DefineVirtualRelation(RangeVar *relation, List *tlist, bool replace,
					  List *options, Query *viewParse)
{
	Oid			viewOid;
	LOCKMODE	lockmode;
	CreateStmt *createStmt = makeNode(CreateStmt);
	List	   *attrList;
	ListCell   *t;

	/*
	 * create a list of ColumnDef nodes based on the names and types of the
	 * (non-junk) targetlist items from the view's SELECT list.
	 */
	attrList = NIL;
	foreach(t, tlist)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(t);

		if (!tle->resjunk)
		{
			ColumnDef  *def = makeColumnDef(tle->resname,
											exprType((Node *) tle->expr),
											exprTypmod((Node *) tle->expr),
											exprCollation((Node *) tle->expr));

			/*
			 * It's possible that the column is of a collatable type but the
			 * collation could not be resolved, so double-check.
			 */
			if (type_is_collatable(exprType((Node *) tle->expr)))
			{
				if (!OidIsValid(def->collOid))
					ereport(ERROR,
							(errcode(ERRCODE_INDETERMINATE_COLLATION),
							 errmsg("could not determine which collation to use for view column \"%s\"",
									def->colname),
							 errhint("Use the COLLATE clause to set the collation explicitly.")));
			}
			else
				Assert(!OidIsValid(def->collOid));

			attrList = lappend(attrList, def);
		}
	}

	/*
	 * Look up, check permissions on, and lock the creation namespace; also
	 * check for a preexisting view with the same name.  This will also set
	 * relation->relpersistence to RELPERSISTENCE_TEMP if the selected
	 * namespace is temporary.
	 */
	lockmode = replace ? AccessExclusiveLock : NoLock;
	(void) RangeVarGetAndCheckCreationNamespace(relation, lockmode, &viewOid);

	if (OidIsValid(viewOid) && replace)
	{
		Relation	rel;
		TupleDesc	descriptor;
		List	   *atcmds = NIL;
		AlterTableCmd *atcmd;
		ObjectAddress address;

		/* Relation is already locked, but we must build a relcache entry. */
		rel = relation_open(viewOid, NoLock);

		/* Make sure it *is* a view. */
		if (rel->rd_rel->relkind != RELKIND_VIEW)
			ereport(ERROR,
					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
					 errmsg("\"%s\" is not a view",
							RelationGetRelationName(rel))));

		/* Also check it's not in use already */
		CheckTableNotInUse(rel, "CREATE OR REPLACE VIEW");

		/*
		 * Due to the namespace visibility rules for temporary objects, we
		 * should only end up replacing a temporary view with another
		 * temporary view, and similarly for permanent views.
		 */
		Assert(relation->relpersistence == rel->rd_rel->relpersistence);

		/*
		 * Create a tuple descriptor to compare against the existing view, and
		 * verify that the old column list is an initial prefix of the new
		 * column list.
		 */
		descriptor = BuildDescForRelation(attrList);
		checkViewTupleDesc(descriptor, rel->rd_att);

		/*
		 * If new attributes have been added, we must add pg_attribute entries
		 * for them.  It is convenient (although overkill) to use the ALTER
		 * TABLE ADD COLUMN infrastructure for this.
		 *
		 * Note that we must do this before updating the query for the view,
		 * since the rules system requires that the correct view columns be in
		 * place when defining the new rules.
		 *
		 * Also note that ALTER TABLE doesn't run parse transformation on
		 * AT_AddColumnToView commands.  The ColumnDef we supply must be ready
		 * to execute as-is.
		 */
		if (list_length(attrList) > rel->rd_att->natts)
		{
			ListCell   *c;
			int			skip = rel->rd_att->natts;

			foreach(c, attrList)
			{
				if (skip > 0)
				{
					skip--;
					continue;
				}
				atcmd = makeNode(AlterTableCmd);
				atcmd->subtype = AT_AddColumnToView;
				atcmd->def = (Node *) lfirst(c);
				atcmds = lappend(atcmds, atcmd);
			}

			/* EventTriggerAlterTableStart called by ProcessUtilitySlow */
			AlterTableInternal(viewOid, atcmds, true);

			/* Make the new view columns visible */
			CommandCounterIncrement();
		}

		/*
		 * Update the query for the view.
		 *
		 * Note that we must do this before updating the view options, because
		 * the new options may not be compatible with the old view query (for
		 * example if we attempt to add the WITH CHECK OPTION, we require that
		 * the new view be automatically updatable, but the old view may not
		 * have been).
		 */
		StoreViewQuery(viewOid, viewParse, replace);

		/* Make the new view query visible */
		CommandCounterIncrement();

		/*
		 * Update the view's options.
		 *
		 * The new options list replaces the existing options list, even if
		 * it's empty.
		 */
		atcmd = makeNode(AlterTableCmd);
		atcmd->subtype = AT_ReplaceRelOptions;
		atcmd->def = (Node *) options;
		atcmds = list_make1(atcmd);

		/* EventTriggerAlterTableStart called by ProcessUtilitySlow */
		AlterTableInternal(viewOid, atcmds, true);

