On Wed, May 17, 2017 at 8:38 PM, Heikki Linnakangas <hlinn...@iki.fi> wrote:
> > As a final note, I found an interesting paper called "Unnesting Arbitrary > Queries", by Thomas Neumann and Alfons Kemper > (http://www.btw-2015.de/res/proceedings/Hauptband/Wiss/Neumann-Unnesting_Arbitrary_Querie.pdf). > It describes a series of transformations that can be applied to de-correlate > (de-lateralize?) any lateral subquery join. The trick is to build a > temporary relation that contains all the distinct outer values of the join, > and pass that relation down to the subquery. So instead of "executing" the > subquery for every outer row, passing the outer values as parameters (using > PostgreSQL terminology), you collect a list of all the outer values first, > and then execute the subquery only once. In the subquery, you perform a join > with the temporary relation. And then the paper describes a bunch of rules > and optimizations, that can optimize away the temporary relation in many > cases. That might be one way to implement the de-lateralization steps in > PostgreSQL. > IIUC what you describe here, in a query involving foreign scan, we could pass down a list of parameters to the foreign server instead of running foreign scan on every parameter in the list. That would improve performance of such queries by a lot. -- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
