Re: Huge shared hit for small table

The index is exceedingly simple:

CREATE UNIQUE INDEX "programPK" ON program(id int8_ops);

From pg_stat_user_indexes:

Staging:

idx_scan: 5826745
idx_tup_read: 52715470
idx_tup_fetch: 52644465

Production:

idx_scan : 7277919087
idx_tup_read: 90612605047
idx_tup_fetch: 5207807880

From: Andres Freund <andres(at)anarazel(dot)de>
Date: Monday, November 4, 2019 at 2:46 PM
To: Scott Rankin <srankin(at)motus(dot)com>
Cc: "pgsql-performance(at)lists(dot)postgresql(dot)org" <pgsql-performance(at)lists(dot)postgresql(dot)org>
Subject: Re: Huge shared hit for small table

Hi,

On 2019-11-04 19:38:40 +0000, Scott Rankin wrote:
> In the staging environment, we get this:
>
> Index Scan using "programPK" on public.program prog (cost=0.29..0.35 rows=1 width=16) (actual time=0.002..0.003 rows=1 loops=21965)
> Output: prog.id<http://prog.id>, prog.version, prog.active<http://prog.active>, prog.created_date, prog.last_modified_date, prog.created_by, prog.last_modified_by, prog.client_id, prog.scheme_id, prog.name<http://prog.name>, prog.legacy_group_id, prog.custom_fields, prog.setup_complete, prog.setup_messages, prog.legacy_program_type
> Index Cond: (prog.id<http://prog.id> = per.program)
> Buffers: shared hit=87860
>
> In the production environment, we get this:
>
> Index Scan using "programPK" on public.program prog (cost=0.29..0.36 rows=1 width=16) (actual time=0.017..4.251 rows=1 loops=21956)
> Output: prog.id<http://prog.id>, prog.version, prog.active<http://prog.active>, prog.created_date, prog.last_modified_date, prog.created_by, prog.last_modified_by, prog.client_id, prog.scheme_id, prog.name<http://prog.name>, prog.legacy_group_id, prog.custom_fields, prog.setup_complete, prog.setup_messages, prog.legacy_program_type
> Index Cond: (prog.id<http://prog.id> = per.program)
> Buffers: shared hit=25437716
>
> The tables in both environments are about the same size (18MB) and the indexes are about the same size (360kb/410kb) – and the shared hits are pretty much the same on the other nodes of the query between the two environments.

It'd be worthwhile to look at the index stats using pgstatindex. Also,
could you show the definition of those indexes?

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Greetings,

Andres Freund

This email message contains information that Motus, LLC considers confidential and/or proprietary, or may later designate as confidential and proprietary. It is intended only for use of the individual or entity named above and should not be forwarded to any other persons or entities without the express consent of Motus, LLC, nor should it be used for any purpose other than in the course of any potential or actual business relationship with Motus, LLC. If the reader of this message is not the intended recipient, or the employee or agent responsible to deliver it to the intended recipient, you are hereby notified that any dissemination, distribution, or copying of this communication is strictly prohibited. If you have received this communication in error, please notify sender immediately and destroy the original message.

Internal Revenue Service regulations require that certain types of written advice include a disclaimer. To the extent the preceding message contains advice relating to a Federal tax issue, unless expressly stated otherwise the advice is not intended or written to be used, and it cannot be used by the recipient or any other taxpayer, for the purpose of avoiding Federal tax penalties, and was not written to support the promotion or marketing of any transaction or matter discussed herein.