我注意到 View 和生成该 View 的底层 Select 语句之间的性能差异。
创建 View 时,它会对所有基础表(约 700 万行)进行表扫描,但 Select 语句不会进行此类扫描。
有人可以解释为什么会有差异吗?我如何才能使 View 像 Select 语句一样执行?
这两个查询的解释计划可以在 Google 表格中找到:Link
选择语句: 更新:我已将问题范围缩小到以下 Select 语句。看来 Group By 或 Group Concat 可能导致了该问题。
SELECT
-- IOHD_REQUEST fields
r.ID as requestIdMultiple
-- KEYVALUE and MULTIPLE fields
, CAST(GROUP_CONCAT(kv6.VALUE)AS CHAR(255)) AS CLIENT_ID
, CAST(GROUP_CONCAT(kv1.VALUE) AS CHAR(255)) AS IDENTIFIER_ID
, CAST(GROUP_CONCAT(ri.IDENTIFIER) AS CHAR(255)) AS IDENTIFIER
, CAST(GROUP_CONCAT(kv7.VALUE) AS CHAR(255)) AS PLATFORM_ID
, CAST(GROUP_CONCAT(kv8.VALUE) AS CHAR(255)) AS VENDOR_ID
FROM IOHD_REQUEST r
LEFT JOIN IOHD_REQUEST_HELPDESK rh ON r.ID = rh.REQUEST_ID
LEFT JOIN IOHD_REQUEST_CLIENT rc ON r.ID = rc.REQUEST_ID
LEFT JOIN IOHD_REQUEST_IDENTIFIER ri ON r.ID = ri.REQUEST_ID
LEFT JOIN IOHD_REQUEST_PLATFORM rp ON r.ID = rp.REQUEST_ID
LEFT JOIN IOHD_REQUEST_VENDOR rv ON r.ID = rv.REQUEST_ID
LEFT JOIN IOHD_REF_KEYVALUE kv1 ON (ri.IDENTIFIER_ID = kv1.KEY AND kv1.TYPE = 'IDENTIFIERTYPE')
LEFT JOIN IOHD_REF_KEYVALUE kv6 ON (rc.CLIENT_ID = kv6.KEY AND kv6.TYPE = 'CLIENT')
LEFT JOIN IOHD_REF_KEYVALUE kv7 ON (rp.PLATFORM_ID = kv7.KEY AND kv7.TYPE = 'PLATFORM')
LEFT JOIN IOHD_REF_KEYVALUE kv8 ON (rv.VENDOR_ID = kv8.KEY AND kv8.TYPE = 'VENDOR')
GROUP BY r.ID;
View 的解释计划中让我困惑的一行如下。计划中的所有后续行在 View 和 select 语句之间都是相同的:
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 PRIMARY <derived2> NULL ALL NULL NULL NULL NULL 7237546 100 NULL
指数
--
-- Indexes for table `IOHD_REF_KEYVALUE`
--
ALTER TABLE `IOHD_REF_KEYVALUE`
ADD PRIMARY KEY (`ID`),
ADD KEY `TYPE` (`TYPE`,`KEY`);
--
-- Indexes for table `IOHD_REQUEST`
--
ALTER TABLE `IOHD_REQUEST`
ADD PRIMARY KEY (`ID`);
--
-- Indexes for table `IOHD_REQUEST_CLIENT`
--
ALTER TABLE `IOHD_REQUEST_CLIENT`
ADD PRIMARY KEY (`ID`),
ADD KEY `REQUEST_ID` (`REQUEST_ID`);
--
-- Indexes for table `IOHD_REQUEST_HELPDESK`
--
ALTER TABLE `IOHD_REQUEST_HELPDESK`
ADD PRIMARY KEY (`ID`),
ADD KEY `REQUEST_ID` (`REQUEST_ID`);
--
-- Indexes for table `IOHD_REQUEST_IDENTIFIER`
--
ALTER TABLE `IOHD_REQUEST_IDENTIFIER`
ADD PRIMARY KEY (`ID`),
ADD KEY `REQUEST_ID` (`REQUEST_ID`);
--
-- Indexes for table `IOHD_REQUEST_PLATFORM`
--
ALTER TABLE `IOHD_REQUEST_PLATFORM`
ADD PRIMARY KEY (`ID`),
ADD KEY `REQUEST_ID` (`REQUEST_ID`);
--
-- Indexes for table `IOHD_REQUEST_VENDOR`
--
ALTER TABLE `IOHD_REQUEST_VENDOR`
ADD PRIMARY KEY (`ID`),
ADD KEY `REQUEST_ID` (`REQUEST_ID`);
最佳答案
哦,键值模式的问题。除了放弃设计模式之外,我们来看看改进索引。
IOHD_REF_KEYVALUE
迫切需要帮助。
PRIMARY KEY(TYPE, KEY)
--(相反的顺序也可以)- 完全摆脱
id
。 - 表必须是 InnoDB。
- 另请参阅 http://mysql.rjweb.org/doc.php/index_cookbook_mysql#speeding_up_wp_postmeta
正在发生“爆炸-内爆”。即 JOIN
会导致更多行; GROUP BY
将其恢复。同时建立了一个巨大的tmp表。部分治愈方法是更换
CAST(GROUP_CONCAT(kv6.VALUE)AS CHAR(255)) AS CLIENT_ID
and
LEFT JOIN IOHD_REF_KEYVALUE kv6 ON (rc.CLIENT_ID = kv6.KEY AND kv6.TYPE = 'CLIENT')
与
( SELECT GROUP_CONCAT(VALUE)
FROM IOHD_REF_KEYVALUE
WHERE KEY = rc.CLIENT_ID
AND TYPE = 'CLIENT'
) AS CLIENT_ID
(类似情况同上。)
修复完可以修复的问题后,请回来寻求更多帮助,但一定要带上 SHOW CREATE TABLE
和 EXPLAIN SELECT ...
关于mysql - 数据库 View 性能问题与底层查询的比较,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/47181826/