Dilip Kumar wrote: > #2. Currently quals are ordered based on cost (refer > order_qual_clauses), But once we pushdown some of the quals, then > those quals will always be executed first. Can this create problem ?
We don't promise order of execution (which is why we can afford to sort on cost), but I think it makes sense to keep a rough ordering based on cost, and not let this push-down affect those ordering decisions too much. I think it's fine to push-down quals that are within the same order of magnitude of the cost of a pushable condition, while keeping any other much-costlier qual (which could otherwise be pushed down) together with non-pushable conditions; this would sort-of guarantee within-order-of- magnitude order of execution of quals. Hopefully an example clarifies what I mean. Let's suppose you have three quals, where qual2 is non-pushable but 1 and 3 are. cost(1)=10, cost(2)=11, cost(3)=12. Currently, they are executed in that order. If you were to compare costs in the straightforward way, you would push only 1 (because 3 is costlier than 2 which is not push-down-able). With fuzzy comparisons, you'd push both 1 and 3, because cost of 3 is close enough to that of qual 2. But if cost(3)=100 then only push qual 1, and let qual 3 be evaluated together with 2 outside the scan node. BTW, should we cost push-down-able quals differently, say discount some fraction of the cost, to reflect the fact that they are cheaper to run? However, since the decision of which ones to push down depends on the cost, and the cost would depend on which ones we push down, it looks rather messy. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
