假设我在MySQL中有一个如下所示的表:
CREATE TABLE mytable (
column_A [...],
column_B [...],
column_C [...],
column_X INT(11) UNSIGNED DEFAULT 0, -- type not really relevant
INDEX a_b_c (column_A, column_B, column_C),
INDEX x (column_X)
);
基本上,它在A、B、C列上有一个索引,在X列上有一个索引.
现在假设我想要执行一个查询,如下所示:
SELECT * FROM mytable
WHERE column_A = <something> AND column_B = <something> AND column_C = <SOMETHING>
ORDER BY column_X
上面的索引是否足以使其尽可能高效,或者在所有四列上也有一个索引会对它有好处吗?