Re: Foreign table scan estimates

Tom Lane wrote:
>> While playing around with ANALYZE on foreign tables, I noticed
>> that the row count estimate for foreign scans is still
>> initialized to 1000 even if there are statistics for the
>> foreign table.  I think that this should be improved.

>> The attached patch illustrates my suggestion.

> I don't think this is appropriate; it will just waste cycles because
> the FDW will have to repeat the calculations after obtaining a real
> estimate of the foreign table size.  If we trusted pg_class.reltuples
> to be up to date, there might be some value in this.  But we don't
> trust that for regular tables (cf. plancat.c), and I don't see why
> we would do so for foreign tables.
> 
> I think on the whole it's better to abdicate responsibility here and
> require the FDW to do something in its GetForeignRelSize function.
> It's not like we'd be saving the FDW a lot of code in the (unlikely)
> case that this is exactly what it would do anyway.

I agree.

Yours,
Laurenz Albe