Converting from MS Access field aliases

Good morning,

Oh joyous day! We are upgrading a legacy database system from MS
Access to PostgreSQL! Yay!

Ok, rejoicing over. Here's our issue and PLEASE point me to the right
place if this has been discussed before.

In MS Access one can reuse field aliases later in the same query. For
example:

SELECT field1 / 2 AS foo,
field2 * 2 AS bar,
foo + bar AS total
WHERE foo < 12;

The first two fields are fine, it's the third that's a problem. The
database reports

ERROR: column "foo" does not exist

This type of situation is happening -many- times in well over fifty
existing SELECT..INTO and INSERT INTO queries. The obvious solution
here is to copy the code around to eliminate the need to reuse "foo"
and "bar" in the query:

SELECT field1 / 2 AS foo,
field2 * 2 AS bar,
(field1 / 2) + (field2 * 2) AS total
WHERE (field1 / 2) < 12;

But this is a bit ugly and cumbersome and in our case, not desirable
since foo and bar get used many times in the remains of the query. To
replace them with the formulae means that debugging is quite
difficult and very prone to errors.

Does anyone have suggestions on how to circumvent this in a more
graceful manner? I mean I could probably find a way to do this with a
couple of queries and some views, or maybe write a function (or more
like 30 functions) to do the work, but both of those only add to the
workload in an undesirable manner. :)

To complicate matters, performance is a concern. We're operating on
upwards of a billion records. Not all at the same time, but the goal
is to run these a series of calculations will be done on all of the
data.

Thanks for any input that you might have.

--Joel