Mysql基础调优

性能分析

explain select ...

总共有这些个字段

性能分析字段

• select_type 字段详解

  
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实例：

explain SELECT * from t_user UNION select * from t_user 

EXPLAIN select * from employee where dept_id in (select id from dept union select id from dept);

• type

  
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  查询性能从最好到最差的排序是:
  System>const>eq_ref>range>index>All

• extra

  
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• 索引优化