Re: Quick estimate of num of rows & table size

On Mon, Nov 5, 2012 at 7:14 PM, Lonni J Friedman <netllama(at)gmail(dot)com> wrote:

> On Mon, Nov 5, 2012 at 2:02 PM, Thalis Kalfigkopoulos
> <tkalfigo(at)gmail(dot)com> wrote:
> > Hi all,
> >
> > I read somewhere that the following query gives a quick estimate of the
> # of
> > rows in a table regardless of the table's size (which would matter in a
> > simple SELECT count(*)?):
> >
> > SELECT (CASE WHEN reltuples > 0 THEN
> > pg_relation_size('mytable')/(8192*relpages/reltuples)
> > ELSE 0
> > END)::bigint AS estimated_row_count
> > FROM pg_class
> > WHERE oid = 'mytable'::regclass;
> >
> > If relpages & reltuples are recorded accurately each time VACUUM is run,
> > wouldn't it be the same to just grab directly the value of reltuples
> like:
> >
> > SELECT reltuples FROM pg_class WHERE oid='mytable'::regclass;
> >
> > In the same manner, are pg_relation_size('mytable') and 8192*relpages the
> > same?
> >
> > I run both assumptions against a freshly VACUUMed table and they seem
> > correct.
>
> This doesn't seem to work for me. I get an estimated row_count of 0
> on a table that I know has millions of rows.
>

Which one doesn't work exactly? The larger query? Are you on a 9.x?

regards,
thalis k.