PostgreSQL reusing computation result in select query

Question

For example using SQL I can do:

SELECT (a+b) as c FROM table WHERE c < 5 AND (c*c+t) > 100;

Is there any way to do that using Postgre

Answer 1

This could be an alternative you might use:

SELECT foo.c
FROM (
    SELECT (a+b) as c FROM table
) as foo
WHERE foo.c < 5 
AND (foo.c*foo.c+t) > 100

From a performance point of view, I think it's not an optimal solution (because of the lack of WHERE clause of foo subquery, hence returning all table records). I don't know if Postgresql does some query optimization there.