Given an arbitrary function fn(x) returning numeric. Question: how often is the function executed?
A. select fn('const'), fn('const'); Answer: Twice. This is not a surprize. B. select v,v from fn('const') as v;  Answer: Once. C. select v.v,v.v from (select fn('const') as v) as v; Answer: Once if declared VOLATILE. Twice if declared STABLE. Now this IS a surprize. It is clear that the system is not allowed to execute the function twice when declared VOLATILE. It IS ALLOWED to execute it twice when STABLE - but to what point, except prolonging execution time? Over all, VOLATILE performs better than STABLE.  I seem to remember that I was not allowed to do this when I coded my SQL, because expressions in the from clause must return SETOF, not a single value. Now it seems to work.