BUG #2016: query working with 8.0 fails with 8.1

The following bug has been logged online:

Bug reference:      2016
Logged by:          Pit Mueller
Email address:      fmi-soft@gmx.de
PostgreSQL version: 8.1 BETA4
Operating system:   WinXP
Description:        query working with 8.0 fails with 8.1
Details:

Using this query to get the primary key for a certain table:

SELECT p.conname AS constraint_name FROM pg_constraint p WHERE p.contype =
'p' AND p.conrelid = pg_class.oid AND pg_class.relname = 'mytablename'

leads to an error:

ERROR:  missing FROM-clause entry for table "pg_class"

This query worked fine with 8.0.3.