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.janes@xxxxxxxxx> 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
