Re: [PATCH] Improve amcheck to also check UNIQUE constraint in btree index.

The documentation in https://www.postgresql.org/docs/devel/amcheck.html#AMCHECK-FUNCTIONS is ambiguous:

"bt_index_check does not verify invariants that span child/parent relationships, but will verify the presence of all heap tuples as index tuples within the index when heapallindexed is true. When checkunique is true bt_index_check will check that no more than one among duplicate entries in unique index is visible. When a routine, lightweight test for corruption is required in a live production environment, using bt_index_check often provides the best trade-off between thoroughness of verification and limiting the impact on application performance and availability."

The second sentence, "When checkunique is true bt_index_check will check that no more than one among duplicate entries in unique index is visible." is not strictly true, as it won't check if the violation spans a page boundary.