Re: cost_agg() with AGG_HASHED does not account for startup costs

On 5 August 2015 at 01:54, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> > During working on allowing the planner to perform GROUP BY before joining
> > I've noticed that cost_agg() completely ignores input_startup_cost
> > when aggstrategy == AGG_HASHED.
>
> Isn't your proposed patch double-counting the input startup cost?
> input_total_cost already includes that charge. The calculation
> reflects the fact that we have to read all of the input before we
> can deliver any aggregated results, so the time to get the first
> input row isn't really interesting.
>
> If this were wrong, the PLAIN costing path would also be wrong, but
> I don't think that either one is.
>
>
Sorry, false alarm, you're right.

This was a bug in my code where I was adding disable_cost to the
startup_cost, but not to the total_cost.

--
David Rowley http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
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