To: Andreas Joseph Krogh <andreak@officenet.no>
cc: pgsql-sql@postgresql.org
Subject: Re: Index usage in order by with multiple columns in order-by-clause 
In-reply-to: <200708101653.13011.andreak@officenet.no> 
References: <200708101653.13011.andreak@officenet.no>
Comments: In-reply-to Andreas Joseph Krogh <andreak@officenet.no>
	message dated "Fri, 10 Aug 2007 16:53:12 +0200"
Date: Fri, 10 Aug 2007 17:30:14 -0400
Message-ID: <14180.1186781414@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Andreas Joseph Krogh <andreak@officenet.no> writes:
> Is there a way I can have multiple columns in the ORDER BY clause, each with
> different ASC/DESC-order and still use an index to speed up sorting?

A btree index isn't magic, it's just an ordered list of entries.  So you
can't just randomly flip the ordering of individual columns.  For
instance, the natural sort order of a 2-column index on (x,y) is like

	x	y

	1	1
	1	2
	1	3
	2	1
	2	2
	2	3
	3	1
	3	2
	3	3

If you scan this index forwards, you get the equivalent of
	ORDER BY x ASC, y ASC
If you scan it backwards, you get the equivalent of
	ORDER BY x DESC, y DESC
But there is no way to get the equivalent of x ASC, y DESC from
a scan of this index, nor x DESC, y ASC.

If you have a specific requirement for one of those combinations,
what you can do is build an index in which one of the columns is
"reverse sorted".  For instance, if we reverse-sort y, the index
ordering looks like

	x	y

	1	3
	1	2
	1	1
	2	3
	2	2
	2	1
	3	3
	3	2
	3	1

Now we can get ORDER BY x ASC, y DESC from a forwards indexscan,
or ORDER BY x DESC, y ASC from a backwards scan.  But there's no
way to get ASC/ASC or DESC/DESC from this index.  If you really need
all four orderings to be available, you're stuck with maintaining
two indexes.

Reverse-sorted index columns are possible but not well supported in
existing PG releases (you need a custom operator class, and the planner
is not all that bright about using them).  8.3 will have full support.

			regards, tom lane