In commit 7012b132d0, which added aggregate
pushdown to postgres_fdw, we didn't account for the evaluation cost and the
selectivity of HAVING quals attached to ForeignPaths performing aggregate
pushdown, as core had never accounted for that for AggPaths and GroupPaths.
And we didn't set these values of the locally-checked quals (ie, fpinfo's
local_conds_cost and local_conds_sel), which were initialized to zeros, but
since estimate_path_cost_size factors in these to estimate the result size
and the evaluation cost of such a ForeignPath when the use_remote_estimate
option is enabled, this caused it to produce underestimated results in that
case.
By commit 7b6c075471 core was changed so that
it accounts for the evaluation cost and the selectivity of HAVING quals in
aggregation paths, so change the postgres_fdw's aggregate pushdown code as
well as such. This not only fixes the underestimation issue mentioned
above, but improves the estimation using local statistics in that function
when that option is disabled.
This would be a bug fix rather than an improvement, but apply it to HEAD
only to avoid destabilizing existing plan choices.
Author: Etsuro Fujita
Discussion: https://postgr.es/m/5BFD3EAD.2060301%40lab.ntt.co.jp
f8f6e44676