在具有相同数据库的类似Amazon RDS PostgreSQL服务器9.6.11版本上,我为一个SQL查询获得了不同的执行计划。
我试图重新创建索引并运行ANALYZE
和VACUUM
。没有任何帮助。
我的查询:
SELECT "users_employee"."id",
(
SELECT U0."created"
FROM "surveys_surveyrequest" U0
WHERE (U0."confirmed" IS NULL
AND U0."skipped" IS NULL
AND U0."from_member_id" = ("users_employee"."id"))
ORDER BY U0."created" ASC
LIMIT 1) AS "earliest_request_date"
FROM "users_employee"
ORDER BY "users_employee"."id" ASC;
问题表信息:
create table surveys_surveyrequest
(
id integer default nextval('public.surveys_surveyrequest_id_seq'::regclass) not null
constraint surveys_surveyrequest_pkey
primary key,
created timestamp with time zone not null,
skipped timestamp with time zone,
from_member_id integer
constraint surveys_surveyreques_from_member_id_81f0e82e_fk_users_emp
references users_employee
deferrable initially deferred,
confirmed timestamp with time zone
);
create index surveys_sur_confirm_48bfa6_idx
on surveys_surveyrequest (confirmed);
create index surveys_sur_created_099976_idx
on surveys_surveyrequest (created);
create index surveys_surveyrequest_70b76ad7
on surveys_surveyrequest (from_member_id);
计划: 答:
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using auth_user_pkey on users_employee (cost=0.28..991903.69 rows=1478 width=12) (actual time=139.054..195486.465 rows=1478 loops=1)
Heap Fetches: 51
Buffers: shared hit=296637323
SubPlan 1
-> Limit (cost=0.42..671.07 rows=1 width=8) (actual time=132.258..132.259 rows=1 loops=1478)
Buffers: shared hit=296637288
-> Index Scan using surveys_sur_created_099976_idx on surveys_surveyrequest u0 (cost=0.42..24143.63 rows=36 width=8) (actual time=132.256..132.256 rows=1 loops=1478)
Filter: ((confirmed IS NULL) AND (skipped IS NULL) AND (from_member_id = users_employee.id))
Rows Removed by Filter: 405780
Buffers: shared hit=296637288
Planning time: 0.188 ms
Execution time: 195487.356 ms
(12 rows)
B:
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using auth_user_pkey on users_employee (cost=0.28..886476.74 rows=1578 width=12) (actual time=0.977..1043.414 rows=1578 loops=1)
Heap Fetches: 0
Buffers: shared hit=98270 read=8
SubPlan 1
-> Limit (cost=561.74..561.74 rows=1 width=8) (actual time=0.658..0.659 rows=1 loops=1578)
Buffers: shared hit=98266 read=5
-> Sort (cost=561.74..561.79 rows=22 width=8) (actual time=0.658..0.658 rows=1 loops=1578)
Sort Key: u0.created
Sort Method: quicksort Memory: 25kB
Buffers: shared hit=98266 read=5
-> Bitmap Heap Scan on surveys_surveyrequest u0 (cost=474.19..561.63 rows=22 width=8) (actual time=0.646..0.652 rows=13 loops=1578)
Recheck Cond: ((from_member_id = users_employee.id) AND (confirmed IS NULL))
Filter: (skipped IS NULL)
Rows Removed by Filter: 3
Heap Blocks: exact=9707
Buffers: shared hit=98266 read=5
-> BitmapAnd (cost=474.19..474.19 rows=23 width=0) (actual time=0.641..0.641 rows=0 loops=1578)
Buffers: shared hit=88562 read=2
-> Bitmap Index Scan on surveys_surveyrequest_70b76ad7 (cost=0.00..11.29 rows=382 width=0) (actual time=0.023..0.023 rows=258 loops=1578)
Index Cond: (from_member_id = users_employee.id)
Buffers: shared hit=5847 read=2
-> Bitmap Index Scan on surveys_sur_confirm_48bfa6_idx (cost=0.00..462.64 rows=24829 width=0) (actual time=0.826..0.826 rows=24756 loops=1165)
Index Cond: (confirmed IS NULL)
Buffers: shared hit=82715
Planning time: 0.234 ms
Execution time: 1043.680 ms
(26 rows)
Time: 1044,547 ms (00:01,045)
我希望生成相同的查询计划,但是不会发生。 可能是什么原因? 如何通过计划B实现执行?
答案 0 :(得分:0)
请检查两个服务器上的配置是否相同,并且所有索引均存在并且未标记为“无效”。
假设是这种情况,不同之处可能是两个数据库中的数据分布不同。请foreach(var control in parentControl.Controls)
{
if(control is Card c)
{
c.Id = 0 /*Some new value*/
}
}
并在两个数据库上运行以下命令:
ANALYZE surveys_surveyrequest
我的猜测是,该值在A上接近1或-1,在B上接近0。
That would explain为什么PostgreSQL偏爱A上的索引扫描。计划A的问题是PostgreSQL必须扫描比其讨价还价更多的索引行,这可能是因为符合条件的行都早已存在索引。
我想说你应该简单地禁用SELECT correlation
FROM pg_stats
WHERE tablename = 'surveys_surveyrequest'
AND attname = 'created';
上的索引扫描,
要么与
surveys_sur_created_099976_idx
或(如果需要将索引用于其他目的),则使用
DROP INDEX surveys_sur_created_099976_idx;