MySQL查询优化一例——也说说 Using intersect
2024-09-01 01:42:17
生产上面有一条sql查询很慢，需要7到8秒左右，简化之后的sql如下所示：
SELECT
mingxi.*
FROM
(
SELECT
btjc01.id,
department.`name` AS btjc01011,
btjc01011 AS obj,
btjc01.inibeginmonth AS beginYearMonth,
lsbtsj.btje AS btjc01030, CASE
WHEN btjc01.inibeginmonth ='2019-10' THEN
'是'
ELSE
'否'
END AS isadd
FROM
btjc01
INNER JOIN department ON department.Id = btjc01.btjc01038
INNER JOIN lsbtsj ON lsbtsj.btrid = btjc01.Id
WHERE
btjc01.btjc01037 = '3'
AND btjc01046 ='江西省'
AND btjc01047 ='九江市'
AND btjc01048 ='修水县'
AND lsbtsj.btqsyf <='2019-10'
AND lsbtsj.btjzyf >='2019-10'
AND beginYearMonth != inibeginmonth
UNION ALL
SELECT
btjc01.id, department.`name` AS btjc01011,
btjc01011 AS obj,
btjc01.beginYearMonth,
btjc01.btjc01030, CASE
WHEN btjc01.inibeginmonth ='2019-10' THEN
'是'
ELSE
'否'
END AS isadd
FROM
btjc01
INNER JOIN department ON department.Id = btjc01.btjc01038
INNER JOIN btgg03 ON btgg03.Id = btjc01.btjc01040
WHERE
btjc01.btjc01037 = '3'
AND btjc01046 ='江西省'
AND btjc01047 ='九江市'
AND btjc01048 ='修水县'
AND (beginYearMonth <='2019-10')
) AS mingxi LIMIT 100,200;
执行计划如下所示：
可以看到 执行计划里面出现了 Using intersect，btjc01046 btjc01048 btjc01037 btjc01047 多个索引来进行数据的过滤。
另外发现一个问题，里面的 union all 子查询 却只需要 0.4秒就可以执行完成：
SELECT
btjc01.id,
department.`name` AS btjc01011,
btjc01011 AS obj,
btjc01.inibeginmonth AS beginYearMonth,
lsbtsj.btje AS btjc01030, CASE
WHEN btjc01.inibeginmonth ='2019-10' THEN
'是'
ELSE
'否'
END AS isadd
FROM
btjc01
INNER JOIN department ON department.Id = btjc01.btjc01038
INNER JOIN lsbtsj ON lsbtsj.btrid = btjc01.Id
WHERE
btjc01.btjc01037 = '3'
AND btjc01046 ='江西省'
AND btjc01047 ='九江市'
AND btjc01048 ='修水县'
AND lsbtsj.btqsyf <='2019-10'
AND lsbtsj.btjzyf >='2019-10'
AND beginYearMonth != inibeginmonth
UNION ALL
SELECT
btjc01.id, department.`name` AS btjc01011,
btjc01011 AS obj,
btjc01.beginYearMonth,
btjc01.btjc01030, CASE
WHEN btjc01.inibeginmonth ='2019-10' THEN
'是'
ELSE
'否'
END AS isadd
FROM
btjc01
INNER JOIN department ON department.Id = btjc01.btjc01038
INNER JOIN btgg03 ON btgg03.Id = btjc01.btjc01040
WHERE
btjc01.btjc01037 = '3'
AND btjc01046 ='江西省'
AND btjc01047 ='九江市'
AND btjc01048 ='修水县'
AND (beginYearMonth <='2019-10')
执行计划如下：
比较一下前面和后面两个的执行计划，可以发现 都是有了 Using intersect 但是呢后面一个却只需要0.4秒，而在外面加上一层 select * from t，时间却需要7到8秒。一直没有明白什么原因。
最后的优化是去掉 Using interset ，使用 force index 手动指定使用索引 btjc01048:
SELECT
mingxi.*
FROM
(
SELECT
btjc01.id,
department.`name` AS btjc01011,
btjc01011 AS obj,
btjc01.inibeginmonth AS beginYearMonth,
lsbtsj.btje AS btjc01030, CASE
WHEN btjc01.inibeginmonth ='2019-10' THEN
'是'
ELSE
'否'
END AS isadd
FROM
btjc01 force index(btjc01048)
INNER JOIN department ON department.Id = btjc01.btjc01038
INNER JOIN lsbtsj ON lsbtsj.btrid = btjc01.Id
WHERE
btjc01.btjc01037 = '3'
AND btjc01046 ='江西省'
AND btjc01047 ='九江市'
AND btjc01048 ='修水县'
AND lsbtsj.btqsyf <='2019-10'
AND lsbtsj.btjzyf >='2019-10'
AND beginYearMonth != inibeginmonth
UNION ALL
SELECT
btjc01.id, department.`name` AS btjc01011,
btjc01011 AS obj,
btjc01.beginYearMonth,
btjc01.btjc01030, CASE
WHEN btjc01.inibeginmonth ='2019-10' THEN
'是'
ELSE
'否'
END AS isadd
FROM
btjc01 force index(btjc01048)
INNER JOIN department ON department.Id = btjc01.btjc01038
INNER JOIN btgg03 ON btgg03.Id = btjc01.btjc01040
WHERE
btjc01.btjc01037 = '3'
AND btjc01046 ='江西省'
AND btjc01047 ='九江市'
AND btjc01048 ='修水县'
AND (beginYearMonth <='2019-10')
) AS mingxi LIMIT 100,200;
执行时间需要1.2秒左右，和0.4秒相比，还是有差距。执行计划如下；
可以看到没有了 Using interset. 我们强制使用索引 btjc01048.
总结：
如果出现 Using interset 需要注意是否mysql优化器选择的是否是最佳的索引方案。是否可以通过force index来选择更优的索引。
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