[GENERAL] Subselects in select expressions

[Royce Ausburn]

I'm having trouble understanding why these two queries produce different 
results:

test=# select (select random()) from generate_series(1,10); -- rows are the same
     ?column?      
-------------------
 0.770797704812139
 0.770797704812139
 0.770797704812139
 0.770797704812139
 0.770797704812139
 0.770797704812139
 0.770797704812139
 0.770797704812139
 0.770797704812139
 0.770797704812139
(10 rows)

test=# select (select random() where generate_series is not null) from 
generate_series(1,10); -- rows are different
      ?column?      
--------------------
  0.561828337144107
 0.0275383000262082
  0.290950470604002
  0.281174722127616
  0.530742571223527
  0.617655908688903
  0.169655770529062
  0.428002137690783
  0.442224354483187
  0.203044794034213
(10 rows)

I understand that it's likely an optimisation thing -- postgres knows that the 
subselect doesn't depend on the FROM rows so it evaluates it only once, but is 
this really correct behaviour?  Ideally, shouldn't postgres know that each 
invocation of random() produces different results and so decide that it should 
execute it for each row? If not, why?

Thanks!

--Royce