指定agg算法的Hint

功能描述

在进行agg算法时可以指定agg的方法。

语法格式

use_hash_agg[(@queryblock)], use_sort_agg[(@queryblock)]

参数说明

@queryblock请参见指定Hint所处于的查询块Queryblock，可省略，表示在当前查询块生效，当不指定时，hint没有括号"()"。

示例

1. 使用hash聚集。

   gaussdb=# explain (costs off) select c1 from t2 where c1 in( select /*+ use_hash_agg */ t1.c1 from t1,t3 where t1.c1=t3.c1 group by 1);
                      QUERY PLAN                   
   ------------------------------------------------
    Hash Semi Join
      Hash Cond: (t2.c1 = t1.c1)
      ->  Seq Scan on t2
      ->  Hash
            ->  HashAggregate
                  Group By Key: t1.c1
                  ->  Hash Join
                        Hash Cond: (t1.c1 = t3.c1)
                        ->  Seq Scan on t1
                        ->  Hash
                              ->  Seq Scan on t3
   (11 rows)

2. 使用use_sort_agg聚集，mergejoin有序。

   gaussdb=# explain (costs off) select c1 from t2 where c1 in( select /*+ use_sort_agg */ t1.c1 from t1,t3 where t1.c1=t3.c1 group by 1);
                          QUERY PLAN                        
   ---------------------------------------------------------
    Hash Semi Join
      Hash Cond: (t2.c1 = t1.c1)
      ->  Seq Scan on t2
      ->  Hash
            ->  Group
                  Group By Key: t1.c1
                  ->  Merge Join
                        Merge Cond: (t1.c1 = t3.c1)
                        ->  Index Only Scan using it1 on t1
                        ->  Sort
                              Sort Key: t3.c1
                              ->  Seq Scan on t3
   (12 rows)