奇怪的SQL语句 求助(分少请原谅)
select A.X, A.Y, B.X, B.M
from A
left join B
on A.X = B.X
where A.X = "XXXX" ---0.015秒
select A.X, A.Y, B.X, B.M
from A
left join B
on A.X = B.X
where B.M = "XXXXXX" -----12秒
select A.X, A.Y, B.X, B.M
from B
left join A
on A.X = B.X
where B.M = "XXXXXX" ----0.015秒
A.X, B.M上都建了索引,B.X为外键,有索引, A表数据百万级 B表十万级
为什么第二条会耗时这么严重?