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; [1]
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.
[1] 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.
