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Hints for Specifying ANY Sublink Pullup

Updated on 2024-08-20 GMT+08:00

Description

A method of optimizing operators is specified when an ANY sublink is pulled up.

Syntax

[no] hashed_sublink[(@queryblock)], material_sublink[(@queryblock)]

Parameters

  • hashed_sublink uses hash table optimization for specified sublinks.
  • material_sublink uses materialized optimization for specified sublinks.

Examples

  • material_sublink
    Table creation statements:
    CREATE TABLE SUPPLIER (
        S_SUPPKEY BIGINT NOT NULL,
        S_NAME CHAR(25) NOT NULL,
        S_ADDRESS VARCHAR(40) NOT NULL,
        S_NATIONKEY INT NOT NULL,
        S_PHONE CHAR(15) NOT NULL,
        S_ACCTBAL DECIMAL(15, 2) NOT NULL,
        S_COMMENT VARCHAR(101) NOT NULL,
        S_STATEKEY INT NOT NULL
    );
    set explain_perf_mode=pretty; -- Open the explain pretty option to view a detailed plan.
    Example:
    -- material_sublink hint
    gaussdb=#  EXPLAIN (blockname on, costs off) SELECT max(s_acctbal) nation_total, s_nationkey
    FROM supplier WHERE s_suppkey < 10000
    GROUP BY s_nationkey having nation_total = ANY (SELECT /*+ material_sublink */ avg(s_acctbal) FROM supplier GROUP BY s_statekey);
     id |                 operation                 | Query Block 
    ----+-------------------------------------------+-------------
      1 | ->  HashAggregate                         | sel$1
      2 |    ->  Seq Scan on supplier@"sel$1"       | sel$1
      3 |    ->  Materialize  [1, SubPlan 1]        | 
      4 |       ->  HashAggregate                   | sel$2
      5 |          ->  Seq Scan on supplier@"sel$2" | sel$2
    (5 rows)
    
     Predicate Information (identified by plan id) 
    -----------------------------------------------
       1 --HashAggregate
             Filter: (SubPlan 1)
       2 --Seq Scan on supplier@"sel$1"
             Filter: (s_suppkey < 10000)
    (4 rows)

    The material operation is performed on the sublink. If the hint is not used, hashed is used.

  • hashed_sublink
    Table creation and preparation:
    set work_mem='64kB';  -- Reduce the work memory to reproduce the scenario.
    set explain_perf_mode = pretty;   -- Open the explain pretty option to view a detailed plan.
    CREATE TABLE nt1 (a int);
    CREATE TABLE nt2 (a int);
    INSERT INTO nt1 VALUES(generate_series(1, 50000));
    INSERT INTO nt2 VALUES(generate_series(1, 50000));
    Example:
    -- hashed_sublink hint
    gaussdb=# EXPLAIN SELECT * FROM nt1 WHERE nt1.a NOT IN (SELECT /*+ hashed_sublink no_expand*/ a FROM nt2);
     id |               operation                | E-rows | E-width |      E-costs       
    ----+----------------------------------------+--------+---------+--------------------
      1 | ->  Seq Scan on nt1                    |  49869 |       4 | 1627.725..3255.450
      2 |    ->  Seq Scan on nt2  [1, SubPlan 1] |  99738 |       4 | 0.000..1378.380
    (2 rows)
    
     Predicate Information (identified by plan id) 
    -----------------------------------------------
       1 --Seq Scan on nt1
             Filter: (NOT (hashed SubPlan 1))
    (2 rows)
    -- work_mem affects plan generation and execution. If you perform other operations, you need to reset it.
    reset work_mem;  -- Roll back the memory settings.

    The hashed operation is performed on the sublink. If the hint is not used, material is used.

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