From: Tom Lane <tgl@sss.pgh.pa.us>
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
Date: Fri, 10 Aug 2007 17:30:14 -0400
Message-ID: <14180.1186781414@sss.pgh.pa.us> (raw)
In-Reply-To: <200708101653.13011.andreak@officenet.no>
References: <200708101653.13011.andreak@officenet.no>
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
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, andreak@officenet.no
Subject: Re: Index usage in order by with multiple columns in order-by-clause
In-Reply-To: <14180.1186781414@sss.pgh.pa.us>
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