更新时间:2024-05-07 GMT+08:00

Stream方式的Hint

功能描述

指明stream使用的方法,可以为broadcast和redistribute,或者直接指定生成gather计划。

语法格式

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[no] broadcast|redistribute( [@queryblock] table_list)
gather( [@queryblock] REL|JOIN|ALL)

参数说明

  • @queryblock 见指定Hint所处的查询块Queryblock章节,可省略,表示在当前查询块生效。
  • broadcast和redistribute
    • no表示hint的stream方式不使用。
    • table_list为进行stream操作的单表或多表join结果集,见参数说明
  • gather
    gather hint可以指定三种计划生成方式:
    • REL:只生成基于基表的gather路径,然后再在CN上执行剩余计划。
    • JOIN:尽可能生成基于join的gather路径,在能下推的join子计划上面(join下面不包含重分布节点)添加gather路径,剩余计划在CN上执行。对于需要重分布节点的join计划则生成不出这种基于join的gather路径,会回退生成基于基表的gather路径。

      在指定Hint(JOIN)后,对于分布表和复制表做连接的情况会导致生成不出来Hint(JOIN)期望的计划,因为优化器已经寻找更优的计划进行替代。

    • ALL:基于最优方式选择Gather Rel或Gather Join路径。

示例

示例中原语句使用如下hint:

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explain
select /*+ no redistribute(store_sales store_returns item store) leading(((store_sales store_returns item store) customer)) */ i_product_name product_name ...

原计划中,(store_sales store_returns item store)和customer做join时,前者做了重分布,此hint表示禁止前者混合表做重分布,但仍然保持join顺序,则生成计划如下所示:

对语句进行Gather Hint指定:

  1. 生成基表Gather计划 /* +GATHER(REL)*/。
    gaussdb=# explain select /*+ GATHER(REL)*/* from t1, t2, t3 where t1.c2 = t2.c2 and t2.c2 = t3.c2;
     id |               operation               | E-rows | E-width | E-costs 
    ----+---------------------------------------+--------+---------+---------
      1 | ->  Hash Join (2,8)                   |     20 |      36 | 44.10
      2 |    ->  Hash Join (3,5)                |     20 |      24 | 29.22
      3 |       ->  Streaming (type: GATHER)    |     20 |      12 | 14.35
      4 |          ->  Seq Scan on t1           |     20 |      12 | 13.13
      5 |       ->  Hash                        |     20 |      12 | 14.35
      6 |          ->  Streaming (type: GATHER) |     20 |      12 | 14.35
      7 |             ->  Seq Scan on t2        |     20 |      12 | 13.13
      8 |    ->  Hash                           |     20 |      12 | 14.35
      9 |       ->  Streaming (type: GATHER)    |     20 |      12 | 14.35
     10 |          ->  Seq Scan on t3           |     20 |      12 | 13.13
    (10 rows)
    
     Predicate Information (identified by plan id) 
    -----------------------------------------------
       1 --Hash Join (2,8)
             Hash Cond: (t1.c2 = t3.c2)
       2 --Hash Join (3,5)
             Hash Cond: (t1.c2 = t2.c2)
    (4 rows)
  2. 生成可下推计划的Join Gather计划 /*+ GATHER(REL)*/。
    gaussdb=# explain select /*+ GATHER(JOIN)*/* from t1, t2, t3 where t1.c1 = t2.c1 and t2.c2 = t3.c2;
     id |             operation              | E-rows | E-width | E-costs 
    ----+------------------------------------+--------+---------+---------
      1 | ->  Hash Join (2,7)                |     20 |      36 | 42.37
      2 |    ->  Streaming (type: GATHER)    |     20 |      24 | 27.49
      3 |       ->  Hash Join (4,5)          |     20 |      24 | 26.56
      4 |          ->  Seq Scan on t1        |     20 |      12 | 13.13
      5 |          ->  Hash                  |     21 |      12 | 13.13
      6 |             ->  Seq Scan on t2     |     20 |      12 | 13.13
      7 |    ->  Hash                        |     20 |      12 | 14.35
      8 |       ->  Streaming (type: GATHER) |     20 |      12 | 14.35
      9 |          ->  Seq Scan on t3        |     20 |      12 | 13.13
    (9 rows)
    
     Predicate Information (identified by plan id) 
    -----------------------------------------------
       1 --Hash Join (2,7)
             Hash Cond: (t2.c2 = t3.c2)
       3 --Hash Join (4,5)
             Hash Cond: (t1.c1 = t2.c1)
    (4 rows)
  3. 生成最优方式的Gather计划 /*+ GATHER(ALL)*/。
    会基于最优方式及规则选择GATHER(REL)或者GATHER(JOIN)路径。
    gaussdb=# explain select /*+ GATHER(ALL)*/* from t1, t2, t3 where t1.c1 = t2.c1 and t2.c2 = t3.c2;
     id |             operation              | E-rows | E-width | E-costs 
    ----+------------------------------------+--------+---------+---------
      1 | ->  Hash Join (2,7)                |     20 |      36 | 42.37
      2 |    ->  Streaming (type: GATHER)    |     20 |      24 | 27.49
      3 |       ->  Hash Join (4,5)          |     20 |      24 | 26.56
      4 |          ->  Seq Scan on t1        |     20 |      12 | 13.13
      5 |          ->  Hash                  |     21 |      12 | 13.13
      6 |             ->  Seq Scan on t2     |     20 |      12 | 13.13
      7 |    ->  Hash                        |     20 |      12 | 14.35
      8 |       ->  Streaming (type: GATHER) |     20 |      12 | 14.35
      9 |          ->  Seq Scan on t3        |     20 |      12 | 13.13
    (9 rows)
    
     Predicate Information (identified by plan id) 
    -----------------------------------------------
       1 --Hash Join (2,7)
             Hash Cond: (t2.c2 = t3.c2)
       3 --Hash Join (4,5)
             Hash Cond: (t1.c1 = t2.c1)
    (4 rows)