；
innodb_row_inserted：执行insert操作插入的行数；
innodb_rows_updated：执行update操作更新的行数；
innodb_rows_deleted：执行deleted操作删除的行数。
针对事务型的应用：
Com_commit：事务提交的次数
Com_rollback：事务回滚的次数
了解数据库基本情况
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。
1.2 定位执行效率较低SQL
1.3 Explain分析低效SQL执行计划
mysql> EXPLAIN
-> SELECT
-> c.name AS CustomerName,
-> COUNT(o.id) AS NumberOfOrders,
-> SUM(o.amount) AS TotalSpent,
-> MAX(o.order_date) AS LastOrderDate
-> FROM
-> customers c
-> JOIN
-> orders o ON c.id = o.customer_id
-> WHERE
-> o.order_date >= DATE_SUB(CURDATE(), INTERVAL 1 YEAR)
-> GROUP BY
-> c.id, c.name
-> HAVING
-> SUM(o.amount) > 1000
-> ORDER BY
-> TotalSpent DESC;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+-------+----------+----------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+-------+----------+----------------------------------------------+
| 1 | SIMPLE | o | NULL | ALL | customer_id | NULL | NULL | NULL | 10294 | 33.33 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | c | NULL | eq_ref | PRIMARY | PRIMARY | 4 | sbtest.o.customer_id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+-------+----------+----------------------------------------------+
2 rows in set, 1 warning (0.00 sec)
table：输出结果集的表
type：表的连接类型，性能由好到差的连接类型为：
possible_keys：表示查询时，可能使用的索引
key：表示实际使用的索引
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
1.4 确定问题并采取相应的优化措施
二、索引问题
2.1 索引的存储分类
2.2 如何使用索引
2.2.1 使用索引
mysql> create index idx_t1 on orders(order_date,amount);
mysql> show index from orders;
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| orders | 0 | PRIMARY | 1 | id | A | 10294 | NULL | NULL | | BTREE | | |
| orders | 1 | idx_cid | 1 | customer_id | A | 6305 | NULL | NULL | | BTREE | | |
| orders | 1 | idx_t1 | 1 | order_date | A | 730 | NULL | NULL | | BTREE | | |
| orders | 1 | idx_t1 | 2 | amount | A | 10003 | NULL | NULL | | BTREE | | |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)
mysql> explain select * from orders where order_date = '2024-5-10';
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+-------+
| 1 | SIMPLE | orders | NULL | ref | idx_t1 | idx_t1 | 3 | const | 13 | 100.00 | NULL |
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from orders where amount = 1851.69;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | orders | NULL | ALL | NULL | NULL | NULL | NULL | 10294 | 10.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> create index idx_name on customers(name);
mysql> explain select * from customers where name like '%1';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | customers | NULL | ALL | NULL | NULL | NULL | NULL | 9809 | 11.11 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from customers where name like 'Customer_101%';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | customers | NULL | ALL | idx_name | NULL | NULL | NULL | 9809 | 49.99 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
2.2.2 存在索引但不使用索引
2.2.3 查看索引使用情况
mysql> show status like 'handler_read%';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| Handler_read_first | 4 |
| Handler_read_key | 4 |
| Handler_read_last | 0 |
| Handler_read_next | 10003 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 10030 |
+-----------------------+-------+
2.3 简单实用的优化方法
2.3.1 定期分析表和检查表
analyze [local | no_write_to_binlog] table table_name1 [, table_name2]...
mysql> analyze table orders;
+---------------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+---------------+---------+----------+----------+
| sbtest.orders | analyze | status | OK |
+---------------+---------+----------+----------+
1 row in set (0.01 sec)
mysql> analyze no_write_to_binlog table orders;
+---------------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+---------------+---------+----------+----------+
| sbtest.orders | analyze | status | OK |
+---------------+---------+----------+----------+
1 row in set (0.02 sec)
·· 检查一个表有没有问题
mysql> check table orders;
+---------------+-------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+---------------+-------+----------+----------+
| sbtest.orders | check | status | OK |
+---------------+-------+----------+----------+
1 row in set (0.03 sec)
2.3.2 定期优化表
mysql> optimize table orders;
+---------------+----------+----------+-------------------------------------------------------------------+
| Table | Op | Msg_type | Msg_text |
+---------------+----------+----------+-------------------------------------------------------------------+
| sbtest.orders | optimize | note | Table does not support optimize, doing recreate + analyze instead |
| sbtest.orders | optimize | status | OK |
+---------------+----------+----------+-------------------------------------------------------------------+
2 rows in set (0.41 sec)
2.4 常用SQL优化
2.4.1 大批量插入数据
alter table table_name disable keys;
loading the data...
alter table table_name enable keys;
2.4.2 优化INSERT语句
2.4.3 优化GROUP BY语句
mysql> explain select address from customers group by address;
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| 1 | SIMPLE | customers | NULL | ALL | NULL | NULL | NULL | NULL | 9809 | 100.00 | Using temporary; Using filesort |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select address from customers group by address order by null;
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | SIMPLE | customers | NULL | ALL | NULL | NULL | NULL | NULL | 9809 | 100.00 | Using temporary |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)
2.4.4 优化ORDER BY语句
2.4.5 优化嵌套查询
2.4.6 优化OR条件
2.4.6 使用SQL提示
SELECT SQL_BUFFER_RESULTS * FROM...
