比较这 2 个查询。将过滤器放在连接条件上或放在 WHERE
子句中更快吗?我一直觉得它在连接标准上更快,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个更易于阅读的意见。
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
我运行了一些测试,结果表明它实际上非常接近,但是 WHERE
子句实际上稍微快一些! =)
我绝对同意在 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。
标准运行时间: 143016 毫秒
已用时间加入标准: 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
最佳答案
在性能方面,它们是相同的(并产生相同的计划)
从逻辑上讲,如果将 INNER JOIN
替换为 LEFT JOIN
,您应该进行仍然有意义的操作。
根据您的情况,这将如下所示:
SELECT *
FROM TableA a
LEFT JOIN
TableXRef x
ON x.TableAID = a.ID
AND a.ID = 1
LEFT JOIN
TableB b
ON x.TableBID = b.ID
或者这个:
SELECT *
FROM TableA a
LEFT JOIN
TableXRef x
ON x.TableAID = a.ID
LEFT JOIN
TableB b
ON b.id = x.TableBID
WHERE a.id = 1
前一个查询不会返回除 1
之外的 a.id
的任何实际匹配项,因此后一个语法(使用 WHERE
)为逻辑上更加一致。
关于sql - 哪种 SQL 查询速度更快?按连接条件或Where 子句过滤?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/2509987/