你的JoinHint为什么不生效-低调大师

你的JoinHint为什么不生效

2023-12-18 380

本文分享自华为云社区《你的JoinHint为什么不生效【绽放吧！GaussDB(DWS)云原生数仓】》，作者：你是猴子请来的救兵吗。

引言

提起数据库的Hint，几乎每个DBA都知道这一强大功能。在GaussDB(DWS)中，Hint可以被用来干预SQL的执行计划，但是在日常工作中，很多开发人员对Hint的缺乏深入了解，经常遇到Hint失效的情况却又束手无策。
本次针对JoinHint从案例着手深入解析JoinHint不生效的原因，以便读者能“知其所以然”。（本文不讨论Hint的基础语法问题）。

问题案例

内核版本 GaussDB 8.1.3

问题描述 两表关联查询，使用hashjoin hint干预join方式，但hint不生效

问题用例

CREATE TABLE workitem (
    language character varying(10),
    userid character varying(240),
    opiontype character varying(240),
    processinstid character varying(240),
    workitemid character varying(240),
    type_name character varying(240),
    type_code character varying(240),
    createtime timestamp without time zone,
    endtime timestamp without time zone,
    notrejecttotal numeric,
    dws_created_time timestamp without time zone
)
WITH (orientation=column, compression=low, colversion=2.0, enable_delta=false)
DISTRIBUTE BY HASH(workitemid);

CREATE  TABLE workday (
    mm timestamp with time zone,
    rn numeric
)
WITH (orientation=column, compression=low, colversion=2.0, enable_delta=false)
DISTRIBUTE BY HASH(mm);

explain
SELECT /*+ hashjoin(c d) */
    c.userid,c.type_name,c.type_code,count(1) num
FROM workitem c INNER JOIN workday d ON c.createtime = d.mm
    WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
    GROUP BY c.userid,c.type_name,c.type_code;

WARNING:  unused hint: HashJoin(c d)
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                      operation                       | E-rows | E-memory | E-width | E-costs
 ----+------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                      |      2 |          |    1502 | 33.12
   2 |    ->  Vector Sonic Hash Aggregate                   |      2 |          |    1502 | 33.12
   3 |       ->  Vector Streaming (type: GATHER)            |      4 |          |    1502 | 33.12
   4 |          ->  Vector Sonic Hash Aggregate             |      4 | 16MB     |    1502 | 27.12
   5 |             ->  Vector Nest Loop (6,8)               |      5 | 1MB      |    1494 | 27.08
   6 |                ->  Vector Streaming(type: BROADCAST) |     14 | 2MB      |       8 | 13.68
   7 |                   ->  CStore Scan on workday d       |      7 | 1MB      |       8 | 13.05
   8 |                ->  Vector Materialize                |      5 | 16MB     |    1502 | 13.09
   9 |                   ->  CStore Scan on workitem c      |      5 | 1MB      |    1502 | 13.08

         RunTime Analyze Information
 -------------------------------------------
         "public.workitem" runtime: 25.794ms
         "public.workday" runtime: 18.098ms

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Nest Loop (6,8)
         Join Filter: (c.createtime = d.mm)
   7 --CStore Scan on workday d
         Filter: (mm >= '2023-09-01 00:00:00'::timestamp without time zone)
         Pushdown Predicate Filter: (mm >= '2023-09-01 00:00:00'::timestamp without time zone)
   9 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5271KB
(33 rows)

问题定位

尝试关闭nestloop路径，来验证是否可以生成hash计划

set enable_nestloop = off;

set enable_mergejoin = off;

set enable_hashjoin = on;

WARNING:  unused hint: HashJoin(c d)
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                      operation                       | E-rows | E-memory | E-width | E-costs
 ----+------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                      |      2 |          |    1502 | 33.12
   2 |    ->  Vector Sonic Hash Aggregate                   |      2 |          |    1502 | 33.12
   3 |       ->  Vector Streaming (type: GATHER)            |      4 |          |    1502 | 33.12
   4 |          ->  Vector Sonic Hash Aggregate             |      4 | 16MB     |    1502 | 27.12
   5 |             ->  Vector Nest Loop (6,8)               |      5 | 1MB      |    1494 | 27.08
   6 |                ->  Vector Streaming(type: BROADCAST) |     14 | 2MB      |       8 | 13.68
   7 |                   ->  CStore Scan on workday d       |      7 | 1MB      |       8 | 13.05
   8 |                ->  Vector Materialize                |      5 | 16MB     |    1502 | 13.09
   9 |                   ->  CStore Scan on workitem c      |      5 | 1MB      |    1502 | 13.08

