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：

1. 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
2. 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！