BUG #8471: subquery where not being applied until outer query

The following bug has been logged on the website:

Bug reference:      8471
Logged by:          Dan Rickner
Email address:      dnrickner@taylor.edu
PostgreSQL version: 9.2.4
Operating system:   CentOS
Description:

Our ERP stores student GPA values as a text string.  I am trying to select
only valid gpa values (a number between 0 and 4.0).  I have a function
called numeric that returns a bool if the value can be converted to a
number.  When I run my query I get errors about values that should not be
considered in my outer where statement.  The inner where is supposed to
filter out the bad data values:


-- function to return if a character string can be converted to a number
create or replace function isnumeric(varchar) returns boolean as $$
declare x numeric;
begin
    x = $1::numeric;
    return true;
exception when others then
    return false;
end
$$
language plpgsql
immutable;


-- test table
create table tbl
(
    id serial not null,
    gpa varchar(6) null
);


-- insert bad data
insert into tbl (gpa) values ('A'), ('2.0'), ('12.5'), ('3.45'), (''),
('-'), ('-2.3'), ('-5');




select *
from
( -- this subquery returrns only numeric values
    select a.id, trunc(a.gpa::numeric, 2) as gpa
    from
    (
        select id, gpa, isnumeric(gpa) as num
        from tbl
    ) as a
    where a.num = true
) as b
-- filter the numeric values to the 4.0 range
where b.gpa between 0.0 and 4.0