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)
