postgres_fdw: Improve cost and size estimation for aggregate pushdown

Enterprise / PostgreSQL - Etsuro Fujita [postgresql.org] - 4 December 2018 08:18 EST

In commit 7012b132d07c2b4ea15b0b3cb1ea9f3278801d98, 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 7b6c07547190f056b0464098bb5a2247129d7aa2 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 postgres_fdw: Improve cost and size estimation for aggregate pushdown.
contrib/postgres_fdw/expected/postgres_fdw.out | 2 +
contrib/postgres_fdw/postgres_fdw.c | 56 ++++++++++++++++++++++----
contrib/postgres_fdw/sql/postgres_fdw.sql | 3 ++
3 files changed, 54 insertions(+), 7 deletions(-)

Upstream: git.postgresql.org


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