group by 優化 加order by null
mysql> explain select uid,sum(times) from tbl_name group by uid\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: tbl_name
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 10000
Extra: Using temporary; Using filesort
1 row in set (0.00 sec)
mysql> explain select uid,sum(times) from tbl_name group by uid order by null\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: tbl_name
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 10000
Extra: Using temporary
1 row in set (0.00 sec)
默認情況下，Group by col會對col字段進行排序，這就是爲什麼第一語句裏面有Using filesort的原因，如果你不需要對col字段進行排序，加上order by null吧，
要快很多，因爲filesort很慢的。