Re: index on to_char(created, 'YYYY') doesn't work - Mailing list pgsql-sql

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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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> > > > 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_accessedtimestamp 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
ALIASFOR $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   |  632118   |    7419   | 1215320   | 1712521   | 2215722   | 2531623   | 2426524   | 2623425   |
2858326  | 2915627   | 2833528   | 2358729   | 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@officenet.no>There will always be someone who agrees with youbut is, inexplicably, a
moron.

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