MySQL 优化器提示(二)之 Optimizer Hints for Naming Query Blocks

原创 aisql 2022-03-06

3023

MySQLOptimizer Hints 的适用范围为以下几种Query block, table,Index,Global
其它三种都是我们常见的类型，就第一个Query block 需要专门定义。

一、定义

QB_NAME(name)
定义很简单以代码举列

select 
/*+
qb_name(qb1)
*/
* from  services  b 
inner join user a on  b.userid  = a.id
where a.tel like '136%'
order  by  b.CreatedTime desc 
limit  10

如上面所示，就定义了一个qb1的Query block 这个Query block 所管辖的表就是 from 后的 b,a两表

当遇到嵌套的时候。可以这样

explain 
select 
/*+ 
qb_name(qb1)
*/
* from order_bill_index a
inner join order_bill_detail b on a.id =b.order_id 
where goods_id in
(
  select
  /*+
  qb_name(qb2)
  */
  c.id  from goods_goods_info c
  inner join  goods_class d on c.class_id = d.id
)

qb1 管辖是a和b表, qb2管辖的是 c和d表

二、使用

Query Blocks 有三种使用情况。分别是

hint_name([@query_block_name])
hint_name([@query_block_name] tbl_name [, tbl_name] ...)
hint_name(tbl_name[@query_block_name] [, tbl_name[@query_block_name]] ...)

a、第一种 hint_name([@query_block_name])

JOIN_FIXED_ORDER 就是这个用法
是指。hint是适用于哪个Query block

explain 
select 
/*+
qb_name(qb1)
JOIN_FIXED_ORDER(@qb1) 
*/
* from  services  b 
inner join user a on  b.userid  = a.id
where a.tel like '136%'
order  by  b.CreatedTime desc 
limit  10

b、第二种 hint_name([@query_block_name] tbl_name [, tbl_name] …)

指定query_block块内的表的顺序。如下面的join_order

explain 
select 
/*+
qb_name(qb1)
JOIN_ORDER(@qb1 a,b) 
*/
* from  services  b 
inner join user a on  b.userid  = a.id
where a.tel like '136%'
order  by  b.CreatedTime desc 
limit  10

c、第三种 hint_name(tbl_name[@query_block_name] [, tbl_name[@query_block_name]] …)

指定表名@query_block_name

explain 
select 
/*+ 
qb_name(qb1)
JOIN_ORDER(a@qb1,b@qb1,a@qb2,b@qb2)
*/
* from order_bill_index a
inner join order_bill_detail b on a.id =b.order_id 
where goods_id in
(
  select
  /*+
  qb_name(qb2)
  */
  a.id  from goods_goods_info a
  inner join  goods_class b on a.class_id = b.id
)