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

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, prog.version, prog.active, prog.created_date, prog.last_modified_date, prog.created_by, prog.last_modified_by, prog.client_id, prog.scheme_id, prog.name, prog.legacy_group_id, prog.custom_fields, prog.setup_complete, prog.setup_messages, prog.legacy_program_type
> Index Cond: (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, prog.version, prog.active, prog.created_date, prog.last_modified_date, prog.created_by, prog.last_modified_by, prog.client_id, prog.scheme_id, prog.name, prog.legacy_group_id, prog.custom_fields, prog.setup_complete, prog.setup_messages, prog.legacy_program_type
> Index Cond: (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