On Fri, Jul 1, 2016 at 10:20 AM, Merlin Moncure <mmonc...@gmail.com> wrote: > Yeah. Also, even if you could parse out those cases, it's major > optimization fence. Consider if you have an ORDER BY clause here: > > SELECT FROM foo WHERE a OR b ORDER BY c; > > ... by pushing inside a union, you're going to be in trouble in real > world cases. That's just a mess and it would add a lot of runtime > analysis of the alternative paths. It's hard for me to believe > rewriting is easier and simpler than rewriting 'false OR x' to 'x'. I > also thing that constant folding strategies are going to render much > more sensible output to EXPLAIN.
I don't think that it's easier and simpler and didn't intend to say otherwise. I do think that I've run across LOTS of queries over the years where rewriting OR using UNION ALL was a lot faster, and I think that case is more likely to occur in practice than FALSE OR WHATEVER. But, I'm just throwing out opinions to see what sticks here; I'm not deeply invested in this. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
