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. -- Thomas Munro http://www.enterprisedb.com
