Re: index on to_char(created, 'YYYY') doesn't work

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On Wednesday 15 January 2003 18:55, Achilleus Mantzios wrote:
> On Wed, 15 Jan 2003, Andreas Joseph Krogh wrote:
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> > On Wednesday 15 January 2003 16:12, you wrote:
> > > On Wed, 15 Jan 2003, Andreas Joseph Krogh wrote:
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> > > > 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 that
> > > > > 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:= 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:= to_char(created, ''YYYY'');
RETURN str_created;
END;
' LANGUAGE 'plpgsql' WITH (iscachable);

create index session_u_idx on session (drus(created)) where username is null;

Now I want to get statistics for number of hits pr. week where users are not
lnogged in(username IS NULL) for the year 2002:

select to_char(created, 'IW') as week, count(session_id) from session WHERE
username IS NULL and drus(created) = '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
run, which makes the browser timeout.
Do I have a design-issue here, should I rather batch-generate the stats in its
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 the
timestamp wrong, should I use another comparation method for getting the year
2002?

- --
Andreas Joseph Krogh <andreak(at)officenet(dot)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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