PostgreSQL enforces SQL uniqueness constraints using unique indexes, which are indexes that disallow multiple entries with identical keys. An access method that supports this feature sets pg_am.amcanunique true. (At present, only b-tree supports it.)
Because of MVCC, it is always necessary to allow duplicate entries to exist physically in an index: the entries might refer to successive versions of a single logical row. The behavior we actually want to enforce is that no MVCC snapshot could include two rows with equal index keys. This breaks down into the following cases that must be checked when inserting a new row into a unique index:
If a conflicting valid row has been deleted by the current transaction, it's okay. (In particular, since an UPDATE always deletes the old row version before inserting the new version, this will allow an UPDATE on a row without changing the key.)
If a conflicting row has been inserted by an as-yet-uncommitted transaction, the would-be inserter must wait to see if that transaction commits. If it rolls back then there is no conflict. If it commits without deleting the conflicting row again, there is a uniqueness violation. (In practice we just wait for the other transaction to end and then redo the visibility check in toto.)
Similarly, if a conflicting valid row has been deleted by an as-yet-uncommitted transaction, the would-be inserter must wait for that transaction to commit or abort, and then repeat the test.
Furthermore, immediately before reporting a uniqueness violation according to the above rules, the access method must recheck the liveness of the row being inserted. If it is committed dead then no violation should be reported. (This case cannot occur during the ordinary scenario of inserting a row that's just been created by the current transaction. It can happen during CREATE UNIQUE INDEX CONCURRENTLY, however.)
We require the index access method to apply these tests itself, which means that it must reach into the heap to check the commit status of any row that is shown to have a duplicate key according to the index contents. This is without a doubt ugly and non-modular, but it saves redundant work: if we did a separate probe then the index lookup for a conflicting row would be essentially repeated while finding the place to insert the new row's index entry. What's more, there is no obvious way to avoid race conditions unless the conflict check is an integral part of insertion of the new index entry.
If the unique constraint is deferrable, there is additional
complexity: we need to be able to insert an index entry for a new
row, but defer any uniqueness-violation error until end of
statement or even later. To avoid unnecessary repeat searches of
the index, the index access method should do a preliminary
uniqueness check during the initial insertion. If this shows that
there is definitely no conflicting live tuple, we are done.
Otherwise, we schedule a recheck to occur when it is time to
enforce the constraint. If, at the time of the recheck, both the
inserted tuple and some other tuple with the same key are live,
then the error must be reported. (Note that for this purpose,
"live" actually means "any tuple in the index entry's HOT chain is
live".) To implement this, the
aminsert function is passed a checkUnique parameter having one of the following
UNIQUE_CHECK_NO indicates that no uniqueness checking should be done (this is not a unique index).
UNIQUE_CHECK_YES indicates that this is a non-deferrable unique index, and the uniqueness check must be done immediately, as described above.
that the unique constraint is deferrable. PostgreSQL will use this mode to insert
each row's index entry. The access method must allow
duplicate entries into the index, and report any potential
duplicates by returning FALSE from
aminsert. For each row for which FALSE is
returned, a deferred recheck will be scheduled.
The access method must identify any rows which might violate the unique constraint, but it is not an error for it to report false positives. This allows the check to be done without waiting for other transactions to finish; conflicts reported here are not treated as errors and will be rechecked later, by which time they may no longer be conflicts.
that this is a deferred recheck of a row that was reported as
a potential uniqueness violation. Although this is
implemented by calling
aminsert, the access method must
not insert a new
index entry in this case. The index entry is already present.
Rather, the access method must check to see if there is
another live index entry. If so, and if the target row is
also still live, report error.
It is recommended that in a UNIQUE_CHECK_EXISTING call, the access method
further verify that the target row actually does have an
existing entry in the index, and report error if not. This is
a good idea because the index tuple values passed to
aminsert will have been
recomputed. If the index definition involves functions that
are not really immutable, we might be checking the wrong area
of the index. Checking that the target row is found in the
recheck verifies that we are scanning for the same tuple
values as were used in the original insertion.