Re: Quick estimate of num of rows & table size

On Mon, Nov 5, 2012 at 9:04 PM, Lonni J Friedman <netllama(at)gmail(dot)com> wrote:

> On Mon, Nov 5, 2012 at 3:56 PM, Thalis Kalfigkopoulos
> <tkalfigo(at)gmail(dot)com> wrote:
> >
> > 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?
>
> doh, sorry. The first/larger doesn't work. As it turns out the 2nd
> actually does work well. I'm on 9.1.x.
>

That's weird. I'm on a 9.1.5 as well and it works fine (!?)
Still the question remains: are they equivalent?