Re: monitoring-stats.html documentation

Robert Haas wrote:
> On Sat, Apr 4, 2009 at 6:08 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> >> http://developer.postgresql.org/pgdocs/postgres/monitoring-stats.html
> >> says: "Note: blocks_fetched minus blocks_hit gives the number of
> >> kernel read() calls issued for the table, index, or database; but the
> >> actual number of physical reads is usually lower due to kernel-level
> >> buffering." ?This seems to imply that anything that increases
> >> blocks_hit should also increase blocks_fetched, but that doesn't seem
> >> to match the actual behavior.
> >
> >> rhaas=# select heap_blks_read, heap_blks_hit from pg_statio_user_tables;
> >
> > It's talking about the underlying pg_stat_get_db_blocks_fetched()
> > function, not heap_blks_read which is just a view field defined as
> >
> > ? ? ? ? ? ?pg_stat_get_blocks_fetched(C.oid) -
> > ? ? ? ? ? ? ? ? ? ?pg_stat_get_blocks_hit(C.oid) AS heap_blks_read,
> >
> > Probably that sentence ought to spell out the full function name
> > instead of abbreviating.
>
> Oh, I see. I misread it, but I agree it could be written in a way
> that would make it less likely to be misread. You could even add a
> sentence explicitly mentioning that the views display the difference
> of the two values.

Done with attached patch; good idea.

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

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