sql-server - MS-SQL AND/OR 是否有条件(执行短路评估)？

Question

我正在创建一个存储过程(简化)

create procedure sp_mysp @p1 bit AS
if (@p1 = 1 AND EXISTS(select  * from mytable1))
BEGIN....END
ELSE IF (@p = 0 AMD EXISTS(select * from mytable2))
BEGIN....END

所以问题是，如果@p1 等于 1，SQL 会一直检查 mytable1 中的数据还是只检查 mytable1 中的数据

(类似于c#中的&&)

Answer 1

无法保证这种行为。
expr1 AND expr2 未发生短路评估的示例是

SET STATISTICS IO ON

IF EXISTS(SELECT COUNT(*) FROM master..spt_monitor HAVING COUNT(*)=2)  
AND EXISTS (SELECT COUNT(*) FROM master..spt_values HAVING COUNT(*)=1)
PRINT 'Y'

EXISTS(SELECT COUNT(*) FROM master..spt_monitor HAVING COUNT(*)=2)是 false (意味着 And -ed 表达式必须是 False )但 IO 结果显示仍然评估了第二个条件。

Table 'spt_values'. Scan count 1, logical reads 14, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'spt_monitor'. Scan count 1, logical reads 1, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

不过，SQL Server 可以做到这一点。我在测试中看到了这个

SET STATISTICS IO ON

DECLARE @p1 BIT = NULL

IF ( @p1 = 1
     AND EXISTS(SELECT *
                FROM   master..spt_values) )
  PRINT '1'

ELSE IF ( @p1 = 0
     AND EXISTS(SELECT *
                FROM   master..spt_values) )
  PRINT '2'

输出是

Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

显示 spt_values从未被访问过。

这是通过执行计划中的传递谓词条件实现的。有一些关于那些here的信息.

If the passthru predicate evaluates to true, the join returns the row .... If the passthru predicate evaluates to false, the join proceeds normally