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Nest Loop (6,8)
         Join Filter: (c.createtime = d.mm)
   7 --CStore Scan on workday d
         Filter: (mm >= '2023-09-01 00:00:00'::timestamp without time zone)
         Pushdown Predicate Filter: (mm >= '2023-09-01 00:00:00'::timestamp without time zone)
   9 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5271KB
(28 rows)

关闭nestloop路径后，仍然生成nestloop计划，且E-costs代价中未添加惩罚代价，说明该场景语句本身不支持hashjoin。

检查关联表达式(c.createtime = d.mm)

，确认是否支持hashjoin。

关联表达式为字段关联，不存在函数嵌套
关联表达式两边数据类型为timestamp without time zone和timestamp with time zone，通过系统表pg_operator确认是否支持hashjoin。

postgres=# select * from pg_operator where oprname = '=' and oprleft = 'timestamp'::regtype and oprright = 'timestamptz'::regtype;
-[ RECORD 1 ]+-------------------------
oprname      | =
oprnamespace | 11
oprowner     | 10
oprkind      | b
oprcanmerge  | t
oprcanhash   | f
oprleft      | 1114
oprright     | 1184
oprresult    | 16
oprcom       | 2542
oprnegate    | 2539
oprcode      | timestamp_eq_timestamptz
oprrest      | eqsel
oprjoin      | eqjoinsel

通过结果确认oprcanhash为false，代表该操作符不支持hash连接；原因是，左边数据不带时区，右边数据带，在比较时要先处理时区问题，不能直接拿存储值进行判断。

改善办法

通过系统表确认timestamp类型的等值关联和timestamptz的等值关联均支持hash连接。

postgres=# select * from pg_operator where oprname = '=' and oprleft = oprright and oprleft in('timestamp'::regtype,'timestamptz'::regtype);
-[ RECORD 1 ]+---------------
oprname      | =
oprnamespace | 11
oprowner     | 10
oprkind      | b
oprcanmerge  | t
oprcanhash   | t
oprleft      | 1184
oprright     | 1184
oprresult    | 16
oprcom       | 1320
oprnegate    | 1321
oprcode      | timestamptz_eq
oprrest      | eqsel
oprjoin      | eqjoinsel
-[ RECORD 2 ]+---------------
oprname      | =
oprnamespace | 11
oprowner     | 10
oprkind      | b
oprcanmerge  | t
oprcanhash   | t
oprleft      | 1114
oprright     | 1114
oprresult    | 16
oprcom       | 2060
oprnegate    | 2061
oprcode      | timestamp_eq
oprrest      | eqsel
oprjoin      | eqjoinsel

在关联条件上添加类型转换，保证两边类型一致，即(c.createtime：：timestamptz = d.mm)或(c.createtime = d.mm：：timestamp)。

postgres=# explain
postgres-# SELECT /*+ hashjoin(c d) */
postgres-#     c.userid,c.type_name,c.type_code,count(1) num
postgres-# FROM workitem c INNER JOIN workday d ON c.createtime::timestamptz = d.mm
postgres-#     WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
postgres-#     GROUP BY c.userid,c.type_name,c.type_code;
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                      operation                       | E-rows | E-memory | E-width | E-costs
 ----+------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                      |      2 |          |    1502 | 34.29
   2 |    ->  Vector Sonic Hash Aggregate                   |      2 |          |    1502 | 34.29
   3 |       ->  Vector Streaming (type: GATHER)            |      4 |          |    1502 | 34.29
   4 |          ->  Vector Sonic Hash Aggregate             |      4 | 16MB     |    1502 | 28.29
   5 |             ->  Vector Sonic Hash Join (6,8)         |      5 | 16MB     |    1494 | 28.25
   6 |                ->  Vector Streaming(type: BROADCAST) |     40 | 2MB      |       8 | 15.06
   7 |                   ->  CStore Scan on workday d       |     20 | 1MB      |       8 | 13.01
   8 |                ->  CStore Scan on workitem c         |      5 | 1MB      |    1502 | 13.08

