[英]Mysql Sub Select Query Optimization
I'm running a query daily to compile stats - but it seems really inefficient. 我每天运行一个查询来编译统计信息-但它似乎效率很低。 This is the Query:
这是查询:
SELECT a.id, tstamp, label_id, (SELECT author_id FROM b WHERE b.tid = a.id ORDER BY b.tstamp DESC LIMIT 1) AS author_id
FROM a, b
WHERE (status = '2' OR status = '3')
AND category != 6
AND a.id = b.tid
AND (b.type = 'C' OR b.type = 'R')
AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.tstamp DESC
LIMIT 500
This query seems to run really slow. 该查询似乎运行得很慢。 Apologies for the crap naming - I've been asked to not reveal the actual table names.
不好意思的命名-我被要求不要透露实际的表名。
The reason there is a sub select is because the outer select gets one row from the table a and it gets a row from table b. 之所以有子选择,是因为外部选择从表a中获得一行,而从表b中获得一行。 But also need to know the latest author_id from table b as well, so I run a subselect to return that one.
但是还需要知道表b中的最新author_id,因此我运行一个subselect来返回该表。 I don't want to run another select inside a php loop - as that is also inefficient.
我不想在php循环内运行另一个选择-因为这也效率低下。
It works correctly - I just need to find a much faster way of getting this data set. 它可以正常工作-我只需要找到一种更快的方法来获取此数据集。
Try: 尝试:
SELECT a.id,
b.tstamp,
label_id,
y.author_id
FROM TABLE_A a
JOIN TABLE_B b ON b.tid = a.id
JOIN (SELECT b.tid,
MAX(b.tstamp) 'm_tstamp'
FROM TABLE_B b
GROUP BY b.tid) x ON x.tid = a.id
JOIN (SELECT b.tid,
b.author_id,
b.tstamp
FROM TABLE_B b
GROUP BY b.tid) y ON y.tid = a.id
AND y.tstamp = x.m_tstamp
WHERE status IN ('2', '3')
AND b.type IN ('C', 'R')
AND category != 6
AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.tstamp DESC
LIMIT 500
If b.tstamp
is unique within b.tid
, take OMG Ponies' solution. 如果
b.tstamp
是内唯一b.tid
,采取OMG小马的解决方案。
Otherwise you could try this solution. 否则,您可以尝试此解决方案。 It sorts the whole result by
b.tstamp DESC
and adds a ranking per author_id
. 它通过
b.tstamp DESC
对整个结果进行排序,并为每个author_id
添加一个排名。 The outer selects takes only the row with rank = 1
, which is the one with the greatest tstamp
per author_id
. 外部选择仅采用
rank = 1
的行,这是每个author_id
具有最大tstamp
author_id
。
SELECT id, tstamp, label_id, author_id
FROM (SELECT id,
tstamp,
label_id,
author_id,
CASE
WHEN @author_id != author_id THEN @row_num := 1
ELSE @row_num := @row_num + 1
END AS rank,
@author_id := b.author_id
FROM a,
b,
(SELECT @row_num := 0, @author_id := NULL) y
WHERE a.id = b.tid
AND (status = '2' OR status = '3')
AND category != 6
AND (b.type = 'C' OR b.type = 'R')
AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.author_id, b.tstamp DESC
) x
WHERE x.rank = 1
LIMIT 500
I have not tried it, so please comment if it does not work. 我还没有尝试过,所以如果它不起作用,请发表评论。
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