[英]Optimize SQL-Query several JOINs
I have an SQL query with a nested join: 我有一个带有嵌套联接的SQL查询:
SELECT rh.host, rh.report, COUNT(results.id), COUNT(results_2.id), COUNT(results_3.id), COUNT(results_4.id)
FROM report_hosts rh
INNER JOIN report_results rr ON rh.report = rr.report
LEFT OUTER JOIN results ON rr.result = results.id AND results.type = 'Hole' AND results.host = rh.host
LEFT OUTER JOIN results results_2 ON rr.result = results_2.id AND results_2.type = 'Warning' AND results_2.host = rh.host
LEFT OUTER JOIN results results_3 ON rr.result = results_3.id AND results_3.type = 'Note' AND results_3.host = rh.host
LEFT OUTER JOIN results results_4 ON rr.result = results_4.id AND results_4.type = 'Log' AND results_4.host = rh.host
GROUP BY rh.host
The query as-is takes about 5sec with 99.7% copying to temp table . 按原样查询大约需要5秒钟,将99.7%的副本复制到temp table 。 An
EXPLAIN
of the full query shows as: 一个
EXPLAIN
完整查询显示为:
+----+-------------+-----------+--------+---------------+---------+---------+-------------------+------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+--------+---------------+---------+---------+-------------------+------+---------------------------------+
| 1 | SIMPLE | rr | ALL | report | NULL | NULL | NULL | 3139 | Using temporary; Using filesort |
| 1 | SIMPLE | rh | ref | report | report | 5 | openvas.rr.report | 167 | Using where |
| 1 | SIMPLE | results | eq_ref | PRIMARY,type | PRIMARY | 4 | openvas.rr.result | 1 | |
| 1 | SIMPLE | results_2 | eq_ref | PRIMARY,type | PRIMARY | 4 | openvas.rr.result | 1 | |
| 1 | SIMPLE | results_3 | eq_ref | PRIMARY,type | PRIMARY | 4 | openvas.rr.result | 1 | |
| 1 | SIMPLE | results_4 | eq_ref | PRIMARY,type | PRIMARY | 4 | openvas.rr.result | 1 | |
+----+-------------+-----------+--------+---------------+---------+---------+-------------------+------+---------------------------------+
When I remove the LEFT JOIN
s, the query executes in about 1s, each LEFT JOIN
adds about one additional second execution time. 当我删除
LEFT JOIN
,查询将在大约1秒内执行,每个LEFT JOIN
增加大约一秒的执行时间。
My question: Can anyone explain, why the copy to temp table task of one join takes longer if there are more LEFT JOIN
s? 我的问题:谁能解释,如果有更多的
LEFT JOIN
,为什么一个LEFT JOIN
的复制到临时表任务会花费更长的时间? Is MySQL copying the temp table several times for each JOIN? MySQL是否为每个JOIN多次复制临时表?
How can I avoid this? 如何避免这种情况? Am I missing an index?
我是否缺少索引?
What I intend to accomplish: I have a table with scanning results of several hosts. 我打算完成的工作:我有一个表,其中包含多个主机的扫描结果。 Each result is classified in types ( "Hole", "Warning", "Note" or "Log").
每个结果均按类型分类(“孔”,“警告”,“注释”或“日志”)。 I want to select each host and the corresponding amount of Holes, Warnings, Notes and Logs.
我要选择每个主机以及相应数量的“孔”,“警告”,“注释”和“日志”。 As a "restriction" I have the fact, that not each host has each type of results.
作为“限制”,我有一个事实,即并非每个主机都有每种类型的结果。
You're joining a single table several times, which effectively is like joining multiple tables. 您要多次联接单个表,这实际上就像联接多个表一样。 You should be able to handle that with some case statements and a where clause instead.
您应该能够使用一些case语句和where子句来处理该问题。 (In fact you may not need the where clause.)
(实际上,您可能不需要where子句。)
SELECT rh.host, rh.report,
COUNT(CASE WHEN results.type = 'Hole' THEN 1 ELSE NULL END) as Holes,
COUNT(CASE WHEN results.type = 'Warning' THEN 1 ELSE NULL END) as Warnings,
COUNT(CASE WHEN results.type = 'Note' THEN 1 ELSE NULL END) as Notes,
COUNT(CASE WHEN results.type = 'Log' THEN 1 ELSE NULL END) as Logs
FROM
report_hosts rh
INNER JOIN
report_results rr
ON
rh.report = rr.report
LEFT OUTER JOIN
results
ON
rr.result = results.id
AND results.host = rh.host
WHERE
results.type = 'Hole'
OR results.type = 'Warning'
OR results.type = 'Note'
OR results.type = 'Log'
GROUP BY rh.host, rh.report
Case statements, IME, are not the greatest performers, but your data bloat from the many joins may offset that and give this better performance. 案例陈述(IME)并不是最出色的执行者,但是您因众多联接而导致的数据膨胀可能会抵消这种情况,从而提高性能。
Using a lot of data (in your case an extra left join
) will mean storing it in memory. 使用大量数据(在您的情况下为额外的
left join
)将意味着将其存储在内存中。
If you deplete the buffers your query will need to be stored to a temporary result table on drive. 如果耗尽缓冲区,则查询将需要存储到驱动器上的临时结果表中。
Try using the same number of left join
s but limiting the number of rows with a limit
. 尝试使用相同数量的
left join
秒,但限制了行数limit
。 It should confirm that the problem lies in the buffers (meaning it will run faster). 它应该确认问题出在缓冲区中(这意味着它将运行得更快)。
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