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Sonic Hash Join (6,8)
         Hash Cond: (d.mm = (c.createtime)::timestamp with time zone)
   8 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5530KB
(24 rows)

postgres=# explain
SELECT /*+ hashjoin(c d) */
    c.userid,c.type_name,c.type_code,count(1) num
FROM workitem c INNER JOIN workday d ON c.createtime = d.mm::timestamp
    WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
    GROUP BY c.userid,c.type_name,c.type_code;
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                      operation                       | E-rows | E-memory | E-width | E-costs
 ----+------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                      |      2 |          |    1502 | 32.91
   2 |    ->  Vector Sonic Hash Aggregate                   |      2 |          |    1502 | 32.91
   3 |       ->  Vector Streaming (type: GATHER)            |      4 |          |    1502 | 32.91
   4 |          ->  Vector Sonic Hash Aggregate             |      4 | 16MB     |    1502 | 26.91
   5 |             ->  Vector Sonic Hash Join (6,8)         |      5 | 16MB     |    1494 | 26.87
   6 |                ->  Vector Streaming(type: BROADCAST) |     14 | 2MB      |       8 | 13.71
   7 |                   ->  CStore Scan on workday d       |      7 | 1MB      |       8 | 13.08
   8 |                ->  CStore Scan on workitem c         |      5 | 1MB      |    1502 | 13.08

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Sonic Hash Join (6,8)
         Hash Cond: ((d.mm)::timestamp without time zone = c.createtime)
   7 --CStore Scan on workday d
         Filter: ((mm)::timestamp without time zone >= '2023-09-01 00:00:00'::timestamp without time zone)
   8 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5530KB
(26 rows)

知识小结

实际使用过程中导致hint生效的原因很多，这里总结排查hashjoin hint步骤以供参考:

检查hint中的表名是否正确、是否存在重名、是否在当前层可见，此类场景通常在explain中会给出提示，自行排查即可。
判断关联hint中的表名是否被提升导致表名不存在，此类场景通常在explain中会给出提示

postgres=# explain
SELECT /*+ hashjoin(c d) */
    c.userid,c.type_name,c.type_code,count(1) num
FROM workitem c INNER JOIN (select * from workday where mm >= '2023-09-01') d ON c.createtime = d.mm::timestamp
    WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
    GROUP BY c.userid,c.type_name,c.type_code;
WARNING:  Error hint: HashJoin(c d), relation name "d" is not found.
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                      operation                       | E-rows | E-memory | E-width | E-costs
 ----+------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                      |      2 |          |    1502 | 32.78
   2 |    ->  Vector Sonic Hash Aggregate                   |      2 |          |    1502 | 32.78
   3 |       ->  Vector Streaming (type: GATHER)            |      4 |          |    1502 | 32.78
   4 |          ->  Vector Sonic Hash Aggregate             |      4 | 16MB     |    1502 | 26.78
   5 |             ->  Vector Sonic Hash Join (6,8)         |      5 | 16MB     |    1494 | 26.74
   6 |                ->  Vector Streaming(type: BROADCAST) |     10 | 2MB      |       8 | 13.58
   7 |                   ->  CStore Scan on workday         |      5 | 1MB      |       8 | 13.11
   8 |                ->  CStore Scan on workitem c         |      5 | 1MB      |    1502 | 13.08

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Sonic Hash Join (6,8)
         Hash Cond: ((workday.mm)::timestamp without time zone = c.createtime)
   7 --CStore Scan on workday
         Filter: ((mm >= '2023-09-01 00:00:00+08'::timestamp with time zone) AND ((mm)::timestamp without time zone >= '2023-09-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: (mm >= '2023-09-01 00:00:00+08'::timestamp with time zone)
   8 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5530KB
(27 rows)

针对此种情况，8.2.0及以上版本可以通过添加no merge hint来禁用子查询提升从而规避hint失效问题。

通过join路径参数验证目标路径是否可生效。

--如通过关闭其他路径参数来验证某一路径是否可达
set enable_nestloop = off;
set enable_mergejoin = off;
set enable_hashjoin = on;

