comparetup_index_btree(const SortTuple *a, const SortTuple *b,
					   Tuplesortstate *state)
{
	/*
	 * This is similar to comparetup_heap(), but expects index tuples.  There
	 * is also special handling for enforcing uniqueness, and special treatment
	 * for equal keys at the end.
	 */
	SortSupport	sortKey = state->sortKeys;
	IndexTuple	tuple1;
	IndexTuple	tuple2;
	int			keysz;
	TupleDesc	tupDes;
	bool		equal_hasnull = false;
	int			nkey;
	int32		compare;
	Datum		datum1,
				datum2;
	bool		isnull1,
				isnull2;


	/* Compare the leading sort key */
	compare = ApplySortComparator(a->datum1, a->isnull1,
								  b->datum1, b->isnull1,
								  sortKey);
	if (compare != 0)
		return compare;

	/* Compare additional sort keys */
	tuple1 = (IndexTuple) a->tuple;
	tuple2 = (IndexTuple) b->tuple;
	keysz = state->nKeys;
	tupDes = RelationGetDescr(state->indexRel);

	if (sortKey->abbrev_converter)
	{
		datum1 = index_getattr(tuple1, 1, tupDes, &isnull1);
		datum2 = index_getattr(tuple2, 1, tupDes, &isnull2);

		compare = ApplySortAbbrevFullComparator(datum1, isnull1,
												datum2, isnull2,
												sortKey);
		if (compare != 0)
			return compare;
	}

	/* they are equal, so we only need to examine one null flag */
	if (a->isnull1)
		equal_hasnull = true;

	sortKey++;
	for (nkey = 2; nkey <= keysz; nkey++, sortKey++)
	{
		datum1 = index_getattr(tuple1, nkey, tupDes, &isnull1);
		datum2 = index_getattr(tuple2, nkey, tupDes, &isnull2);

		compare = ApplySortComparator(datum1, isnull1,
									  datum2, isnull2,
									  sortKey);
		if (compare != 0)
			return compare;		/* done when we find unequal attributes */

		/* they are equal, so we only need to examine one null flag */
		if (isnull1)
			equal_hasnull = true;
	}

	/*
	 * If btree has asked us to enforce uniqueness, complain if two equal
	 * tuples are detected (unless there was at least one NULL field).
	 *
	 * It is sufficient to make the test here, because if two tuples are equal
	 * they *must* get compared at some stage of the sort --- otherwise the
	 * sort algorithm wouldn't have checked whether one must appear before the
	 * other.
	 */
	if (state->enforceUnique && !equal_hasnull)
	{
		Datum		values[INDEX_MAX_KEYS];
		bool		isnull[INDEX_MAX_KEYS];

		/*
		 * Some rather brain-dead implementations of qsort (such as the one in
		 * QNX 4) will sometimes call the comparison routine to compare a
		 * value to itself, but we always use our own implementation, which
		 * does not.
		 */
		Assert(tuple1 != tuple2);

		index_deform_tuple(tuple1, tupDes, values, isnull);
		ereport(ERROR,
				(errcode(ERRCODE_UNIQUE_VIOLATION),
				 errmsg("could not create unique index \"%s\"",
						RelationGetRelationName(state->indexRel)),
				 errdetail("Key %s is duplicated.",
						   BuildIndexValueDescription(state->indexRel,
													  values, isnull)),
				 errtableconstraint(state->heapRel,
								 RelationGetRelationName(state->indexRel))));
	}

	/*
	 * If key values are equal, we sort on ItemPointer.  This does not affect
	 * validity of the finished index, but it may be useful to have index
	 * scans in physical order.
	 */
	{
		BlockNumber blk1 = ItemPointerGetBlockNumber(&tuple1->t_tid);
		BlockNumber blk2 = ItemPointerGetBlockNumber(&tuple2->t_tid);

		if (blk1 != blk2)
			return (blk1 < blk2) ? -1 : 1;
	}
	{
		OffsetNumber pos1 = ItemPointerGetOffsetNumber(&tuple1->t_tid);
		OffsetNumber pos2 = ItemPointerGetOffsetNumber(&tuple2->t_tid);

		if (pos1 != pos2)
			return (pos1 < pos2) ? -1 : 1;
	}

	return 0;
}