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

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>
> 
> *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>


Use:

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.klaver@aklaver.com

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.klaver@aklaver.com 
> <mailto:adrian.klaver@aklaver.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.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
> 

-- 
Adrian Klaver
adrian.klaver@aklaver.com