b898eb6367
Dating back to commit f10b63923, our grammar has allowed "USING" to optionally appear before an opclass name in CREATE INDEX (and, lately, some related places such as ON CONFLICT specifications). Nikolay Shaplov noticed that this syntax existed but wasn't documented, and proposed documenting it. But what seems like a better idea is to remove the production, thereby making the code match the docs not vice versa. This isn't our usual modus operandi for such cases, but there are a couple of good reasons to proceed this way: * So far as I can find, this syntax has never been documented anywhere. It isn't relied on by any of our own code or test cases, and there seems little reason to suppose that it's been used in the wild either. * Documenting it would mean that there would be two separate uses of USING in the CREATE INDEX syntax, the other being "USING access_method". That can lead to nothing but confusion. So, let's just remove it. On the off chance that somebody somewhere is using it, this isn't something to back-patch, but we can fix it in HEAD. Discussion: <1593237.l7oKHRpxSe@nataraj-amd64>
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