Re: dead tuple difference between pgstattuple and pg_stat_user_tables

On Fri, Aug 23, 2024 at 10:26 AM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 8/23/24 09:14, Matthew Tice wrote:
> > Hi All,
> >
> > I'm trying to understand why there's a difference between what
> > pgstattuple reports and pg_stat_user_tables reports (for the number of
> > dead tuples).
> >
> > As I understand, pgstattuple and pgstattuple_approx return the exact
> > number of dead tuples (as noted in the documentation) and based on an
>
> 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."

>
> > This is a Google Alloy DB instance running:
>
> https://cloud.google.com/alloydb/docs/overview
>
> "AlloyDB for PostgreSQL is a fully managed, PostgreSQL-compatible
> database service that's designed for your most demanding workloads,
> including hybrid transactional and analytical processing. AlloyDB pairs
> a Google-built database engine with a cloud-based, multi-node
> architecture to deliver enterprise-grade performance, reliability, and
> availability."
>
> Where the important parts are 'PostgreSQL-compatible' and 'Google-built
> database engine'. You probably need to reach out to Google to see what
> that means for this situation.
>
> Got it, thanks Adrian.

>
> > > select version();
> > -[ RECORD 1 ]-------------------------
> > version | PostgreSQL 14.10 on x86_64-pc-linux-gnu, compiled by Debian
> > clang version 12.0.1, 64-bit
> > SELECT 1
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>
>