> -----Original Message----- > From: Tom Lane [mailto:t...@sss.pgh.pa.us] > Sent: Thursday, August 27, 2015 9:03 AM > To: Qingqing Zhou > Cc: Kaigai Kouhei(海外 浩平); Greg Stark; PostgreSQL-development > Subject: Re: [HACKERS] Our trial to TPC-DS but optimizer made unreasonable > plan > > Qingqing Zhou <zhouqq.postg...@gmail.com> writes: > > Above two queries essentially the same, but the second one is a > > non-optimal plan. The reason is that how my patch works: it put a > > substitution in front of SS_process_ctes(): > > > /* > > * If there is a WITH list, process each WITH query and build an initplan > > ! * SubPlan structure for it. Before we process ctes, try to subsitute with > > ! * subqueries to benefits from global optimization. > > */ > > if (parse->cteList) > > + { > > + substitute_ctes_with_subqueries(root); > > SS_process_ctes(root); > > + } > > > AFAICS, the substitution only handles cteList within a query block, so > > it does not go across the subquery boundary. I can see this is an > > issue but can't see a nice way to fix it. Anybody has some recipe? > > It seems like you're doing this in fundamentally the wrong place. > > What I had in mind in <38448.1430519...@sss.pgh.pa.us> was to convert CTEs > into plain subqueries during the prepjointree phase, either just before > or as part of the pull_up_subqueries pass (since you'd want the converted > subquery to be flattened if possible). If you do it later than that, > then you'll have to reinvent a whole bunch of wheels to provide behavior > similar to regular subquery optimization. > Hmm... My suggestion might not be reasonable. Sorry.
-- NEC Business Creation Division / PG-Strom Project KaiGai Kohei <kai...@ak.jp.nec.com> -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
