SUM( A+B) vs SUM(A) + SUM(B)

Question

我偶然发现我的一位同事写了一个脚本，并找到了如下一行

SELECT SUM(A) + SUM(B) FROM TABLE

我提出了一种不同的方法

SELECT SUM( A + B ) FROM TABLE

哪条线最有效率？

Answer 1

对于带有30M+行的InnoDB/MySQL5.6表，下面是FLOATs结果。

select SUM(x + y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 1 query: 8,596 sec. */

select SUM(x + y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 1 query: 8,799 sec. */

select SUM(x) + SUM(y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 2 queries: 8,720 sec. */

select SUM(x) + SUM(y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 2 queries: 8,580 sec. */

select SUM(x + y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 1 query: 8,642 sec. */

一般来说，时间的范围是相同的，所以我认为这种差异是没有意义的。