Great, thanks for the links and useful past discussions! I figured I wasn't the first to stumble across this, and it's interesting to see the issue arise with ORDER BY [VOLATILE FUNC] as well.
My question was not so much about changing behavior as it was about understanding what is desired, especially in light of the fact that subqueries behave differently. From my reading of the links you provided, it seems that even the notion of "desired" here is itself dubious and that there is a case for reevaluating RANDOM() everywhere and a case for not doing that. Given this murkiness, is it fair then to say that drawing parallels between how GROUP BY subquery is handled is moot?
-Paul-
I wanted to surface a discussion in [1] regarding the expected behavior of GROUP BY with VOLATILE expressions. There seems to be a discrepancy between how volatile functions (RANDOM(), also confirmed with TIMEOFDAY()) and subqueries are evaluated in groups. In the examples below, volatile functions do not always appear to be evaluated per-call (evidenced by looking at EXPLAIN or results) whereas scalar subqueries always appear to be independently evaluated.
Based on the docs, "A query using a volatile function will re-evaluate the function at every row where its value is needed," it seems that the handling of subqueries is correct and that each call to RANDOM() should be evaluated (not the current behavior). But, what is correct/anticipated?
The observed behaviors are unlikely to change. Prior discussions can be found regarding this:
David J.