Re: Help in vetting outcome of "vacuumdb --analyze-in-stages" - during DB Upgrade from EC2- PGS - Community Edn ver 13.X to 14.X

On Sun, Feb 16, 2025 at 8:13 AM Y_Bharani_mbsv <mailbsv(at)yahoo(dot)com> wrote:

> Team
> Good Morning.
> As part of DB upgrade from EC2 - PGS - community Edn Ver 13.X to 14.X
> I followed steps of "pg_upgrade" and had executed the last step (post
> successful db migration)
>
> vacuumdb --analyze-in-stages
>
> and later noticed an caveat
> url = https://www.postgresql.org/docs/current/app-vacuumdb.html
>
>
> --analyze-in-stages
>
> Only calculate statistics for use by the optimizer (no vacuum), like
> --analyze-only. Run three stages of analyze; the first stage uses the
> lowest possible statistics target (see default_statistics_target
> <https://www.postgresql.org/docs/current/runtime-config-query.html#GUC-DEFAULT-STATISTICS-TARGET>)
> to produce usable statistics faster, and subsequent stages build the full
> statistics.
>
> This option is only useful to analyze a database that currently has no
> statistics or has wholly incorrect ones, such as if it is newly populated
> from a restored dump or by pg_upgrade. *Be aware that running with this
> option in a database with existing statistics may cause the query optimizer
> choices to become transiently worse due to the low statistics targets of
> the early stages.*
>
> How to overcome the issue to avoid "transiently worse"
>

"Transiently" means "temporarily".

And since pg_upgrade does not carry over optimizer statistics, query
optimizer choices would be transiently worse *anyway* until the ANALYZE
completes.

> Later, I too did
> a) vacuum(full,verbose,skip_locked) ... each table wise
>

Why? It certainly didn't do what you think it did.

(This is why giving "rewrite the whole table" the name VACUUM FULL was a
horrible idea.)

> b) analyze (verbose,skip_locked) .. each table wise
> Any guidance
>

You wasted much time and effort. Best to have just waited until
the --analyze-in-stages had completed.

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