On Tue, Oct 25, 2016 at 10:35 PM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> 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.
Ok, Thanks for clarification.
>
> 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.
After putting more thought on this, IMHO it need not to be so
complicated. Currently we are talking about pushing only "var op
const", and cost of all such functions are very low and fixed "1".
Do we really need to take care of any user defined function which is
declared with very low cost ?
Because while building index conditions also we don't take care of
such things. Index conditions will always we evaluated first then only
filter will be applied.
Am I missing something ?
--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com
