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

On Thu, Oct 16, 2025 at 01:38:19AM -0400, Tom Lane wrote:
> Michael Paquier <michael@paquier.xyz> writes:
>> Hmm.  Why should it be always OK to preserve the stats of an index
>> when one of its attributes is changed so as a relation is rewritten?
>
> Right offhand, this proposal seems utterly unsafe, to the point of
> maybe introducing security-grade bugs.  I see that the patch compares
> opfamilies but that seems insufficient, since "same opfamily" does not
> mean "binary compatible".  We could easily be restoring stats whose
> binary content is incompatible with the new column type.

The point of the thread is about copying the aggregated numbers stored
in pgstats.  These numbers have a fixed size, for contents in
PgStat_StatTabEntry.  The point of the patch is about copying these
entries in the pgstats hash table across rewrites, so I am not sure to
follow your argument.

My point was slightly different: I am questioning if a reset does not
make more sense in most cases as an attribute type change may cause
the planner to choose a different Path, making the new stats generated
leading to decisions that are inconsistent when aggregated with the
numbers copied across the rewrites.
--
Michael