MySQL中使用UNION进行两表合并，去重导致效率低下，请问如何优化

Question

MySQL中使用UNION进行两表合并，去重导致效率低下，请问如何优化

Answer 1

不应用ALL关键字就可以删除重复的行，例如：select col1,col2 from t1unionselect col1,col2 from t2unionselect col1,col2 from t3;多表联合并保留所有的行：select col1,col2 from t1union allselect col1,col2 from t2union allselect col1,col2 from t3;

Answer 2

MySQL中使用UNION进行两表合并，去重导致效率低下，请问如何优化mysql> explain select * from wp_options limit 1\G;id: 1select_type: SIMPLEtable: wp_optionspartitions: NULLtype: ALLpossible_keys: NULLkey: NULLkey_len: NULLref: NULLrows: 136filtered: 100.00Extra: NULL1 row in set, 1 warning (0.01 sec)mysql> show session status like 'Handler_read%';