Re: Query planner / Analyse statistics bad estimate rows=1 with maximum statistics 10000 on PostgreSQL 10.2

Hi Jeff,

Thanks for your help. That is exactly what is happening.

I have a long running job which deletes all of the common_student table and
then repopulates it. It takes long time to load all the other data and
commit the transaction. I didn't think the delete inside the transaction
would have any effect until it is commited or rolled back.

I will have to rewrite the application so it updates the existing rows
rather than deleting all and then inserting.

Thanks again for helping me understand what's happening here.

Proof:

db=> explain analyze select * from common_student where school_id = 36;
QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on common_student (cost=88.30..3846.49 rows=1533
width=384) (actual time=4.852..7.065 rows=1388 loops=1)
Recheck Cond: (school_id = 36)
Heap Blocks: exact=67
-> Bitmap Index Scan on idx_common_student_sid (cost=0.00..87.91
rows=1533 width=0) (actual time=4.817..4.817 rows=1388 loops=1)
Index Cond: (school_id = 36)
Planning time: 0.097 ms
Execution time: 8.084 ms
(7 rows)

db=> /* At this point I have started a long running transaction that
deletes all of common_student for school_id 36 */ ;

db=> analyse verbose common_student(school_id);
INFO: analyzing "public.common_student"
INFO: "common_student": scanned 7322 of 7322 pages, containing 65431 live
rows and 8060 dead rows; 56818 rows in sample, 65431 estimated total rows
ANALYZE
db=> explain analyze select * from common_student where school_id = 36;
QUERY
PLAN
--------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using idx_common_student_sid on common_student
(cost=0.41..8.43 rows=1 width=384) (actual time=0.017..1.498 rows=1388
loops=1)
Index Cond: (school_id = 36)
Planning time: 0.098 ms
Execution time: 2.583 ms
(4 rows)

db=> /* At this point I have killed the long running transaction that
deletes all of common_student for school_id 36 */ ;
db=> vacuum analyze common_student;
VACUUM
db=> explain analyze select * from common_student where school_id = 36;
QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on common_student (cost=79.17..3357.79 rows=1388
width=383) (actual time=0.088..1.302 rows=1388 loops=1)
Recheck Cond: (school_id = 36)
Heap Blocks: exact=67
-> Bitmap Index Scan on idx_common_student_sid (cost=0.00..78.83
rows=1388 width=0) (actual time=0.077..0.077 rows=1388 loops=1)
Index Cond: (school_id = 36)
Planning time: 0.327 ms
Execution time: 2.311 ms
(7 rows)

On Sun, 23 Dec 2018 at 02:57 Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:

>
>> - Does the analyse output below mean that it only scanned 51538 of 65463
>> rows in the table? Is school_id 36 just being missed in the sample? (This
>> happens when the analyse is repeated )
>>
>
> Is there a transaction which had deleted all of school_id=36, and then was
> just left open indefinitely without either committing or rolling back?
>
> That would explain it, and I don't know of anything else that could. The
> deleted but not committed tuples are still live, but don't get sampled.
>
> Cheers,
>
> Jeff
>