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Re: Avoiding planning redundant backwards merges

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-hackers(at)postgreSQL(dot)org>
Subject: Re: Avoiding planning redundant backwards merges
Date: 2007-10-27 07:33:17
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> The idea I'm toying with is to make pathkeys_useful_for_merging()
> consider only ASC pathkeys as useful for merging --- that is, only
> pathkeys with pk_strategy = BTLessStrategyNumber.  This would mean that
> only forward scans on ASC indexes and backward scans on DESC indexes
> would be considered to have "interesting" sort orders, and therefore
> in cases without any ORDER BY clause to worry about, the other indexscan
> path would not survive the initial competition in add_path.  It'd be
> seen as having the same cost and worse ordering, and would be dropped.

So the case that wouldn't be covered would be if you have a descending index
on one table and an ascending index on another table and try to merge join

  Gregory Stark

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