检查关联条件中是否存在volatile函数。

postgres=# create or replace function gettimediff(timestamp) returns interval language sql as 'select $1-timeofday()::timestamp' volatile;
CREATE FUNCTION
postgres=# explain
SELECT /*+ hashjoin(c d) */
    c.userid,c.type_name,c.type_code,count(1) num
FROM workitem c INNER JOIN workday d ON gettimediff(c.createtime) = gettimediff(d.mm::timestamp)
    WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
    GROUP BY c.userid,c.type_name,c.type_code;
WARNING:  unused hint: HashJoin(c d)
                                                                               QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                          operation                          | E-rows | E-width | E-costs
 ----+-------------------------------------------------------------+--------+---------+---------
   1 | ->  HashAggregate                                           |      5 |    1502 | 3.10
   2 |    ->  Nested Loop (3,4)                                    |      5 |    1494 | 3.00
   3 |       ->  Data Node Scan on workitem "_REMOTE_TABLE_QUERY_" |      5 |    1502 | 0.00
   4 |       ->  Data Node Scan on workday "_REMOTE_TABLE_QUERY_"  |     20 |       8 | 0.00

                                                              Predicate Information (identified by plan id)
 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------
   2 --Nested Loop (3,4)
         Join Filter: ((c.createtime - (timeofday())::timestamp without time zone) = ((d.mm)::timestamp without time zone - (timeofday())::timestamp without time zone))
(11 rows)

检查关联条件中两表字段是否在等号两侧，若不是则进行调整。

postgres=# explain
SELECT /*+ hashjoin(c d) */
    c.userid,c.type_name,c.type_code,count(1) num
FROM workitem c INNER JOIN workday d ON ifnull(c.createtime,d.mm) = now()
    WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
    GROUP BY c.userid,c.type_name,c.type_code;
WARNING:  unused hint: HashJoin(c d)
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                      operation                       | E-rows | E-memory | E-width | E-costs
 ----+------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                      |      1 |          |    1502 | 35.37
   2 |    ->  Vector Sonic Hash Aggregate                   |      1 |          |    1502 | 35.37
   3 |       ->  Vector Streaming (type: GATHER)            |      2 |          |    1502 | 35.37
   4 |          ->  Vector Sonic Hash Aggregate             |      2 | 16MB     |    1502 | 29.37
   5 |             ->  Vector Nest Loop (6,8)               |      2 | 1MB      |    1494 | 29.35
   6 |                ->  Vector Streaming(type: BROADCAST) |     40 | 2MB      |       8 | 15.06
   7 |                   ->  CStore Scan on workday d       |     20 | 1MB      |       8 | 13.01
   8 |                ->  Vector Materialize                |      5 | 16MB     |    1502 | 13.09
   9 |                   ->  CStore Scan on workitem c      |      5 | 1MB      |    1502 | 13.08

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Nest Loop (6,8)
         Join Filter: (COALESCE((c.createtime)::timestamp with time zone, d.mm) = now())
   9 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5275KB
(25 rows)

检查关联条件是否为等值关联，若不是则进行调整。

postgres=# explain
SELECT /*+ hashjoin(c d) */
    c.userid,c.type_name,c.type_code,count(1) num
FROM workitem c INNER JOIN workday d ON c.createtime::timestamptz > d.mm
    WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
    GROUP BY c.userid,c.type_name,c.type_code;
WARNING:  unused hint: HashJoin(c d)
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                      operation                       | E-rows | E-memory | E-width | E-costs
 ----+------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                      |      5 |          |    1502 | 35.41
   2 |    ->  Vector Sonic Hash Aggregate                   |      5 |          |    1502 | 35.41
   3 |       ->  Vector Streaming (type: GATHER)            |     10 |          |    1502 | 35.41
   4 |          ->  Vector Sonic Hash Aggregate             |     10 | 16MB     |    1502 | 29.41
   5 |             ->  Vector Nest Loop (6,8)               |     33 | 1MB      |    1494 | 29.20
   6 |                ->  Vector Streaming(type: BROADCAST) |     40 | 2MB      |       8 | 15.06
   7 |                   ->  CStore Scan on workday d       |     20 | 1MB      |       8 | 13.01
   8 |                ->  Vector Materialize                |      5 | 16MB     |    1502 | 13.09
   9 |                   ->  CStore Scan on workitem c      |      5 | 1MB      |    1502 | 13.08

