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 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:

You didnt try it!!

Change your to_char(created, ''YYYY'')||$2 to
to_char(created, ''YYYY'')||(coalesce($2,'''')
(provided there is no user named mister '' :)

then perform your query like:

select to_char(created, 'IW') as week, count(session_id) from session
WHERE drus(created,username) = '2002' group by week ORDER BY
week;

do a explain analyze to see index and performance issues.

> 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@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