From: Tom Lane <tgl@sss.pgh.pa.us>
To: Achilleas Mantzios - cloud <a.mantzios@cloud.gatewaynet.com>
cc: pgsql-performance@lists.postgresql.org
Subject: Re: pgsql 10.23 , different systems, same table , same plan,
 different Buffers: shared hit
In-reply-to: <24a36ca8-b5c0-d39f-c5f6-47cf2ef51abd@cloud.gatewaynet.com>
References: <24a36ca8-b5c0-d39f-c5f6-47cf2ef51abd@cloud.gatewaynet.com>
Comments: In-reply-to Achilleas Mantzios - cloud
 <a.mantzios@cloud.gatewaynet.com>
	message dated "Fri, 15 Sep 2023 17:30:39 +0300"
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Date: Fri, 15 Sep 2023 11:23:49 -0400
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Achilleas Mantzios - cloud <a.mantzios@cloud.gatewaynet.com> writes:
> *FreeBSD*
> 
>    ->  Index Only Scan using mail_vessel_addressbook_address_regex_idx 
> on mail_vessel_addressbook  (cost=0.42..2912.06 rows=620 width=32) 
> (actual time=96.704..96.705 rows=1 loops=1)
>          Filter: ('foo@bar.com'::text ~* address_regex)
>          Rows Removed by Filter: 14738
>          Heap Fetches: 0
>          Buffers: shared hit=71
> 
> *Linux*
> 
>    ->  Index Only Scan using mail_vessel_addressbook_address_regex_idx 
> on mail_vessel_addressbook  (cost=0.42..2913.04 rows=620 width=32) 
> (actual time=1768.724..1768.725 rows=1 loops=1)
>          Filter: ('foo@bar.com'::text ~* address_regex)
>          Rows Removed by Filter: 97781
>          Heap Fetches: 0
>          Buffers: shared hit=530

> The file in FreeBSD came by pg_dump from the linux system, I am puzzled 
> why this huge difference in Buffers: shared hit.

The "rows removed" value is also quite a bit different, so it's not
just a matter of buffer touches --- there's evidently some real difference
in how much of the index is being scanned.  I speculate that you are
using different collations on the two systems, and FreeBSD's collation
happens to place the first matching row earlier in the index.

			regards, tom lane