pgsql-bugs(at)postgresql(dot)org writes:
> Unable to identify an operator '=' for types 'numeric' and 'double precision'
At the moment you have to cast the literal to numeric (or cast the
column to float8, if you'd rather use float arithmetic, which I
imagine you wouldn't). The existing parser doesn't see any reason
to prefer either choice over the other, so it throws up its hands.
In 7.3 I expect this will behave the way you'd like.
regards, tom lane