Re: [HACKERS] Account for cost and selectivity of HAVING quals

Moin,

On Tue, October 31, 2017 5:45 pm, Tom Lane wrote:
> Pursuant to the discussion at
> https://www.postgresql.org/message-id/20171029112420.8920B5FB05@mx.zeyos.com
> here's a patch to fix the planner so that eval costs and selectivity of
> HAVING quals are factored into the appropriate plan node numbers.
> Perhaps unsurprisingly, this doesn't change the results of any
> existing regression tests.
>
> It's slightly annoying that this approach will result in calculating
> the eval costs and selectivity several times, once for each aggregation
> plan type we consider.  I thought about inserting RestrictInfo nodes
> into the havingQual so that those numbers could be cached, but that turned
> out to break various code that doesn't expect to see such nodes there.
> I'm not sure it's worth going to the trouble of fixing that; in the big
> scheme of things, the redundant calculations likely don't cost much, since
> we aren't going to have relevant statistics.
>
> Comments?  If anyone wants to do a real review of this, I'm happy to stick
> it into the upcoming CF; but without an expression of interest, I'll just
> push it.  I don't think there's anything terribly controversial here.

Not a review, but the patch has this:


+                 total_cost += qual_cost.startup + output_tuples *
qual_cost.per_tuple;
+
+                 output_tuples = clamp_row_est(output_tuples *

...

That looks odd to me, it first uses output_tuples in a formula, then
overwrites the value with a new value. Should these lines be swapped?

Best regards,

Tels


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