5fc438fb25
While poking around in the aggregate logic, I noticed that commit 8ed3f11bb broke the logic in nodeAgg.c that purports to detect nested aggregates, by moving initialization of regular aggregate argument expressions out of the code segment that checks for that. You could argue that this check is unnecessary, but it's not much code so I'm inclined to keep it as a backstop against parser and planner bugs. However, there's certainly zero value in checking only some of the subexpressions. We can make the check complete again, and as a bonus make it a good deal more bulletproof against future mistakes of the same ilk, by moving it out to the outermost level of ExecInitAgg. This means we need to check only once per Agg node not once per aggregate, which also seems like a good thing --- if the check does find something wrong, it's not urgent that we report it before the plan node initialization finishes. Since this requires remembering the original length of the aggs list, I deleted a long-obsolete stanza that changed numaggs from 0 to 1. That's so old it predates our decision that palloc(0) is a valid operation, in (digs...) 2004, see commit 24a1e20f1. In passing improve a few comments. Back-patch to v10, just in case.
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