Re: Preserve index stats during ALTER TABLE ... TYPE ...

Hi,

On Mon, Oct 20, 2025 at 10:53:37AM +0900, Michael Paquier wrote:
> On Thu, Oct 16, 2025 at 03:39:59PM -0500, Sami Imseih wrote:
> > This sounds like a good enhancement. This will also take care of the
> > index stats being preserved on a table in the case an index is dropped.
> >
> > But that means we will need some new fields to aggregate index access
> > in PgStat_StatTabEntry, which may not be so good in
> > terms of memory and performance.
>
> Putting aside the should-we-preserve-index-stats-on-relation-rewrite
> problem for a minute.

Okay.

> FWIW, I think that aiming at less memory per entry is better in the
> long term, because we are that it's going to be cheaper. One thing
> that's been itching me quite a bit with pgstat_relation.c lately is
> that PgStat_StatTabEntry is being used by both tables and indexes, but
> we don't care about the most of its fields for indexes. The ones I
> can see as used for indexes are:
> - blocks_hit
> - blocks_fetched
> - reset_time
> - tuples_returned
> - tuples_fetched
> - lastscan
> - numscan
>
> This means that we don't care about the business around HOT, vacuum
> (we could care about the vacuum timings for individual index
> cleanups), analyze, live/dead tuples.

Exactly, and that's one of the reasons why the "Split index and table statistics
into different types of stats" work ([1]) started.

> It may be time to do a clean split, even if the current state of
> business in pgstat.h is a kind of historical thing.

Yeah, but maybe it would make more sense to look at this once the relfilenode
stats one ([2]) is done? (see [3]).

[1]: https://www.postgresql.org/message-id/f572abe7-a1bb-e13b-48c7-2ca150546822%40gmail.com
[2]: https://www.postgresql.org/message-id/flat/ZlGYokUIlERemvpB(at)ip-10-97-1-34(dot)eu-west-3(dot)compute(dot)internal
[3]: https://www.postgresql.org/message-id/20230105002733.ealhzubjaiqis6ua%40awork3.anarazel.de

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com