# 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 见[指定Hint所处于的查询块Queryblock](https://support.huaweicloud.com/centralized-devg-v3-gaussdb/gaussdb-42-0294.html#ZH-CN_TOPIC_0000002235339121)，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)
   ```