Thread: a lot of shared buffers hit when planning for a simple query with primary access path
a lot of shared buffers hit when planning for a simple query with primary access path
From
James Pang
Date:
Hi,
Query Identifier: -1350604566224020319
Planning:
Buffers: shared hit=110246 <<< here planning need access a lot of buffers
Planning Time: 81.850 ms
Execution Time: 0.034 ms
a simple SQL "select ... from tablex where id1=34215670 and id2=59403938282;
id1 and i2 are bigint and primary key.
Index Cond: ((tablex.id2 = ' 5940393828299'::bigint) AND (tablex.id1 = ' 34215670 '::bigint))
Buffers: shared hit=2Query Identifier: -1350604566224020319
Planning:
Buffers: shared hit=110246 <<< here planning need access a lot of buffers
Planning Time: 81.850 ms
Execution Time: 0.034 ms
could you help why planning need a lot of shared buffers access ? this table has 4 indexes. and I tested similar SQL with another table has 4 compound indexes and that table only show very small shared buffers hit when planning.
this table has a lot of "update" and "delete" .
Thanks,
James
Re: a lot of shared buffers hit when planning for a simple query with primary access path
From
David Rowley
Date:
On Mon, 1 Jul 2024 at 21:45, James Pang <jamespang886@gmail.com> wrote: > Buffers: shared hit=110246 <<< here planning need access a lot of buffers > Planning Time: 81.850 ms > Execution Time: 0.034 ms > > could you help why planning need a lot of shared buffers access ? Perhaps you have lots of bloat in your system catalogue tables. That could happen if you make heavy use of temporary tables. There are many other reasons too. It's maybe worth doing some vacuum work on the catalogue tables. David
Re: a lot of shared buffers hit when planning for a simple query with primary access path
From
Pavel Stehule
Date:
Hi
po 1. 7. 2024 v 12:10 odesílatel David Rowley <dgrowleyml@gmail.com> napsal:
On Mon, 1 Jul 2024 at 21:45, James Pang <jamespang886@gmail.com> wrote:
> Buffers: shared hit=110246 <<< here planning need access a lot of buffers
> Planning Time: 81.850 ms
> Execution Time: 0.034 ms
>
> could you help why planning need a lot of shared buffers access ?
Perhaps you have lots of bloat in your system catalogue tables. That
could happen if you make heavy use of temporary tables. There are many
other reasons too. It's maybe worth doing some vacuum work on the
catalogue tables.
The planners get min/max range from indexes. So some user's indexes can be bloated too with similar effect
Regards
Pavel
David
Re: a lot of shared buffers hit when planning for a simple query with primary access path
From
David Rowley
Date:
On Mon, 1 Jul 2024 at 22:20, Pavel Stehule <pavel.stehule@gmail.com> wrote: > The planners get min/max range from indexes. So some user's indexes can be bloated too with similar effect I considered that, but it doesn't apply to this query as there are no range quals. David
Re: a lot of shared buffers hit when planning for a simple query with primary access path
From
James Pang
Date:
we have a daily job to do vacuumdb including catalog tables, and in same database , I did similar query with where=pk on another table and shared buffer access is very small, if catalog table bloat, should see similar shared buffer access when planning for other tables ,right? How to get more details about this planning ?
relname | last_vacuum | last_analyze
-------------------------+-------------------------------+-------------------------------
pg_statistic | 2024-06-30 01:13:08.703291+00 |
pg_attribute | 2024-06-30 01:14:48.061235+00 | 2024-07-01 01:11:49.377759+00
pg_class | 2024-06-30 01:15:09.984027+00 | 2024-07-01 01:12:05.160881+00
pg_type | 2024-06-30 01:15:11.139648+00 | 2024-07-01 01:12:05.32726+00
...
(62 rows)
-------------------------+-------------------------------+-------------------------------
pg_statistic | 2024-06-30 01:13:08.703291+00 |
pg_attribute | 2024-06-30 01:14:48.061235+00 | 2024-07-01 01:11:49.377759+00
pg_class | 2024-06-30 01:15:09.984027+00 | 2024-07-01 01:12:05.160881+00
pg_type | 2024-06-30 01:15:11.139648+00 | 2024-07-01 01:12:05.32726+00
...
(62 rows)
David Rowley <dgrowleyml@gmail.com> 於 2024年7月1日週一 下午6:52寫道：
On Mon, 1 Jul 2024 at 22:20, Pavel Stehule <pavel.stehule@gmail.com> wrote:
> The planners get min/max range from indexes. So some user's indexes can be bloated too with similar effect
I considered that, but it doesn't apply to this query as there are no
range quals.
David
Re: a lot of shared buffers hit when planning for a simple query with primary access path
From
Andrei Lepikhov
Date:
On 1/7/2024 17:58, James Pang wrote: > we have a daily job to do vacuumdb including catalog tables, and > in same database , I did similar query with where=pk on another table > and shared buffer access is very small, if catalog table bloat, should > see similar shared buffer access when planning for other tables ,right? > How to get more details about this planning ? > > relname | last_vacuum | > last_analyze > -------------------------+-------------------------------+------------------------------- > pg_statistic | 2024-06-30 01:13:08.703291+00 | > pg_attribute | 2024-06-30 01:14:48.061235+00 | 2024-07-01 > 01:11:49.377759+00 > pg_class | 2024-06-30 01:15:09.984027+00 | 2024-07-01 > 01:12:05.160881+00 > pg_type | 2024-06-30 01:15:11.139648+00 | 2024-07-01 > 01:12:05.32726+00 > ... > (62 rows) > > David Rowley <dgrowleyml@gmail.com <mailto:dgrowleyml@gmail.com>> 於 > 2024年7月1日週一 下午6:52寫道： > > On Mon, 1 Jul 2024 at 22:20, Pavel Stehule <pavel.stehule@gmail.com > <mailto:pavel.stehule@gmail.com>> wrote: > > The planners get min/max range from indexes. So some user's > indexes can be bloated too with similar effect > > I considered that, but it doesn't apply to this query as there are no > range quals. > > David > Don't forget about extended statistics as well - it also could be used. -- regards, Andrei Lepikhov