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

>> Hence, the planner may decide
>> to treat a given index differently (doesn't it?  Tuple width or
>> whole-row references come into mind).

> I do think so, yes.

The planner may also treat the index differently after
ALTER INDEX ... ALTER COLUMN ... SET STATISTICS ...; ANALYZE,
but in

I am not sure the planner aspect is a good reason to not preserve
cumulative stats for an index.

In the case where the table is not rewritten, Isn't that a clear case in
which stats should be preserved?

> > Keeping the past stats may
> > actually lead to confusing conclusions when overlapping them with some
> > of the new number generated under the new type?  Could there be more
> > benefits in always resetting them as we do now?
>
> The issue is that these stats are also exposed at the table level
> (idx_scan, last_idx_scan, idx_tup_fetch in pg_stat_all_tables).
> That's valuable information for understanding table access patterns
> that is currently lost.
>
> It would make more sense to reset the index stats if table level
> stats were tracked independently from the underlying index stats.

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.

--
Sami