Implicit typecast behavior

Hi,

I am looking for a way to make postgresql function as Oracle and other databases do with implicit typecasts.  For example, in the query below:

SELECT RECID, RANK FROM MYTABLE WHERE RANK > 12

RANK is defined as a VARCHAR and will be implicitly cast to NUMBER, but when I run this in Postgresql I get an error.
No operator matches the given name and argument type(s). You might need to add explicit type casts.

I know it will work If I do:

SELECT RECID, RANK FROM MYTABLE WHERE CAST(RANK AS NUMERIC) > CAST (12 AS NUMERIC)

But I want it to be done implicitly like other databases.  I have tried the following

CREATE FUNCTION tonumeric(varchar)
  RETURNS numeric
  STRICT IMMUTABLE LANGUAGE SQL AS
'SELECT cast($1 as numeric);';

CREATE CAST (varchar AS numeric) WITH FUNCTION tonumeric(varchar) AS IMPLICIT;

But this query:
SELECT RECID, RANK FROM MYTABLE WHERE RANK > CAST (12 AS NUMERIC);
Returns the following.
SQL function "tonumeric" statement 1

Because the numeric is being passed most likely.  Is there a way to do this correctly such that "RANK" will be converted to NUMERIC without me explicitly having to CAST it to numeric?

Thanks,
Rick

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