Re: Foreign table scan estimates

"Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at> writes:
> 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.

A different line of thought would be to refactor the definition of
GetForeignRelSize so that it's supposed to set rel->tuples and then
after that we do the selectivity calculation to set rel->rows.
But that doesn't seem attractive to me either; it saves a few lines
for trivial FDWs but makes life impossible for complex ones. The
FDW might well have a better idea than the core code does about how
to calculate selectivity for remote tables.

regards, tom lane