Thread: Multicolumn indexes and ORDER BY

I have a multicolumn index on two columns. If i use the columns in ORDER BY
like this:
  ORDER BY col1, col2;

The index is used. But, if one column is sorted DESC it is not used:
  ORDER BY col1 DESC, col2;

How can i make this work ?

Regards,
Jernej Kos.
--
Kostko <kostko@jweb-network.net>
JWeb-Network

On Wed, Jun 16, 2004 at 07:12:26AM +0200, Jernej Kos wrote:
> I have a multicolumn index on two columns. If i use the columns in ORDER BY
> like this:
>   ORDER BY col1, col2;
>
> The index is used. But, if one column is sorted DESC it is not used:
>   ORDER BY col1 DESC, col2;
>
> How can i make this work ?

Try:

ORDER BY col1 DESC, col2 desc;

Hope this helps,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

Yes i tried that already - and as you said, it works. But i need to have one
column sorted DESC and one ASC. Is there any way this could be done ?

Regards,
Jernej Kos.

On Wednesday 16 of June 2004 08:12, Martijn van Oosterhout wrote:
> On Wed, Jun 16, 2004 at 07:12:26AM +0200, Jernej Kos wrote:
> > I have a multicolumn index on two columns. If i use the columns in ORDER
> > BY like this:
> >   ORDER BY col1, col2;
> >
> > The index is used. But, if one column is sorted DESC it is not used:
> >   ORDER BY col1 DESC, col2;
> >
> > How can i make this work ?
>
> Try:
>
> ORDER BY col1 DESC, col2 desc;
>
> Hope this helps,

--
Jernej Kos <kostko@jweb-network.net>
JWeb-Network

Jernej Kos <kostko@jweb-network.net> writes:
> Yes i tried that already - and as you said, it works. But i need to have one
> column sorted DESC and one ASC. Is there any way this could be done ?

Not easily.  You could look into building a "reverse sort" operator
class for one index column or the other.  There is discussion of how
to do this in the archives, but I don't know of anyone having actually
gotten off their duff and done it.  For reasonable index performance
this would require writing at least one function in C (a pretty trivial
one, but nonetheless a C function).

            regards, tom lane

Jernej Kos <kostko@jweb-network.net> writes:
> Well, writing a C function is not a problem ;) So where could i find any
> documentation regarding this matter ?

Read the "Interfacing Extensions To Indexes" docs chapter.  A crude
example for integers would go like

regression=# create function revcmp(int,int) returns int as
regression-# 'select $2 - $1' language sql;
CREATE FUNCTION
regression=# create operator class rev_int_ops for type int using btree as
regression-# operator 1 > ,
regression-# operator 2 >= ,
regression-# operator 3 = ,
regression-# operator 4 <= ,
regression-# operator 5 < ,
regression-# function 1 revcmp(int,int);
CREATE OPERATOR CLASS

(compare the operator order here to the "standard" btree order shown in
the docs --- we're swapping < for > and <= for >=)

This actually works:

regression=# create table foo (f1 int, f2 int);
CREATE TABLE
regression=# create index fooi on foo (f1, f2 rev_int_ops);
CREATE INDEX
regression=# explain select * from foo order by f1, f2 desc;
                             QUERY PLAN
--------------------------------------------------------------------
 Index Scan using fooi on foo  (cost=0.00..52.00 rows=1000 width=8)
(1 row)

regression=# explain select * from foo order by f1 desc, f2 asc;
                                 QUERY PLAN
-----------------------------------------------------------------------------
 Index Scan Backward using fooi on foo  (cost=0.00..52.00 rows=1000 width=8)
(1 row)

but index performance would be pretty sucky without reducing the
comparator function to C.  Also I didn't consider overflow when writing
this comparator function, so the above would probably fall over if faced
with index entries outside +/- 1 billion or so.

At the C level it'd probably be best to call the standard comparator
function for the type and then negate its result, viz
    PG_RETURN_INT32(- DatumGetInt32(btint4cmp(fcinfo)));
which reduces what might otherwise be a bit complicated to trivial
boilerplate.

We have previously discussed putting together a contrib package that
implements reverse-sort opclasses of this kind for all the standard
datatypes.  If you feel like doing the legwork, the submission would
be gratefully accepted ...

            regards, tom lane