Re: PoC: Partial sort

On Thu, Feb 6, 2014 at 5:31 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> Hmm, sounds a little steep.  Why is it so expensive?  I'm probably
> missing something here, because I would have thought that planner
> support for partial sorts would consist mostly of considering the same
> sorts we consider today, but with the costs reduced by the batching.

I guess it's because the patch undoes some optimizations in the
mergejoin planner wrt caching merge clauses and adds a whole lot of
code to find_mergeclauses_for_pathkeys. In other code paths the
overhead does seem to be negligible.

Notice the removal of:
/* Select the right mergeclauses, if we didn't already */
/*
 * Avoid rebuilding clause list if we already made one;
 * saves memory in big join trees...
 */

Regards,
Marti