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Nest Loop (6,8)
         Join Filter: ((c.createtime)::timestamp with time zone > d.mm)
   9 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5281KB
(25 rows)

检查关联关系两侧的数据类型，并通过pg_operator.oprcanhash确认是否支持hash连接，若不支持则需改写为支持的操作符。

postgres=# select * from pg_operator where oprname = '=' and oprleft = 'timestamp'::regtype and oprright = 'timestamptz'::regtype;
-[ RECORD 1 ]+-------------------------
oprname      | =
oprnamespace | 11
oprowner     | 10
oprkind      | b
oprcanmerge  | t
oprcanhash   | f
oprleft      | 1114
oprright     | 1184
oprresult    | 16
oprcom       | 2542
oprnegate    | 2539
oprcode      | timestamp_eq_timestamptz
oprrest      | eqsel
oprjoin      | eqjoinsel

如果是指定join顺序的hint，如leading(c e),会存在逻辑本身冲突导致hint失败的情况。

postgres=# explain SELECT /*+ leading(c e) */
    c.userid,c.type_name,c.type_code,count(1) num
FROM workitem c LEFT JOIN workday d ON c.createtime = d.mm LEFT JOIN workday e ON d.mm = e.mm
    WHERE c.createtime >= '2023-09-01' AND c.endtime < '2023-10-01'
    GROUP BY c.userid,c.type_name,c.type_code;
WARNING:  unused hint: Leading(c e)
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  id |                        operation                        | E-rows | E-memory | E-width | E-costs
 ----+---------------------------------------------------------+--------+----------+---------+---------
   1 | ->  Row Adapter                                         |      2 |          |    1502 | 47.97
   2 |    ->  Vector Sonic Hash Aggregate                      |      2 |          |    1502 | 47.97
   3 |       ->  Vector Streaming (type: GATHER)               |      4 |          |    1502 | 47.97
   4 |          ->  Vector Sonic Hash Aggregate                |      4 | 16MB     |    1502 | 41.97
   5 |             ->  Vector Nest Loop Left Join (6, 7)       |      5 | 1MB      |    1494 | 41.93
   6 |                ->  CStore Scan on workitem c            |      5 | 1MB      |    1502 | 13.08
   7 |                ->  Vector Materialize                   |     40 | 16MB     |       8 | 28.00
   8 |                   ->  Vector Streaming(type: BROADCAST) |     40 | 2MB      |       8 | 27.90
   9 |                      ->  Vector Hash Left Join (10, 11) |     20 | 16MB     |       8 | 26.32
  10 |                         ->  CStore Scan on workday d    |     20 | 1MB      |       8 | 13.01
  11 |                         ->  CStore Scan on workday e    |     20 | 1MB      |       8 | 13.01

                                                               Predicate Information (identified by plan id)
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   5 --Vector Nest Loop Left Join (6, 7)
         Join Filter: (c.createtime = d.mm)
   6 --CStore Scan on workitem c
         Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
         Pushdown Predicate Filter: ((createtime >= '2023-09-01 00:00:00'::timestamp without time zone) AND (endtime < '2023-10-01 00:00:00'::timestamp without time zone))
   9 --Vector Hash Left Join (10, 11)
         Hash Cond: (d.mm = e.mm)

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 5274KB
(29 rows)

检查查询语句中，from表数量是否超出from_collapse_limit，以及join表数量是否超出join_collapse_limit。超出时存在一定概率使hint无法生效。

比默认值小的数值将降低规划时间，但是可能生成差的执行计划。

postgres=# show from_collapse_limit;
 from_collapse_limit
---------------------
 8
(1 row)

postgres=# show join_collapse_limit;
 join_collapse_limit
---------------------
 8
(1 row)

检查查询语句中，from表数量是否超出geqo_threshold（geqo开启的前提下），如果超出会使用基因查询优化来生成计划，存在很大概率使hint无法生效。

对于简单的查询，通常用详尽搜索方法，当涉及多个表的查询的时候，用GEQO可以更好的管理查询。

postgres=# show geqo_threshold;
 geqo_threshold
----------------
 12
(1 row)

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原文链接：https://my.oschina.net/u/4526289/blog/10322251

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