Re: Significant Execution Time Difference Between PG13.14 and PG16.4 for Query on information_schema Tables.

On 8/26/24 15:41, nikhil raj wrote:
> Hi Adrian,
>
> Thanks for the quick response.
>
> I've already performed a vacuum, reindex, and analyze on the entire
> database, but the issue persists. As you can see from the execution
> plan, the time difference in PostgreSQL 16 is still significantly
> higher, even after all maintenance activities have been completed.
>
> It seems there might be a bug in PostgreSQL 16 where the performance of
> queries on *information_schema* tables is degraded. As both the tables
> are postgres system tables
>
> https://explain.depesz.com/s/bdO6b <https://explain.depesz.com/s/bdO6b>
> :-PG13 <https://explain.depesz.com/s/bdO6b>
>
> https://explain.depesz.com/s/bpAU <https://explain.depesz.com/s/bpAU>
> :- PG16 <https://explain.depesz.com/s/bpAU>

What I see is Postgres 13:

Nested Loop (cost=9.54..119.02 rows=1 width=128) (actual
time=1.038..288.777 rows=1 loops=1)

Join Filter: (("*SELECT* 1".constraint_name)::name = "*SELECT*
1_1".conname)
Rows Removed by Join Filter: 935
Buffers: shared hit=34,675

vs Postgres 16

Nested Loop (cost=62.84..538.22 rows=1 width=128) (actual
time=1,905.153..14,006.921 rows=1 loops=1)

Join Filter: ("*SELECT* 1".conname = ("*SELECT*
1_1".constraint_name)::name)
Rows Removed by Join Filter: 997
Buffers: shared hit=5,153,054

So either switching this

("*SELECT* 1".constraint_name)::name = "*SELECT* 1_1".conname

to

"*SELECT* 1".conname = ("*SELECT* 1_1".constraint_name)::name

is more of a change then I would expect.

Or

Buffers: shared hit=34,675

vs

Buffers: shared hit=5,153,054

indicates a hardware/configuration difference.

Are both instances running on the same machine?

Is the configuration for both the same?

>
> On Tue 27 Aug, 2024, 3:40 AM Adrian Klaver, <adrian(dot)klaver(at)aklaver(dot)com
> <mailto:adrian(dot)klaver(at)aklaver(dot)com>> wrote:
>
> On 8/26/24 14:49, nikhil raj wrote:
> > Hi All,
> >
> > I've encountered a noticeable difference in execution time and query
> > execution plan row counts between PostgreSQL 13 and PostgreSQL 16
> when
> > running a query on |information_schema| tables. Surprisingly,
> PostgreSQL
> > 16 is performing slower than PostgreSQL 13.
>
> Did you run ANALYZE on the Postgres 16 instance?
>
> > *4PostgreSQL 13.14 (PostgreSQL 13.14 on x86_64-pc-linux-gnu,
> compiled by
> > gcc 11.4.0, 64-bit)*
> > Execution plan: PG13.14 Execution Plan
> > <https://explain.dalibo.com/plan/ag1a62a9d47dg29d
> <https://explain.dalibo.com/plan/ag1a62a9d47dg29d>>
> >
> > *PostgreSQL 16.4 (PostgreSQL 16.4 on x86_64-pc-linux-gnu,
> compiled by
> > gcc 11.4.0, 64-bit)*
> > Execution plan: PG16.4 Execution Plan
> > <https://explain.dalibo.com/plan/4c66fdfbf2hf9ed2
> <https://explain.dalibo.com/plan/4c66fdfbf2hf9ed2>>
>
>
> Use:
>
> https://explain.depesz.com/ <https://explain.depesz.com/>
>
> It is easier to follow it's output.
>
> >
> >
> > Has anyone else experienced similar behavior or could provide
> insights
> > into why PostgreSQL 16 might be slower for this query? Any advice or
> > suggestions for optimization would be greatly appreciated.
>
> Yes when ANALYZE was not run on a new instance.
>
> >
> > Thank you!
> >
> > NOTE:-  PFA the raw file of explain and analyze below.
> >
> >
> >
> >
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com