In
http://www.postgresql.org/docs/9.0/static/tutorial-window.html
it say
"Although avg will produce the same result no matter what order it
processes the partition's rows in, this is not true of all window
functions. When needed, you can control that order using ORDER BY within
OVER."
While it's true that avg() produce the same result no matter what order. A
ORDER BY clause will affect what rows are included in the computation and
thus change the result (the default window frame is
RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW). So one can not in
general add an ORDER BY to the example in the tutorial and get the same
result as without an ORDER BY.
Maybe we can find some better wording of the above?
/Dennis