Re: pg_buffercache query example results misleading, grouping by just relname, needs schema_name

On Wed, Feb 12, 2020 at 11:55:51PM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/12/pgbuffercache.html
> Description:
>
> The pg_buffercache query example results are misleading. The "group by" uses
> just by relname. It needs to include pg_namespace.nspname, without it, if
> the same object exists in multiple schemas, the buffer count is summed for
> those multiple distinct objects.
> In: https://www.postgresql.org/docs/12/pgbuffercache.html
> Alternative SQL (the count is now correct for tables in multiple schemas):
> SELECT ts.nspname AS schema_name,c.relname, count(*) AS buffers
> FROM pg_buffercache b INNER JOIN pg_class c
> ON b.relfilenode = pg_relation_filenode(c.oid) AND
> b.reldatabase IN (0, (SELECT oid FROM pg_database
> WHERE datname = current_database()))
> JOIN pg_namespace ts ON ts.oid = c.relnamespace
> GROUP BY ts.nspname,c.relname
> ORDER BY buffers DESC
> LIMIT 10;
>
> Example Results:
> Current Query returns 1 row with buffer count summed for 3 tables:
> relname buffers
> tab1 72401
>
> Modified Query:
> schema_name relname buffers
> schema1 tab1 1883
> schema2 tab1 69961
> schema3 tab1 557

Very good point! Patch attached.

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Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

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