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

Hi Mark,

It's look very weird.
Can you try something like this (check that you have enough disk space for
second copy of common_student before):

create table test_table AS SELECT * from common_student;
Vacuum analyze test_table;
explain analyze select * from test_table where school_id = 36;
drop table test_table;
create table test_table AS SELECT * from common_student ORDER BY school_id;
Vacuum analyze test_table;
explain analyze select * from test_table where school_id = 36;
drop table test_table;

And provide results of both explain analyze queries.

On Sat, Dec 22, 2018 at 3:39 AM Mark <mwchambers(at)gmail(dot)com> wrote:

> Hi All,
>
> I'm seeing some very slow queries and it looks like the query planner is
> deciding to do a 'Nested Loop Left Join'
> which is slow. When I SET enable_nestloop=OFF for testing it does a 'Hash
> Left Join' which is much faster.
>
> I think I can see the cause of the problem in the examples below. I can't
> make sense of the statistics being written
> by ANALYSE or the planners row estimates.
>
> Can anyone help me understand....
>
> - Why the row estimate I get in the query below for school_id = 36 is 1 ?
> ( I expect it to be higher)
> - Why does '1' appear in the most_common_vals when it is actually the
> least common value.
> - Why doesn't 36 appear in the most_common_vals (it is more common than 1)
> - 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 )
>
> Any help with understanding what's happening here would be much
> appreciated.
>
> I hope I've provided enough information below.
>
> Thanks,
>
> Mark
>
> db=> explain analyse select * from common_student where school_id = 36 ;
> QUERY
> PLAN
>
> --------------------------------------------------------------------------------------------------------------------------------------------
> Index Scan using idx_common_student_sid on common_student
> (cost=0.41..8.39 rows=1 width=385) (actual time=0.264..1.691 rows=1388
> loops=1)
> Index Cond: (school_id = 36)
> Planning time: 0.087 ms
> Execution time: 2.706 ms
> (4 rows)
>
> db=> select tablename,attname,most_common_vals,histogram_bounds,n_distinct
> from pg_stats where attname='school_id' and tablename='common_stude
> nt';
> tablename | attname |
> most_common_vals
> | histogram_bounds | n_distinct
>
> ----------------+-----------+----------------------------------------------------------------------------------------------------------------------------------------+------------------+------------
> common_student | school_id |
> {79,60,25,61,59,69,86,77,64,33,56,78,58,81,41,97,22,67,38,23,3,72,92,93,48,24,96,26,75,90,70,52,51,21,14,91,83,54,85,11,68,94,53,88,1}
> | | 45
> (1 row)
>
> db=> select count(distinct(school_id)) from common_student ;
> count
> -------
> 55
> (1 row)
>
> db=> alter table common_student alter column school_id set statistics
> 100000;
> WARNING: lowering statistics target to 10000
> ALTER TABLE
> db=> analyse verbose common_student(school_id);
> INFO: analyzing "public.common_student"
> INFO: "common_student": scanned 7322 of 7322 pages, containing 65463 live
> rows and 49026 dead rows; 51538 rows in sample, 65463 estimated total rows
> ANALYZE
> db=> select tablename,attname,most_common_vals,histogram_bounds,n_distinct
> from pg_stats where attname='school_id' and tablename='common_stude
> nt';
> tablename | attname |
> most_common_vals
> | histogram_bounds | n_distinct
>
> ----------------+-----------+----------------------------------------------------------------------------------------------------------------------------------------+------------------+------------
> common_student | school_id |
> {79,60,25,61,59,69,86,77,64,33,56,78,58,81,41,97,22,67,38,23,3,72,92,93,48,24,96,26,75,90,70,52,51,21,14,91,83,54,85,11,68,94,53,88,1}
> | | 45
> (1 row)
>
> db=> explain analyse select * from common_student where school_id = 36 ;
> QUERY
> PLAN
>
> --------------------------------------------------------------------------------------------------------------------------------------------
> Index Scan using idx_common_student_sid on common_student
> (cost=0.41..8.39 rows=1 width=385) (actual time=0.542..4.022 rows=1388
> loops=1)
> Index Cond: (school_id = 36)
> Planning time: 0.334 ms
> Execution time: 6.542 ms
> (4 rows)
>
> db=> select school_id, count(*) from common_student group by school_id
> order by count(*) limit 6 ;
> school_id | count
> -----------+-------
> 1 | 50
> 88 | 161
> 53 | 252
> 94 | 422
> 31 | 434
> 68 | 454
> (6 rows)
>
> dvpjxbzc=> select school_id, count(*) from common_student where school_id
> = 36 group by school_id ;
> school_id | count
> -----------+-------
> 36 | 1388
> (1 row)
>
> db=> explain analyse select * from common_student where school_id = 1 ;
> QUERY PLAN
>
>
> ----------------------------------------------------------------------------------------------------------------------------------
> Bitmap Heap Scan on common_student (cost=4.91..243.76 rows=64 width=385)
> (actual time=0.128..0.267 rows=50 loops=1)
> Recheck Cond: (school_id = 1)
> Heap Blocks: exact=16
> -> Bitmap Index Scan on "unique common_student" (cost=0.00..4.90
> rows=64 width=0) (actual time=0.110..0.110 rows=50 loops=1)
> Index Cond: (school_id = 1)
> Planning time: 0.177 ms
> Execution time: 0.414 ms
> (7 rows)
>
> db=> select VERSION();
> version
>
>
> -----------------------------------------------------------------------------
> PostgreSQL 10.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.2,
> 64-bit
> (1 row)
>
>

--
Maxim Boguk
Senior Postgresql DBA
https://dataegret.com/

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когда я так делаю ещё раз?"