Refactor broken CREATE TABLE IF NOT EXISTS support.

Per bug #5988, reported by Marko Tiikkaja, and further analyzed by Tom
Lane, the previous coding was broken in several respects: even if the
target table already existed, a subsequent CREATE TABLE IF NOT EXISTS
might try to add additional constraints or sequences-for-serial
specified in the new CREATE TABLE statement.

In passing, this also fixes a minor information leak: it's no longer
possible to figure out whether a schema to which you don't have CREATE
access contains a sequence named like "x_y_seq" by attempting to create a
table in that schema called "x" with a serial column called "y".

Some more refactoring of this code in the future might be warranted,
but that will need to wait for a later major release.

Author: robertmhaas

src/backend/bootstrap/bootparse.y

@@ -247,8 +247,7 @@ Boot_CreateStmt:
 													  ONCOMMIT_NOOP,
 													  (Datum) 0,
 													  false,
-													  true,
-													  false);
+													  true);
 						elog(DEBUG4, "relation created with oid %u", id);
 					}
 					do_end();

src/backend/catalog/heap.c

@@ -973,8 +973,7 @@ heap_create_with_catalog(const char *relname,
 						 OnCommitAction oncommit,
 						 Datum reloptions,
 						 bool use_user_acl,
-						 bool allow_system_table_mods,
-						 bool if_not_exists)
+						 bool allow_system_table_mods)
 {
 	Relation	pg_class_desc;
 	Relation	new_rel_desc;
@@ -994,26 +993,14 @@ heap_create_with_catalog(const char *relname,
 	CheckAttributeNamesTypes(tupdesc, relkind, allow_system_table_mods);
 
 	/*
-	 * If the relation already exists, it's an error, unless the user
-	 * specifies "IF NOT EXISTS".  In that case, we just print a notice and do
-	 * nothing further.
+	 * This would fail later on anyway, if the relation already exists.  But
+	 * by catching it here we can emit a nicer error message.
 	 */
 	existing_relid = get_relname_relid(relname, relnamespace);
 	if (existing_relid != InvalidOid)
-	{
-		if (if_not_exists)
-		{
-			ereport(NOTICE,
-					(errcode(ERRCODE_DUPLICATE_TABLE),
-					 errmsg("relation \"%s\" already exists, skipping",
-							relname)));
-			heap_close(pg_class_desc, RowExclusiveLock);
-			return InvalidOid;
-		}
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_TABLE),
 				 errmsg("relation \"%s\" already exists", relname)));
-	}
 
 	/*
 	 * Since we are going to create a rowtype as well, also check for

src/backend/catalog/namespace.c

@@ -360,6 +360,35 @@ RangeVarGetCreationNamespace(const RangeVar *newRelation)
 	return namespaceId;
 }
 
+/*
+ * RangeVarGetAndCheckCreationNamespace
+ *      As RangeVarGetCreationNamespace, but with a permissions check.
+ */
+Oid
+RangeVarGetAndCheckCreationNamespace(const RangeVar *newRelation)
+{
+	Oid			namespaceId;
+
+	namespaceId = RangeVarGetCreationNamespace(newRelation);
+
+	/*
+	 * Check we have permission to create there. Skip check if bootstrapping,
+	 * since permissions machinery may not be working yet.
+	 */
+	if (!IsBootstrapProcessingMode())
+	{
+		AclResult	aclresult;
+
+		aclresult = pg_namespace_aclcheck(namespaceId, GetUserId(),
+										  ACL_CREATE);
+		if (aclresult != ACLCHECK_OK)
+			aclcheck_error(aclresult, ACL_KIND_NAMESPACE,
+						   get_namespace_name(namespaceId));
+	}
+
+	return namespaceId;
+}
+
 /*
  * RelnameGetRelid
  *		Try to resolve an unqualified relation name.

src/backend/catalog/toasting.c

@@ -227,8 +227,7 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid, Datum reloptio
 										   ONCOMMIT_NOOP,
 										   reloptions,
 										   false,
-										   true,
-										   false);
+										   true);
 	Assert(toast_relid != InvalidOid);
 
 	/* make the toast relation visible, else heap_open will fail */

src/backend/commands/cluster.c

@@ -646,8 +646,7 @@ make_new_heap(Oid OIDOldHeap, Oid NewTableSpace)
 										  ONCOMMIT_NOOP,
 										  reloptions,
 										  false,
-										  true,
-										  false);
+										  true);
 	Assert(OIDNewHeap != InvalidOid);
 
 	ReleaseSysCache(tuple);

src/backend/commands/tablecmds.c

@@ -439,22 +439,10 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId)
 				 errmsg("cannot create temporary table within security-restricted operation")));
 
