记一次mysql的union all的简单优化

公司后台最近需要把两张表数据进行统一分页查询,一张表100多万,一张表10多万

首先就想到了union all ,但感觉会很慢, 果不其然, 第一页就花了十几秒, 开始的SQL是这样的:

-- 总数
select count(*) from
 (
select id,order_no,state,create_time from order1
 union all 
select id,order_no,state,create_time from order2
 ) ua where state=1 

-- 分页
select * from
 (
select id,order_no,state,create_time from order1
 union all 
select id,order_no,state,create_time from order2
 ) ua where state=1 
order by create_time desc 
limit 0,10;

细思极恐之后, 决定还是要优化一下, 效果不错, 最重要的两个优化如下:

1,  如果有筛选条件, 务必先分别过滤查询,分而治之然后再进行union

-- 总数
select count(*) from
 (
select id from order1 where state=1 
 union all 
select id from order2 where state=1 
 ) ua ;

-- 分页
select * from
 (
select id,order_no,state,create_time from order1 where state=1
 union all 
select id,order_no,state,create_time from order2 where state=1
 ) ua 
order by create_time desc 
limit 0,10;

2, 如果没有任何过滤条件, 务必充分利用“覆盖索引”- Covering Index

覆盖索引,简单理解就是 select 字段(身上有索引) from table , 这种情况不会去遍历表, 使用explain解释覆盖索引语句时,会显示type为index , extra为Using index

当无条件分页时, 首先利用覆盖索引快速查出一页的id, 然后再用in去查询出该页的行数据

-- 覆盖索引, id是主键, create_time身上有索引
select * from
 (
select id,create_time from order1 
 union all 
select id,create_time from order2 
 ) ua order by create_time desc
limit 0,10;

-- 根据上面的结果id,查找指定行
select * from
 (
select id,order_no,state,create_time from order1 where id in (...)
 union all 
select id,order_no,state,create_time from order2 where id in (...)
 ) ua