Re: Inserting a needed record before a select - Mailing list pgsql-general

On Mon, 14 Apr 2003, Jay O'Connor wrote:

> All,
>
> Hello, I'm new to this list but I'm looking for some information.
>
>
> What I need to do is to find a way to automallically insert a record if it
> doesn'
> t exist, prior to a SELECT.
>
> Basically, I'm tracking some information based on date and the client
> application when retrieving the information knows will that 'current' is
> the record with today's date.
>
> What I'd like is a way of of specifiying that when the SELECT is done,
> there will be a check that a record exists for the current date and that if
> it doesn't exist, a record will be inserted into the table for the current
> date and then the SELECT will return the recors including the newly
> inserted one.
>
> What I've tried so far is a function that checks for the record, inserts it
> if needed, and then returns a cursor that the client can use, but I was
> wondering if there was something more direct.  I thought of a RULE but I
> don't see that they can do processing that expressive.
>
> Here's an example: mytable contains the date based stats information with
> another id.  All the stats columns are defaulted so just inserting a new
> record with the right id and date is sufficient.
> =============
>
> CREATE FUNCTION getdatestats (varchar) RETURNS refcursor AS '
> DECLARE
>     myid ALIAS FOR $1;
>     today date;
>     ref refcursor;
>     dummyrow RECORD;
>
> BEGIN
>
>     ref := 'statscursor';
>     SELECT INTO dummyrow * FROM mytable where id = AND statdate = today;
>     IF NOT FOUND
>         INSERT INTO  mytable (id, statdate) VALUES (myid, today)
>     END IF;
>
>     OPEN ref for SELECT * FROM mytable WHERE id=myid;
>
>     RETURN ref;
> END;
> ' LANGUAGE PLPGSQL
>
> =============
>
>
> Any other ideas?  This works but I was wondering if it was possible to do
> it without the client needing to do the transaction/cursor handling
> (considering I'm only holding up to 30 days worth of data)

You're over halfway there.  Now you just need to do it in a trigger.
Basically, in older versions of pgsql, you can change your
function to return an opaque (in 7.3. it's now a trigger type, not sure
about 7.2) and just check every time somebody selects from the table for
the row and stick it in.  Then make a trigger on it.

\h create trigger
Command:     CREATE TRIGGER
Description: define a new trigger
Syntax:
CREATE TRIGGER name { BEFORE | AFTER } { event [OR ...] }
    ON table FOR EACH { ROW | STATEMENT }
    EXECUTE PROCEDURE func ( arguments )

So, after you've got a function that returns the right type,

create trigger somename before select on tablename for each statement
execute insertdefaultrow(rowid).