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This change prevents us from doing inappropriate subquery flattening in cases such as dangerous functions hidden inside a sub-SELECT in the targetlist of another sub-SELECT. That could result in unexpected behavior due to multiple evaluations of a volatile function, as in a recent complaint from Etienne Dube. It's been questionable from the very beginning whether these functions should look into subqueries (as noted in their comments), and this case seems to provide proof that they should. Because the new code only descends into SubLinks, not SubPlans or InitPlans, the change only affects the planner's behavior during prepjointree processing and not later on --- for example, you can still get it to use a volatile function in an indexqual if you wrap the function in (SELECT ...). That's a historical behavior, for sure, but it's reasonable given that the executor's evaluation rules for subplans don't depend on whether there are volatile functions inside them. In any case, we need to constrain the behavioral change as narrowly as we can to make this reasonable to back-patch. |
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