On Tue, Dec 19, 2017 at 6:54 AM, Thomas Munro <thomas.mu...@enterprisedb.com> wrote: > On Mon, Dec 18, 2017 at 9:29 AM, Robert Haas <robertmh...@gmail.com> wrote: >> I went through the callers to create_sort_path and the only one that >> looks like it can pass a limit is the one you and Jeff already >> identified. So I think the question is just whether >> create_gather_merge_path needs a similar fix. > > I might be missing something, but it looks like there are no cases > where we have a limit_tuples value we could use AND we're relying on > create_gather_merge_path's own ability to create sort paths. So I > suspect there is no reason to change create_gather_merge_path itself > to deal with tuple limits. I looked at each of its callers: > > 1. create_ordered_paths is the case the patch posted earlier covers: > it has a useful limit_tuples value but it creates the sort path itself > first, so there is no need for create_gather_merge_path to be aware of > it. > > 2. create_grouping_paths doesn't have limit_tuples value because > grouping always inhibits limits. > > 3. generate_gather_paths is in turn called by: > > 3.1. standard_joinsearch can't use limits (at least in general) since > it's dealing with a join. > > 3.2. geco's merge_clump is also about joins, so ditto. > > 3.3. set_rel_pathlist will consider only pathkeys from existing index > scans that set_plain_rel_pathlist found, not creating new pathkeys by > sorting.
Well, it might be good future-proofing, but at least it's good to know that it's not a active bug. I pushed your earlier fix, which turns out to be a revert of commit dc02c7bca4dccf7de278cdc6b3325a829e75b252. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
