postgres_fdw: Add ORDER BY to some remote SQL queries

Enterprise / PostgreSQL - Robert Haas [postgresql.org] - 3 November 2015 12:04 UTC

If the join problem's entire ORDER BY clause can be pushed to the remote server, consider a path that adds this ORDER BY clause. If use_remote_estimate is on, we cost this path using an additional remote EXPLAIN. If not, we just estimate that the path costs 20% more, which is intended to be large enough that we won't request a remote sort when it's not helpful, but small enough that we'll have the remote side do the sort when in doubt. In some cases, the remote sort might actually be free, because the remote query plan might happen to produce output that is ordered the way we need, but without remote estimates we have no way of knowing that.

It might also be useful to request sorted output from the remote side if it enables an efficient merge join, but this patch doesn't attempt to handle that case.

Ashutosh Bapat with revisions by me. Also reviewed by Fabrízio de Royes Mello and Jeevan Chalke.

f18c944 postgres_fdw: Add ORDER BY to some remote SQL queries.
contrib/postgres_fdw/deparse.c | 56 +++++++++++-
contrib/postgres_fdw/expected/postgres_fdw.out | 64 ++++++++-----
contrib/postgres_fdw/postgres_fdw.c | 116 +++++++++++++++++++++++-
contrib/postgres_fdw/postgres_fdw.h | 3 +
contrib/postgres_fdw/sql/postgres_fdw.sql | 13 ++-
5 files changed, 222 insertions(+), 30 deletions(-)

Upstream: git.postgresql.org

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