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

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)????

>
> Can't I have multicolumn-indexes with functions? Any idea how to rewrite that
> so it works?
> Here is my session table:
> 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
> );
>
> Here is my query I wish to optimize using indexes:
> SELECT to_char(created, 'IW') as week, count(session_id) from session WHERE
> username IS NULL and to_char(created, 'YYYY') = '2002' group by week ORDER BY
> week;
>
> Any hints on optimizing this query, index-usage etc?
>
> - --
> 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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==================================================================
Achilleus Mantzios
S/W Engineer
IT dept
Dynacom Tankers Mngmt
Nikis 4, Glyfada
Athens 16610
Greece
tel:    +30-10-8981112
fax:    +30-10-8981877
email:  achill@matrix.gatewaynet.com       mantzios@softlab.ece.ntua.gr