Re: Quick estimate of num of rows & table size

On Mon, Nov 5, 2012 at 2:02 PM, Thalis Kalfigkopoulos
<tkalfigo@gmail.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.