On 05/04/2017 12:34 PM, David G. Johnston wrote: > On Thu, May 4, 2017 at 9:22 AM, Andrew Dunstan > <andrew.duns...@2ndquadrant.com > <mailto:andrew.duns...@2ndquadrant.com>>wrote: > > > Yeah, the idea that this won't cause possibly significant pain is > quite wrong. Quite by accident I came across an example just this > morning where rewriting as a CTE makes a big improvement. > > I wrote this query: > > select (json_populate_record(null::mytype, myjson)).* > from mytable; > > > It turned out that this was an order of magnitude faster: > > with r as > ( > select json_populate_record(null::mytype, myjson) as x > from mytable > ) > select (x).* > from r; > > > Except I suspect we at least have a chance to detect the above and > not de-optimize it by evaluating "json_populate_record" once for every > column in mytype. > > The now idiomatic solution to the above is to use LATERAL so the > above CTE is no longer actually a required workaround.
Hadn't though about LATERAL, good point. Still, there will be other cases. cheers andrew -- Andrew Dunstan https://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers