From: | Lonni J Friedman <netllama(at)gmail(dot)com> |
---|---|
To: | Thalis Kalfigkopoulos <tkalfigo(at)gmail(dot)com> |
Cc: | "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: Quick estimate of num of rows & table size |
Date: | 2012-11-05 22:14:07 |
Message-ID: | CAP=oouHENneb0Jq2ADDmVqHGj=oKx76s3RNvkSvE7qnCW7s9ag@mail.gmail.com |
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Lists: | pgsql-general |
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.
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