你的JoinHint为什么不生效
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)
你的JoinHint为什么不生效的更多相关文章
- excel 日期/数字格式不生效需要但双击才会生效的解决办法
原因: Excel2007设置过单元格格式后,并不能立即生效必须挨个双击单元格,才能生效.数据行很多.效率太低. 原因:主要是一些从网上拷贝过来的日期或数字excel默认为文本格式或特殊-中文数字格式 ...
- Git忽略.gitignore规则不生效的解决办法
在git中如果想忽略掉某个文件,不让这个文件提交到版本库中,可以使用修改根目录中 .gitignore 文件的方法(如无,则需自己手工建立此文件). 这个文件每一行保存了一个匹配的规则例如: # 此为 ...
- supervisor监管进程max file descriptor配置不生效的问题
配置了 sudo vim /etc/security/limits.conf * soft nofile * hard nofile 单独起进程没问题, 放到supervisor下监管启动,则报错 ...
- 做web开发和测试，修改hosts指定某个域名访问某个特定的IP后，如何使hosts立即生效的方法
本文转自SUN'S BLOG,原文地址:http://whosmall.com/post/143 hosts的配置方法: 在windows系统中,找到C:\windows\system32\drive ...
- 嵌入式 Linux下永久生效环境变量bashrc
嵌入式 Linux下永久生效环境变量bashrc 1) .bashrc文件 在linux系统普通用户目录(cd /home/xxx)或root用户目录(cd /root)下,用指令ls -al可以看到 ...
- Apache commons-configuration setDelimiterParsingDisable不生效的处理
Apache commons-configuration setDelimiterParsingDisable不生效的处理 项目中有用到commons-configuration,版本1.9. 配置初 ...
- jquery动态添加的html，第三方插件无法生效的情况
今天一个问题纠结了半天,问题如下图 问题大致就是如上,新增的内容死活点不起,插件没有生效,在一个装逼前端群里面问,给我的答案是叫我去了解事件委托,了解一下事件冒泡!! 好吧,我一上午加半个下午的时间 ...
- TextView属性android:ellipsize="marquee"不生效的解决办法
最近自己在写自己的第一个app,过程中遇到了这个问题,查了不少帖子,经过尝试发现,这种问题一般分为两类: 1. TextView的Text值赋值后不更改,很多帖子上说如下写法就可以生效: <Te ...
- MVC项目中ExecutionTimeout不生效的解决方案
我们做web服务器端开发时,经常会遇到一个需求场景,因为某些耗时处理造成页面的响应处理时间超长,技术角度就想能否给页面处理程序一个指定的超时时间,服务端处理程序执行时间超过这个指定的超时时间则中断处理 ...
- 让/etc/profile文件修改后立即生效
方法1： 让/etc/profile文件修改后立即生效 ,可以使用如下命令: # . /etc/profile 注意: . 和 /etc/profile 有空格 方法2： 让/etc/profile ...
随机推荐
- CUDA C编程权威指南：2.2-给核函数计时
本文主要通过例子介绍了如何给核函数计时的思路和实现.实现例子代码参考文献[7],只需要把相应章节对应的CMakeLists.txt文件拷贝到CMake项目根目录下面即可运行. 1.用CPU计时器计 ...
- Python 利用pandas 和 matplotlib绘制柱状图
当你需要展示数据时,图表是一个非常有用的工具.Python 中的 pandas 和 matplotlib 库提供了丰富的功能,可以帮助你轻松地绘制各种类型的图表.本文将介绍如何使用这两个库,绘制一个店 ...
- 【Vue3响应式原理#02】Proxy and Reflect
专栏分享:vue2源码专栏,vue3源码专栏,vue router源码专栏,玩具项目专栏,硬核推荐 欢迎各位ITer关注点赞收藏 背景 以下是柏成根据Vue3官方课程整理的响应式书面文档 - 第二节, ...
- OpenGL 基础光照详解
1. 光照 显示世界中,光照环境往往是相对复杂的.因为假设太阳作为世界的唯一光源,那么太阳光照在物体A上A将阳光进行反射后,A又做为一个新的光源共同作用于另一个物体B.所以于B来讲光源是复杂的.然而这 ...
- Java 面试题之 Logback 打印日志是如何获取当前方法名称的？
在 Java 中,有四种方法可以获取当前正在执行方法体的方法名称,分别是: 使用 Thread.currentThread().getStackTrace() 方法 使用异常对象的 getStackT ...
- 鸿蒙开发学习（一）之ArkTS
目录 TypeScript语法 基础 module ArkTS 基本UI描述 基本概念 状态管理 页面级变量的状态管理 @State @Prop @Link 应用级变量的状态管理 开发入门 应用模型 ...
- Codeforces Round #538 (Div. 2) F. Please, another Queries on Array?
原题链接 F. Please, another Queries on Array? 这道题让求\(\phi(\prod\limits_{i = l}^r a_i)\),然后我们化简一下. 设\(P\) ...
- python之封装及私有方法
目录 封装 简洁 私有方法 封装:提高程序的安全性 将属性和方法包装到类对象中,在方法内部对属性进行操作,在类对象外部调用方法,使得程序更加简洁 在python中,如果该属性不希望在类对象外部被访问, ...
- JAVA学习9/29
1.继承extends//关键字 1.1.测试:子类继承父类后,能使用子类对象调用父类方法吗? 可以,因为子类继承父类后,这个该方法就属于子类了. 当然可以使用子类对象来调用 1.2.在实际开发中,满 ...
- STM32外设：信号转换器 ADC、DAC
主要外设: ADC:Analog to Digital Converter 模数转换器 DAC:Digital to Analog Converter 数模转换器 ADC_IN` 主要功能:测外部引脚 ...