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

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.

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.

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.
--
Michael