Hi Andrus,
>
> How to add column validation to table column which forces this ?
You're looking for a constraint - I presume you know what conditions you want to check for - I've done a similar thing recently ensuring the first character in a column is a decimal point, or that if one column's value is something, another column's value is within a certain range etc.
http://www.postgresql.org/docs/8.4/static/ddl-constraints.html (assuming you're running 8.4)
Regards,
Andy