From: Andreas Joseph Krogh <andreak@officenet.no>
Organization: OfficeNet AS
To: Achilleus Mantzios <achill@matrix.gatewaynet.com>
Subject: Re: index on to_char(created, 'YYYY') doesn't work
Date: Wed, 15 Jan 2003 15:04:46 +0000
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Cc: pgsql-sql@postgresql.org
References: <Pine.LNX.4.44.0301151651120.3008-100000@matrix.gatewaynet.com>
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On Wednesday 15 January 2003 18:55, Achilleus Mantzios wrote:
> On Wed, 15 Jan 2003, Andreas Joseph Krogh wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > On Wednesday 15 January 2003 16:12, you wrote:
> > > On Wed, 15 Jan 2003, Andreas Joseph Krogh wrote:
> > > > -----BEGIN PGP SIGNED MESSAGE-----
> > > > Hash: SHA1
> > > >
> > > > On Wednesday 15 January 2003 11:37, you wrote:
> > > > > The following does not work:
> > > > >
> > > > > create index session_u_idx on session (to_char(created, 'YYYY'));
> > > > > ERROR:  parser: parse error at or near "'YYYY'" at character 57
> > > > >
> > > > > Can I make a function to do this and index using the result of th=
at
> > > > > funtion? Do anyone have an example of such a function?
> > > >
> > > > I tried the following function:
> > > > - -----------------------------------------------------------------
> > > > create function drus (timestamp) returns varchar AS'
> > > >     DECLARE
> > > >         str_created VARCHAR;
> > > >         created ALIAS FOR $1;
> > > >     BEGIN
> > > >         str_created:=3D to_char(created, ''YYYY'');
> > > >         RETURN str_created;
> > > >     END;
> > > > ' LANGUAGE 'plpgsql';
> > >
> > > add
> > > WITH (iscachable)
> >
> > Thank you, not _that_ works:-)
> > But now this doesn't work:
> > create index session_u_idx on session (drus(created), username);
>
> Functinal indexes are single column indexes.
>
> Why dont you change your function to:
>
> create function drus (timestamp,varchar) returns varchar A
>
> and return the concatenation of to_char(created, ''YYYY'')||$2
>
> and then create the index as usual (passing the date and the username
> as params to your function)????

OK, thank you.
Let me explain what I want to do:
I have the following schema for gathering statistics from the web:

CREATE TABLE session (
	session_id varchar(256) NOT NULL PRIMARY KEY,
	created timestamp DEFAULT 'now' NOT NULL,
	last_accessed timestamp NOT NULL,
	destroyed timestamp NOT NULL,
	username varchar -- Allow sessions from not logged in users
);

create or replace function drus (timestamp) returns varchar AS'
    DECLARE
        str_created VARCHAR;
        created ALIAS FOR $1;
    BEGIN
        str_created:=3D to_char(created, ''YYYY'');
        RETURN str_created;
    END;
' LANGUAGE 'plpgsql' WITH (iscachable);

create index session_u_idx on session (drus(created)) where username is nul=
l;

Now I want to get statistics for number of hits pr. week where users are no=
t=20
lnogged in(username IS NULL) for the year 2002:

select to_char(created, 'IW') as week, count(session_id) from session WHERE=
=20
username IS NULL and drus(created) =3D '2002' group by week ORDER BY week;
 week | count
- ------+-------
 01   |  6321
 18   |    74
 19   | 12153
 20   | 17125
 21   | 22157
 22   | 25316
 23   | 24265
 24   | 26234
 25   | 28583
 26   | 29156
 27   | 28335
 28   | 23587
 29   | 23203

This table is quite large(900 000 rows) and the query takes several minutes=
 to=20
run, which makes the browser timeout.
Do I have a design-issue here, should I rather batch-generate the stats in =
its=20
own table so I don't have to process all the data(900 000 rows) each time.

Is there any way to optimize/rewrite this query? Is the use of to_char on t=
he=20
timestamp wrong, should I use another comparation method for getting the ye=
ar=20
2002?

- --=20
Andreas Joseph Krogh <andreak@officenet.no>
	There will always be someone who agrees with you
	but is, inexplicably, a moron.

gpg public_key: http://dev.officenet.no/~andreak/public_key.asc
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