MySQL EXPLAIN EXTENDED
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查询MySQL SQL语句的执行计划，一般都是EXPLAIN SQL_STATMENT。EXPLAIN EXTENDED比EXPLAIN查询出的结果多了filtered列，filtered列展示了表的过滤条件过滤后预估的百分比，这给DBA优化工作带来了一定程度的判断根据。
EXPLAIN EXTENDED的结果中，会有warning提示，通过show warnings\G命令，可以看到优化器对原有SQL的查询重写优化过后的SQL。
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mysql> explain select * from a\G ERROR 2006 (HY000): MySQL server has gone away No connection. Trying to reconnect... Connection id: 12 Current database: test *************************** 1. row *************************** id: 1 select_type: SIMPLE table: a type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 4 Extra: 1 row in set (0.01 sec) mysql> explain extended select * from a\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: a type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 4 filtered: 100.00 Extra: 1 row in set, 1 warning (0.00 sec) mysql> show warnings\G *************************** 1. row *************************** Level: Note Code: 1003 Message: select `test`.`a`.`id` AS `id`,`test`.`a`.`name` AS `name` from `test`.`a` //查询重写优化过后的SQL 1 row in set (0.00 sec) |