		/*
		 * There is very little to do here to update the view's dependencies.
		 * Most view-level dependency relationships, such as those on the
		 * owner, schema, and associated composite type, aren't changing.
		 * Because we don't allow changing type or collation of an existing
		 * view column, those dependencies of the existing columns don't
		 * change either, while the AT_AddColumnToView machinery took care of
		 * adding such dependencies for new view columns.  The dependencies of
		 * the view's query could have changed arbitrarily, but that was dealt
		 * with inside StoreViewQuery.  What remains is only to check that
		 * view replacement is allowed when we're creating an extension.
		 */
		ObjectAddressSet(address, RelationRelationId, viewOid);

		recordDependencyOnCurrentExtension(&address, true);

		/*
		 * Seems okay, so return the OID of the pre-existing view.
		 */
		relation_close(rel, NoLock);	/* keep the lock! */

		return address;
	}
	else
	{
		ObjectAddress address;

		/*
		 * Set the parameters for keys/inheritance etc. All of these are
		 * uninteresting for views...
		 */
		createStmt->relation = relation;
		createStmt->tableElts = attrList;
		createStmt->inhRelations = NIL;
		createStmt->constraints = NIL;
		createStmt->options = options;
		createStmt->oncommit = ONCOMMIT_NOOP;
		createStmt->tablespacename = NULL;
		createStmt->if_not_exists = false;

		/*
		 * Create the relation (this will error out if there's an existing
		 * view, so we don't need more code to complain if "replace" is
		 * false).
		 */
		address = DefineRelation(createStmt, RELKIND_VIEW, InvalidOid, NULL,
								 NULL);
		Assert(address.objectId != InvalidOid);

		/* Make the new view relation visible */
		CommandCounterIncrement();

		/* Store the query for the view */
		StoreViewQuery(address.objectId, viewParse, replace);

		return address;
	}
}
DefineVirtualRelation(RangeVar *relation, List *tlist, bool replace,
					  List *options, Query *viewParse)
{
	Oid			viewOid;
	LOCKMODE	lockmode;
	CreateStmt *createStmt = makeNode(CreateStmt);
	List	   *attrList;
	ListCell   *t;

	 
	attrList = NIL;
	foreach(t, tlist)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(t);

		if (!tle->resjunk)
		{
			ColumnDef  *def = makeColumnDef(tle->resname,
											exprType((Node *) tle->expr),
											exprTypmod((Node *) tle->expr),
											exprCollation((Node *) tle->expr));

			 
			if (type_is_collatable(exprType((Node *) tle->expr)))
			{
				if (!OidIsValid(def->collOid))
					ereport(ERROR,
							(errcode(ERRCODE_INDETERMINATE_COLLATION),
							 errmsg("could not determine which collation to use for view column \"%s\"",
									def->colname),
							 errhint("Use the COLLATE clause to set the collation explicitly.")));
			}
			else
				Assert(!OidIsValid(def->collOid));

			attrList = lappend(attrList, def);
		}
	}

	 
	lockmode = replace ? AccessExclusiveLock : NoLock;
	(void) RangeVarGetAndCheckCreationNamespace(relation, lockmode, &viewOid);

	if (OidIsValid(viewOid) && replace)
	{
		Relation	rel;
		TupleDesc	descriptor;
		List	   *atcmds = NIL;
		AlterTableCmd *atcmd;
		ObjectAddress address;

		 
		rel = relation_open(viewOid, NoLock);

		 
		if (rel->rd_rel->relkind != RELKIND_VIEW)
			ereport(ERROR,
					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
					 errmsg("\"%s\" is not a view",
							RelationGetRelationName(rel))));

		 
		CheckTableNotInUse(rel, "CREATE OR REPLACE VIEW");

		 
		Assert(relation->relpersistence == rel->rd_rel->relpersistence);

		 
		descriptor = BuildDescForRelation(attrList);
		checkViewTupleDesc(descriptor, rel->rd_att);

		 
		if (list_length(attrList) > rel->rd_att->natts)
		{
			ListCell   *c;
			int			skip = rel->rd_att->natts;

			foreach(c, attrList)
			{
				if (skip > 0)
				{
					skip--;
					continue;
				}
				atcmd = makeNode(AlterTableCmd);
				atcmd->subtype = AT_AddColumnToView;
				atcmd->def = (Node *) lfirst(c);
				atcmds = lappend(atcmds, atcmd);
			}

			 
			AlterTableInternal(viewOid, atcmds, true);

			 
			CommandCounterIncrement();
		}

		 
		StoreViewQuery(viewOid, viewParse, replace);

		 
		CommandCounterIncrement();

		 
		atcmd = makeNode(AlterTableCmd);
		atcmd->subtype = AT_ReplaceRelOptions;
		atcmd->def = (Node *) options;
		atcmds = list_make1(atcmd);

		 
		AlterTableInternal(viewOid, atcmds, true);

		 
		ObjectAddressSet(address, RelationRelationId, viewOid);

		recordDependencyOnCurrentExtension(&address, true);

		 
		relation_close(rel, NoLock);	 

		return address;
	}
	else
	{
		ObjectAddress address;

		 
		createStmt->relation = relation;
		createStmt->tableElts = attrList;
		createStmt->inhRelations = NIL;
		createStmt->constraints = NIL;
		createStmt->options = options;
		createStmt->oncommit = ONCOMMIT_NOOP;
		createStmt->tablespacename = NULL;
		createStmt->if_not_exists = false;

		 
		address = DefineRelation(createStmt, RELKIND_VIEW, InvalidOid, NULL,
								 NULL);
		Assert(address.objectId != InvalidOid);

		 
		CommandCounterIncrement();

		 
		StoreViewQuery(address.objectId, viewParse, replace);

		return address;
	}
}