Re: dead tuple difference between pgstattuple and pg_stat_user_tables

On 8/23/24 09:33, Matthew Tice wrote:
>
>
> On Fri, Aug 23, 2024 at 10:26 AM Adrian Klaver
> <adrian(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>> wrote:

> https://www.postgresql.org/docs/current/pgstattuple.html
> <https://www.postgresql.org/docs/current/pgstattuple.html>
>
> pgstattuple_approx(regclass) returns record
>
>      pgstattuple_approx is a faster alternative to pgstattuple that
> returns approximate results.
>
> Not sure how you get exact count out of that?
>
>
> Maybe the wording is a little confusing to me. Under the section
> for pgstattuple_approx:
> "pgstattuple_approx tries to avoid the full-table scan and returns exact
> dead tuple statistics along with an approximation of the number and size
> of live tuples and free space."

Yeah, see what you mean.

The part that bears more investigating for this case is:

"It does this by skipping pages that have only visible tuples according
to the visibility map (if a page has the corresponding VM bit set, then
it is assumed to contain no dead tuples).

Wondering if PostgreSQl-compatible covers this?

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com