lightdb21.3-支持语句级hint

Posted 2022-06-27 紫无之紫

lightdb21.3-新增支持语句级hint及支持qb_name

之前版本，优化器提示作用于整条SQL，从21.3版本开始，lightdb支持优化器提示只作用于当前查询块。

规则如下：

• 定义在SQL开头的优化器提示仍作用于整条SQL

  chuhx@postgres=# explain(costs false) select * from t2 ,(select * from t2) x where t2.id=x.id;
             QUERY PLAN            
  ---------------------------------
   Hash Join
     Hash Cond: (t2.id = t2_1.id)
     ->  Seq Scan on t2
     ->  Hash
           ->  Seq Scan on t2 t2_1
  (5 rows)
  
  chuhx@postgres=# /*+index(t2)*/explain(costs false) select * from t2 ,(select * from t2) x where t2.id=x.id;
                  QUERY PLAN                 
  -------------------------------------------
   Merge Join
     Merge Cond: (t2.id = t2_1.id)
     ->  Index Scan using t2_pkey on t2
     ->  Index Scan using t2_pkey on t2 t2_1
  (4 rows)
  
  chuhx@postgres=# 
  
  

• 定义在select,delete,update,insert 后的优化器提示只作用于当前sql

  chuhx@postgres=# explain(costs false) select/*+index(t2)*/ * from t2 ,(select * from t2) x where t2.id=x.id;
                    QUERY PLAN                  
  ----------------------------------------------
   Hash Join
     Hash Cond: (t2.id = t2_1.id)
     ->  Index Scan using t2_pkey on t2 @"lt#0"
     ->  Hash
           ->  Seq Scan on t2 t2_1
  (5 rows)
  
  chuhx@postgres=# explain(costs false) select * from t2 ,(select /*+index(t2)*/* from t2) x where t2.id=x.id;
                      QUERY PLAN                     
  ---------------------------------------------------
   Hash Join
     Hash Cond: (t2_1.id = t2.id)
     ->  Index Scan using t2_pkey on t2 t2_1 @"lt#0"
     ->  Hash
           ->  Seq Scan on t2 @"lt#1"
  (5 rows)
  
  chuhx@postgres=# 
  
  

跨查询块使用需要结合qb_name

qb_name

新增支持qb_name 来对其他查询块的对象使用优化器提示。在查询块中使用qb_name(xxx) 表示当前查询块的块名为xxx, 然后可以在其他查询块中引用以对此查询块中的对象生效。

引用方式有两种：

1. hashjoin(t1 t2@qb) 跟在表名后面。
2. hashjoin(@qb t1 t2) 相当于hashjoin(t1@qb t2@qb)

下面是具体的用例：

chuhx@postgres=# explain(costs false) select /*+nestloop(t2 t2@qb)*/* from t2 ,(select/*+qb_name(qb)*/ * from t2) x where t2.id=x.id;
                  QUERY PLAN                   
-----------------------------------------------
 Nested Loop
   ->  Seq Scan on t2 @"lt#0"
   ->  Index Scan using t2_pkey on t2 t2_1 @qb
         Index Cond: (id = t2.id)
(4 rows)

chuhx@postgres=# 

chuhx@postgres=# explain(costs false) select /*+nestloop(@qb t2 t3)*/* from t3 ,(select/*+no_merge qb_name(qb)*/ t2.id from t2,t3 where t2.id=t3.id) x where t3.id=x.id;
LOG:  pg_hint_plan:
used hint:
NestLoop(@qb t2@qb t3@qb)
no_merge
not used hint:
duplication hint:
error hint:

                        QUERY PLAN                        
----------------------------------------------------------
 Hash Join
   Hash Cond: (t2.id = t3.id)
   ->  Nested Loop
         ->  Seq Scan on t2 @qb
         ->  Index Only Scan using t3_pkey on t3 t3_1 @qb
               Index Cond: (id = t2.id)
   ->  Hash
         ->  Seq Scan on t3 @"lt#0"
(8 rows)

参考文档

官方文档

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