MySQL查询性能、EXPLAIN执行计划
首先,是这么一段执行时间24秒的sql:
SQL 1:
SELECT
d.id,
d.project_code,
d.push_type,
d.push_name,
d.push_to,
d.push_from,
d.push_way,
tr.isread,
d.push_content,
d.push_attachment,
d.add_time,
d.push_status,
d.push_reviewer,
push_time,
d.reviewer_time,
d.push_class,
d.rel_object,
p.short_name AS project_name,
u1. NAME AS push_to_name,
u2. NAME AS push_from_name,
u3. NAME AS push_reviewer_name
FROM tb_push_data d
LEFT JOIN tb_project p ON p.short_code = d.project_code
LEFT JOIN tb_user u1 ON u1. CODE = d.push_to
LEFT JOIN tb_user u2 ON u2. CODE = d.push_from
LEFT JOIN tb_user u3 ON u3. CODE = d.push_reviewer
LEFT JOIN tb_push_log tr ON tr.pushdata_id = d.id
WHERE 1 = 1
AND d.add_time BETWEEN '2017-01-01' and '2018-01-11'
GROUP BY d.id
LIMIT 0,100
受影响的行: 0
时间: 24.933s
可以看到,仅仅是100条数据,查询时间竟然高达24秒,于是我尝试将左外连接改为交叉连接:
SQL 2:
SELECT
d.id,
d.project_code,
d.push_type,
d.push_name,
d.push_to,
d.push_from,
d.push_way,
tr.isread,
d.push_content,
d.push_attachment,
d.add_time,
d.push_status,
d.push_reviewer,
push_time,
d.reviewer_time,
d.push_class,
d.rel_object,
p.short_name AS project_name,
u1. NAME AS push_to_name,
u2. NAME AS push_from_name,
u3. NAME AS push_reviewer_name
FROM
tb_push_data d,
tb_project p,
tb_user u1,
tb_user u2,
tb_user u3,
tb_push_log tr
WHERE 1 = 1
and p.short_code = d.project_code
and u1. CODE = d.push_to
and u2. CODE = d.push_from
and u3. CODE = d.push_reviewer
and tr.pushdata_id = d.id
AND d.add_time BETWEEN '2017-01-01' and '2018-01-11'
GROUP BY d.id
LIMIT 0,100
受影响的行: 0
时间: 0.189s
这一下查询时间终于算是正常了,看来很多人说的数据库查询不用外键不用join还是有道理的。
但是这又出现了一个问题,因为SQL 2没有使用外连接,比如我某条数据是没有推送人的,那这条记录都显示不出来了。
那么,针对目前的这种情况,该如何优化?
首先,我陷入了两个误区:
- 这个限制了只返回100条,并不是满足条件的只有100条,所以不能说数据量小
- 至于
LEFT JOIN
和交叉连接
,两者意义不一样,查询计划不一样,满足条件的数据量也不一样,所以没有可比性。需要的数据出不来了,所以明显是少了,少了数据,速度变更快不正常么?
解决方案
对于这种优化的问题,应该先拿到执行计划:EXPLAIN
也就是在查询sql的前面加上EXPLAIN
,得到如下结果:
可以看到,LEFT JOIN tb_push_log tr ON tr.pushdata_id = d.id
这个没走索引,而且看起来数据量不少,那么就要先在上面加索引试试:
alter table tb_push_log add key(pushdata_id)
问题解决!