Re: Is this possible in a trigger?

Fernando wrote:
> I want to keep a history of changes on a field in a table.  This will
> be the case in multiple tables.
>
> Can I create a trigger that loops the OLD and NEW values and compares
> the values and if they are different creates a change string as follows:
>
> e.g;
>
> FOR EACH field IN NEW
>     IF field.value <> OLD.field.name THEN
>        changes := changes
>             || field.name
>             || ' was: '
>             || OLD.field.value
>             || ' now is: '
>             || field.value
>             || '\n\r';
>     END IF
> END FOR;
>
> Your help is really appreciated.
You can't in plpgsql.  It doesn't have the equivalent of a walkable
fields collection.  Its possible in some other procedure languages (I've
seen it done in C).

Having said that, you might be able to create new and old temp tables
and then use the system tables to walk the columns list executing sql to
check for differences.

something like

   create temp table oldblah as select old.*;
   create temp table newblah as select new.*;
   for arecord in
        select columnname
        from pg_??columns??
        join pg_??tables?? on ??columns??.xxx = ??tables??.yyy
       where tablename = oldblah and pg_table_is_visible
   loop

        execute 'select old.' || arecord.columname || '::text , new. '
|| arecord.columname || '::text' ||
                    ' from oldblah old, newblah new ' ||
                    ' where oldblah.' || arecord.columnname || ' <>
newblah.' ||arecord.columnname    into oldval,newval;

       changes := changes || arecord.columnname || ' was ' || oldval ||
' now ' || newval;
   end loop;
   execute 'drop table oldblah';
   execute 'drop table newblah';

performance could be awful though.

klint.

--
Klint Gore
Database Manager
Sheep CRC
A.G.B.U.
University of New England
Armidale NSW 2350

Ph: 02 6773 3789
Fax: 02 6773 3266
EMail: kgore4@une.edu.au