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=1loops=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=1loops=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