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We require the partition key to be a subset of the set of columns being made unique, so that physically-separate indexes on the different partitions are sufficient to enforce the uniqueness constraint. The existing code checked that the listed columns appear, but did not inquire into the index semantics, which is a serious oversight given that different index opclasses might enforce completely different notions of uniqueness. Ideally, perhaps, we'd just match the partition key opfamily to the index opfamily. But hash partitioning uses hash opfamilies which we can't directly match to btree opfamilies. Hence, look up the equality operator in each family, and accept if it's the same operator. This should be okay in a fairly general sense, since the equality operator ought to precisely represent the opfamily's notion of uniqueness. A remaining weak spot is that we don't have a cross-index-AM notion of which opfamily member is "equality". But we know which one to use for hash and btree AMs, and those are the only two that are relevant here at present. (Any non-core AMs that know how to enforce equality are out of luck, for now.) Back-patch to v11 where this feature was introduced. Guancheng Luo, revised a bit by me Discussion: https://postgr.es/m/D9C3CEF7-04E8-47A1-8300-CA1DCD5ED40D@gmail.com
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