Hi Jeff, (2019/02/21 6:19), Jeff Janes wrote:
On Wed, Jan 30, 2019 at 6:12 AM Etsuro Fujita <fujita.ets...@lab.ntt.co.jp <mailto:fujita.ets...@lab.ntt.co.jp>> wrote:(2018/12/28 15:50), Etsuro Fujita wrote: > Attached is a new version of the > patch. Here is an updated version of the patch set. Changes are: * In the previous version, LIMIT without OFFSET was not performed remotely as the costs was calculated the same as the costs of performing it locally. (Actually, such LIMIT was performed remotely in a case in the postgres_fdw regression test, but that was due to a bug :(.) I think we should prefer performing such LIMIT remotely as that avoids extra row fetches from the remote that performing it locally might cause, so I tweaked the costs estimated in estimate_path_cost_size(), to ensure that we'll prefer performing such LIMIT remotely.
With your tweaks, I'm still not seeing the ORDER-less LIMIT get pushed down when using use_remote_estimate in a simple test case, either with this set of patches, nor in the V4 set. However, without use_remote_estimate, the LIMIT is now getting pushed with these patches when it does not in HEAD.
Good catch! I think the root cause of that is: when using use_remote_estimate, remote estimates obtained through remote EXPLAIN are rounded off to two decimal places, which I completely overlooked. Will fix. I think I can post a new version early next week.
See attached test case, to be run in new database named 'foo'.
Thanks for the review and the test case! Best regards, Etsuro Fujita
