From: Andreas Joseph Krogh <andreak@officenet.no>
Organization: OfficeNet AS
To: pgsql-sql@postgresql.org
Subject: Index usage in order by with multiple columns in order-by-clause
Date: Fri, 10 Aug 2007 16:53:12 +0200
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I have the following test-case:

CREATE TABLE test(
name varchar PRIMARY KEY,
value varchar NOT NULL,
created timestamp not null
);

create index test_lowernamevalue_idx ON test ((lower(name) || lower(value))=
);
create index test_lowernamevaluecreated_idx ON test ((lower(name) ||=20
lower(value)), created);

andreak=3D# EXPLAIN ANALYZE select * from test order by lower(name) ||=20
lower(value) ASC, created ASC;
                                                               QUERY PLAN
=2D------------------------------------------------------------------------=
=2D---------------------------------------------------------------
 Index Scan using test_lowernamevaluecreated_idx on test  (cost=3D0.00..61.=
58=20
rows=3D770 width=3D72) (actual time=3D0.013..0.013 rows=3D0 loops=3D1)
 Total runtime: 0.127 ms
(2 rows)

andreak=3D# EXPLAIN ANALYZE select * from test order by lower(name) ||=20
lower(value) ASC, created DESC;
                                               QUERY PLAN
=2D------------------------------------------------------------------------=
=2D------------------------------
 Sort  (cost=3D60.39..62.32 rows=3D770 width=3D72) (actual time=3D0.034..0.=
034 rows=3D0=20
loops=3D1)
   Sort Key: (lower((name)::text) || lower((value)::text)), created
   ->  Seq Scan on test  (cost=3D0.00..23.47 rows=3D770 width=3D72) (actual=
=20
time=3D0.004..0.004 rows=3D0 loops=3D1)
 Total runtime: 0.123 ms
(4 rows)

As the EXPLAIN-output shows, the index is not used when sort-ordering diffe=
rs=20
in the two order-by-columns.
Is there a way I can have multiple columns in the ORDER BY clause, each wit=
h=20
different ASC/DESC-order and still use an index to speed up sorting?

In my application I often have a need to sort by more than 3 columns, so I'=
m=20
really wondering if there is a way to make sorting of multiple columsn (eac=
h=20
which may have different sort-order) use an index? Preferrably without havi=
ng=20
to create 2^N indexes.

=2D-=20
Andreas Joseph Krogh <andreak@officenet.no>
Senior Software Developer / Manager
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