On Wed, Sep 23, 2015 at 8:38 AM, Etienne Champetier <champetier.etie...@gmail.com> wrote: > Two questions: > 1) Is it normal to have such a big difference between OR and UNION and > should i always prefer UNION when possible?
yes, it's unfortunate but true. I consider automatic transformation of these expressions to possible enhancements to the planner. Although the fact it hasn't already been done hints at high difficulty or complicated trade-offs. Note, these queries are not the same. OR/UNION ALL should give the same result regardless of input but UNION has a deduplication step that gives a different answer. Avoid pure UNION unless it's very specifically what you want; it can act as an optimization fence in more complex queries. > 2) How can UNION ALL be slower than UNION, it's not doing the HashAggregate > but is 0.2s slower? likely measurement noise. take 10 samples and take a median. merlin -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
