Re: BUG #14495: Cost of comparator is not taken into account in sorting

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: zszabo(at)chemaxon(dot)com
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #14495: Cost of comparator is not taken into account in sorting
Date: 2017-01-16 14:43:32
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zszabo(at)chemaxon(dot)com writes:
> Our problem is that the cost of sorting by the Molecule column does not take
> into account the cost of the comparison by molecule_relevance_compare() at
> all!

Yeah, cost_sort just uses cpu_operator_cost as the estimated cost per
comparison. It's unlikely anyone is going to be very excited about
changing that. It would be quite expensive to look up a function-specific
cost on every call, and cost_sort is called often enough during planning
that that expense would be a problem. Also, some call sites don't supply
enough information to do such a lookup at all; requiring them to provide
it would add more expense, and complication. So I don't really see us
adding that much overhead to support such a corner case as an
unduly-expensive sort comparator.

regards, tom lane

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