how to avoid repeating expensive computation in select

I have been searching through the docs and mailing list and haven't found a way to do this, so I thought I would ask the community.

I would like to know if there is a way in PostgreSQL to avoid repeating an expensive computation in a SELECT where the result is needed both as a returned value and as an expression in the WHERE clause.

As a simple example, consider the following query on a table with 'id' and 'value' columns, and an expensive computation represented as a function:

SELECT id, expensivefunc(value) AS score FROM mytable
WHERE id LIKE '%z%' AND expensivefunc(value) > 0.5;

It would be great if I could find a way to only compute expensivefunc(value) at most once per row, and not at all if the other WHERE constraints are not satisfied.

For this simple case I know that I could rewrite the SELECT as something like the following:

WITH other_where AS (
SELECT id, value FROM mytable WHERE id LIKE '%z%'
), calc_scores AS (
SELECT id, expensivefunc(value) AS score FROM other_where
)
SELECT id, score from calc_scores WHERE score > 0.5;

This works in this simple case, but my guess is that it probably adds a lot of overhead (is this true?), and I also have to deal with much more complicated scenarios with multiple expensive calculations that may not fit into this kind of rewrite.

Does anyone know of a simpler way to accomplish this?

For example, it would be great if there were a function that could reference the Nth select list item so it is only computed once, like:

SELECT id, expensivefunc(value) AS score FROM mytable
WHERE id LIKE '%z%' AND sel_list_item(2) > 0.5;

or if there were temporary variables in the WHERE expressions like:

SELECT id, tmp1 AS score FROM mytable
WHERE id LIKE '%z%' AND (tmp1 = expensivefunc(value)) > 0.5;

Any ideas anyone!

Thanks in advance!
Bob