【MySQL优化】MySQL GROUP BY 操作的优化_MySQL
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MySQL GROUP BY 操作的优化
默认情况下, MySQL 在执行 GROUP BY col1 , col2.... 操作的时候,会按照 GROUP BY 字段的顺序进行排序。如果显式包括一个包含相同的列的 ORDER BY 子句,则对 MySQL 的实际执行性能没有什么额外的影响。
如果查询包括 GROUP BY 操作, 但是不需要对结果进行排序,或者对默认的排序结果不满意,希望获得结果后再由程序进一步处理的时候,可以指定 ORDER BY NULL 禁止排序,从而避免排序结果的消耗。
下面介绍的例子对比了开启 / 关闭 GROUP BY 排序的执行计划:
mysql> desc select dep,pos,avg(sal) from employee group by dep,pos G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: employee
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 10
Extra: Using temporary; Using filesort
1 row in set (0.00 sec)
mysql> desc select dep,pos,avg(sal) from employee group by dep,pos order by null G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: employee
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 10
Extra: Using temporary
1 row in set (0.00 sec)
从执行计划可以看到,使用了 ORDER BY NULL 的 SQL 减少了文件排序的步骤,当返回结果集很大时,对于 GROUP BY 的性能是有很大改善的。
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