ATExecAddInherit(Relation child_rel, RangeVar *parent, LOCKMODE lockmode)
{
	Relation	parent_rel,
				catalogRelation;
	SysScanDesc scan;
	ScanKeyData key;
	HeapTuple	inheritsTuple;
	int32		inhseqno;
	List	   *children;

	/*
	 * A self-exclusive lock is needed here.  See the similar case in
	 * MergeAttributes() for a full explanation.
	 */
	parent_rel = heap_openrv(parent, ShareUpdateExclusiveLock);

	/*
	 * Must be owner of both parent and child -- child was checked by
	 * ATSimplePermissions call in ATPrepCmd
	 */
	ATSimplePermissions(parent_rel, ATT_TABLE);

	/* Permanent rels cannot inherit from temporary ones */
	if (parent_rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP &&
		child_rel->rd_rel->relpersistence != RELPERSISTENCE_TEMP)
		ereport(ERROR,
				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
				 errmsg("cannot inherit from temporary relation \"%s\"",
						RelationGetRelationName(parent_rel))));

	/* If parent rel is temp, it must belong to this session */
	if (parent_rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP &&
		!parent_rel->rd_islocaltemp)
		ereport(ERROR,
				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
		errmsg("cannot inherit from temporary relation of another session")));

	/* Ditto for the child */
	if (child_rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP &&
		!child_rel->rd_islocaltemp)
		ereport(ERROR,
				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
		 errmsg("cannot inherit to temporary relation of another session")));

	/*
	 * Check for duplicates in the list of parents, and determine the highest
	 * inhseqno already present; we'll use the next one for the new parent.
	 * (Note: get RowExclusiveLock because we will write pg_inherits below.)
	 *
	 * Note: we do not reject the case where the child already inherits from
	 * the parent indirectly; CREATE TABLE doesn't reject comparable cases.
	 */
	catalogRelation = heap_open(InheritsRelationId, RowExclusiveLock);
	ScanKeyInit(&key,
				Anum_pg_inherits_inhrelid,
				BTEqualStrategyNumber, F_OIDEQ,
				ObjectIdGetDatum(RelationGetRelid(child_rel)));
	scan = systable_beginscan(catalogRelation, InheritsRelidSeqnoIndexId,
							  true, NULL, 1, &key);

	/* inhseqno sequences start at 1 */
	inhseqno = 0;
	while (HeapTupleIsValid(inheritsTuple = systable_getnext(scan)))
	{
		Form_pg_inherits inh = (Form_pg_inherits) GETSTRUCT(inheritsTuple);

		if (inh->inhparent == RelationGetRelid(parent_rel))
			ereport(ERROR,
					(errcode(ERRCODE_DUPLICATE_TABLE),
			 errmsg("relation \"%s\" would be inherited from more than once",
					RelationGetRelationName(parent_rel))));
		if (inh->inhseqno > inhseqno)
			inhseqno = inh->inhseqno;
	}
	systable_endscan(scan);

	/*
	 * Prevent circularity by seeing if proposed parent inherits from child.
	 * (In particular, this disallows making a rel inherit from itself.)
	 *
	 * This is not completely bulletproof because of race conditions: in
	 * multi-level inheritance trees, someone else could concurrently be
	 * making another inheritance link that closes the loop but does not join
	 * either of the rels we have locked.  Preventing that seems to require
	 * exclusive locks on the entire inheritance tree, which is a cure worse
	 * than the disease.  find_all_inheritors() will cope with circularity
	 * anyway, so don't sweat it too much.
	 *
	 * We use weakest lock we can on child's children, namely AccessShareLock.
	 */
	children = find_all_inheritors(RelationGetRelid(child_rel),
								   AccessShareLock, NULL);

	if (list_member_oid(children, RelationGetRelid(parent_rel)))
		ereport(ERROR,
				(errcode(ERRCODE_DUPLICATE_TABLE),
				 errmsg("circular inheritance not allowed"),
				 errdetail("\"%s\" is already a child of \"%s\".",
						   parent->relname,
						   RelationGetRelationName(child_rel))));

	/* If parent has OIDs then child must have OIDs */
	if (parent_rel->rd_rel->relhasoids && !child_rel->rd_rel->relhasoids)
		ereport(ERROR,
				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
				 errmsg("table \"%s\" without OIDs cannot inherit from table \"%s\" with OIDs",
						RelationGetRelationName(child_rel),
						RelationGetRelationName(parent_rel))));

	/* Match up the columns and bump attinhcount as needed */
	MergeAttributesIntoExisting(child_rel, parent_rel);

	/* Match up the constraints and bump coninhcount as needed */
	MergeConstraintsIntoExisting(child_rel, parent_rel);

	/*
	 * OK, it looks valid.	Make the catalog entries that show inheritance.
	 */
	StoreCatalogInheritance1(RelationGetRelid(child_rel),
							 RelationGetRelid(parent_rel),
							 inhseqno + 1,
							 catalogRelation);

	/* Now we're done with pg_inherits */
	heap_close(catalogRelation, RowExclusiveLock);

	/* keep our lock on the parent relation until commit */
	heap_close(parent_rel, NoLock);
}