Yeb Havinga <yebhavi...@gmail.com> writes: > Tom and Robert, thank you both for your replies. I think I'm having some > blind spots and maybe false assumptions regarding the overal work in the > optimizer, as it is not clear to me what 'the same sort of query' would > look like. I was under the impression that using cost to select the best > paths is only done per simple query, and fail to see how a total > combined plan with pulled up subquery could be compared on cost with a > total plan where the subquery is still a separate subplan, since the > range tables / simple-queries to compare are different.
Well, you could look at what planagg.c does to decide whether an indexscan optimization of MIN/MAX is worthwhile, or at the calculations in planner.c that decide which way to do grouping/aggregation/ordering. It could be fairly expensive to handle this type of problem because of the need to cost out two fundamentally different scan/join trees, but we're assuming the queries are expensive enough to make that worthwhile ... regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers