Re: pgsql: Fix an oversight in the 8.2 patch that improved mergejoin

On Sat, 2008-09-06 at 13:06 +0100, Gregory Stark wrote:
> tgl@postgresql.org (Tom Lane) writes:
>
> > (The materialize protects the sort from having to support mark/restore,
> > allowing it to do its final merge pass on-the-fly.) We neglected to teach
> > cost_mergejoin about that hack, so it was failing to include the
> > materialize's costs in the estimated cost of the mergejoin.
>
> Is that right? The materialize is just doing the same writing that the final
> pass of the sort would have been doing. Did we discount the costs for sort for
> that skipping writing that final pass when that was done?

IIRC the cost of the sort didn't include the final merge, so when we
avoided the final merge the cost model for the sort became accurate.
Perhaps we should add something when we don't do that.

It seems reasonable than an extra node should cost something anyhow, and
the per tuple cost is the current standard way of indicating that extra
cost.

--
 Simon Riggs           www.2ndQuadrant.com
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