使用合并

Question

有人可以向我解释为什么第二个查询比第一个查询要快得多吗？ （第一个需要大约40秒，第二个小于0.1）

SELECT releases.feature_code, releases.platform_id, version, release_date, general_available, uri, filesize, display_name, feature_name.full_name,
IF ((SELECT EXISTS(SELECT 1 FROM release_exceptions WHERE 
releases.feature_code = release_exceptions.feature_code
 AND releases.platform_id = release_exceptions.platform_id 
 AND releases.version = release_exceptions.version) = 1), 
1, 0) as release_exceptions

FROM releases

LEFT OUTER JOIN platform USING(platform_id)
LEFT OUTER JOIN feature_name USING(feature_code)

ORDER BY version DESC;

~~~

QUERY 2 ------------------

~~~

SELECT feature_code, version, release_date, general_available, platform_id, uri, filesize, display_name, feature_name.full_name, COALESCE(release_exceptions, 0) AS release_exceptions

FROM mus.releases

LEFT OUTER JOIN (
SELECT DISTINCT feature_code, version, platform_id, 1 AS release_exceptions
FROM mus.release_exceptions
) release_exceptions
USING (feature_code, version, platform_id)

LEFT OUTER JOIN platform USING(platform_id)
LEFT OUTER JOIN feature_name USING(feature_code)

;

它生成release_exceptions列的方式似乎是速度的巨大差异。我的同事写下了第二个，但无法解释为什么它的速度要快得多。

http://imgur.com/dXLuo6S,noRYV7v#0

第一张图片是第一个查询 （注意 - 在发布后，feature_code，platform_id和版本被添加为索引，这导致第一个稍快一点）

Answer 1

它不是导致加速的煤炭。这是删除子查询。第一个运行每行查询

第二个执行单个子查询