【发布时间】：2011-01-31 09:15:42
【问题描述】：
比较这两个查询。将过滤器放在连接条件或WHERE 子句中是否更快。我一直觉得加入标准更快，因为它会尽快减少结果集，但我不确定。
我将构建一些测试来查看，但我也想获得关于哪些更易于阅读的意见。
查询 1
SELECT *
FROM TableA a
INNER JOIN TableXRef x
ON a.ID = x.TableAID
INNER JOIN TableB b
ON x.TableBID = b.ID
WHERE a.ID = 1 /* <-- Filter here? */
查询 2
SELECT *
FROM TableA a
INNER JOIN TableXRef x
ON a.ID = x.TableAID
AND a.ID = 1 /* <-- Or filter here? */
INNER JOIN TableB b
ON x.TableBID = b.ID
编辑
我进行了一些测试，结果表明它实际上非常接近，但WHERE 子句实际上稍微快一点！ =)
我完全同意在WHERE 子句上应用过滤器更有意义，我只是对性能影响感到好奇。
经过时间标准： 143016 毫秒
经过时间加入标准： 143256 毫秒
测试
SET NOCOUNT ON;
DECLARE @num INT,
@iter INT
SELECT @num = 1000, -- Number of records in TableA and TableB, the cross table is populated with a CROSS JOIN from A to B
@iter = 1000 -- Number of select iterations to perform
DECLARE @a TABLE (
id INT
)
DECLARE @b TABLE (
id INT
)
DECLARE @x TABLE (
aid INT,
bid INT
)
DECLARE @num_curr INT
SELECT @num_curr = 1
WHILE (@num_curr <= @num)
BEGIN
INSERT @a (id) SELECT @num_curr
INSERT @b (id) SELECT @num_curr
SELECT @num_curr = @num_curr + 1
END
INSERT @x (aid, bid)
SELECT a.id,
b.id
FROM @a a
CROSS JOIN @b b
/*
TEST
*/
DECLARE @begin_where DATETIME,
@end_where DATETIME,
@count_where INT,
@begin_join DATETIME,
@end_join DATETIME,
@count_join INT,
@curr INT,
@aid INT
DECLARE @temp TABLE (
curr INT,
aid INT,
bid INT
)
DELETE FROM @temp
SELECT @curr = 0,
@aid = 50
SELECT @begin_where = CURRENT_TIMESTAMP
WHILE (@curr < @iter)
BEGIN
INSERT @temp (curr, aid, bid)
SELECT @curr,
aid,
bid
FROM @a a
INNER JOIN @x x
ON a.id = x.aid
INNER JOIN @b b
ON x.bid = b.id
WHERE a.id = @aid
SELECT @curr = @curr + 1
END
SELECT @end_where = CURRENT_TIMESTAMP
SELECT @count_where = COUNT(1) FROM @temp
DELETE FROM @temp
SELECT @curr = 0
SELECT @begin_join = CURRENT_TIMESTAMP
WHILE (@curr < @iter)
BEGIN
INSERT @temp (curr, aid, bid)
SELECT @curr,
aid,
bid
FROM @a a
INNER JOIN @x x
ON a.id = x.aid
AND a.id = @aid
INNER JOIN @b b
ON x.bid = b.id
SELECT @curr = @curr + 1
END
SELECT @end_join = CURRENT_TIMESTAMP
SELECT @count_join = COUNT(1) FROM @temp
DELETE FROM @temp
SELECT @count_where AS count_where,
@count_join AS count_join,
DATEDIFF(millisecond, @begin_where, @end_where) AS elapsed_where,
DATEDIFF(millisecond, @begin_join, @end_join) AS elapsed_join
【问题讨论】：
-
根据数据，WHERE vs JOIN 标准可以返回不同的结果集。
-
@OMG Ponies 非常正确，但很多时候情况并非如此。
-
我不会将低于 5% 的差异称为差异——它们是相同的。您希望 2%% 差异的显着性更好地运行测试 1000 次，以确保它不仅仅是随机的。
-
好处是在加入之前过滤数据，所以如果它是 x.ID，那么你会比使用 a.ID 更有可能看到改进
标签： sql sql-server tsql sql-server-2008