BUG #6612: Functions can be called inside CHECK statements

The following bug has been logged on the website:

Bug reference: 6612
Logged by: Ari Entlich
Email address: atrigent(at)ccs(dot)neu(dot)edu
PostgreSQL version: 9.1.2
Operating system: Windows?
Description:

Seeing as Postgres does not allow sub-queries in CHECK constraints yet, it
doesn't make any sense to me for it to allow function calls, since functions
can perform queries. Additionally, if a function is called from a check
constraint and that function executes a query, the change that caused the
check constraint to fire does not appear to be "visible" to the query.
Therefore, calling functions in check constraints does not have the ability
to check whether that change is valid, making it mostly useless.