Re: monitoring-stats.html documentation

On Sat, Apr 4, 2009 at 6:08 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.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.

...Robert