 	/*
-	 * Look up the namespace in which we are supposed to create the relation.
-	 * Check we have permission to create there. Skip check if bootstrapping,
-	 * since permissions machinery may not be working yet.
+	 * Look up the namespace in which we are supposed to create the relation,
+	 * and check we have permission to create there.
 	 */
-	namespaceId = RangeVarGetCreationNamespace(stmt->relation);
-
-	if (!IsBootstrapProcessingMode())
-	{
-		AclResult	aclresult;
-
-		aclresult = pg_namespace_aclcheck(namespaceId, GetUserId(),
-										  ACL_CREATE);
-		if (aclresult != ACLCHECK_OK)
-			aclcheck_error(aclresult, ACL_KIND_NAMESPACE,
-						   get_namespace_name(namespaceId));
-	}
+	namespaceId = RangeVarGetAndCheckCreationNamespace(stmt->relation);
 
 	/*
 	 * Select tablespace to use.  If not specified, use default tablespace
@@ -602,16 +590,7 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId)
 										  stmt->oncommit,
 										  reloptions,
 										  true,
-										  allowSystemTableMods,
-										  stmt->if_not_exists);
-
-	/*
-	 * If heap_create_with_catalog returns InvalidOid, it means that the user
-	 * specified "IF NOT EXISTS" and the relation already exists.  In that
-	 * case we do nothing further.
-	 */
-	if (relationId == InvalidOid)
-		return InvalidOid;
+										  allowSystemTableMods);
 
 	/* Store inheritance information for new rel. */
 	StoreCatalogInheritance(relationId, inheritOids);

src/backend/executor/execMain.c

@@ -2341,7 +2341,6 @@ OpenIntoRel(QueryDesc *queryDesc)
 	Oid			namespaceId;
 	Oid			tablespaceId;
 	Datum		reloptions;
-	AclResult	aclresult;
 	Oid			intoRelationId;
 	TupleDesc	tupdesc;
 	DR_intorel *myState;
@@ -2378,13 +2377,7 @@ OpenIntoRel(QueryDesc *queryDesc)
 	 * Find namespace to create in, check its permissions
 	 */
 	intoName = into->rel->relname;
-	namespaceId = RangeVarGetCreationNamespace(into->rel);
-
-	aclresult = pg_namespace_aclcheck(namespaceId, GetUserId(),
-									  ACL_CREATE);
-	if (aclresult != ACLCHECK_OK)
-		aclcheck_error(aclresult, ACL_KIND_NAMESPACE,
-					   get_namespace_name(namespaceId));
+	namespaceId = RangeVarGetAndCheckCreationNamespace(into->rel);
 
 	/*
 	 * Select tablespace to use.  If not specified, use default tablespace
@@ -2444,8 +2437,7 @@ OpenIntoRel(QueryDesc *queryDesc)
 											  into->onCommit,
 											  reloptions,
 											  true,
-											  allowSystemTableMods,
-											  false);
+											  allowSystemTableMods);
 	Assert(intoRelationId != InvalidOid);
 
 	FreeTupleDesc(tupdesc);

src/backend/parser/parse_utilcmd.c

@@ -148,13 +148,41 @@ transformCreateStmt(CreateStmt *stmt, const char *queryString)
 	List	   *result;
 	List	   *save_alist;
 	ListCell   *elements;
+	Oid			namespaceid;
 
 	/*
 	 * We must not scribble on the passed-in CreateStmt, so copy it.  (This is
 	 * overkill, but easy.)
 	 */
 	stmt = (CreateStmt *) copyObject(stmt);
 
+	/*
+	 * Look up the creation namespace.  This also checks permissions on the
+	 * target namespace, so that we throw any permissions error as early as
+	 * possible.
+	 */
+	namespaceid = RangeVarGetAndCheckCreationNamespace(stmt->relation);
+
+	/*
+	 * If the relation already exists and the user specified "IF NOT EXISTS",
+	 * bail out with a NOTICE.
+	 */
+	if (stmt->if_not_exists)
+	{
+		Oid		existing_relid;
+
+		existing_relid = get_relname_relid(stmt->relation->relname,
+										   namespaceid);
+		if (existing_relid != InvalidOid)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_TABLE),
+					 errmsg("relation \"%s\" already exists, skipping",
+						 stmt->relation->relname)));
+			return NIL;
+		}
+	}
+
 	/*
 	 * If the target relation name isn't schema-qualified, make it so.  This
 	 * prevents some corner cases in which added-on rewritten commands might
@@ -164,11 +192,7 @@ transformCreateStmt(CreateStmt *stmt, const char *queryString)
 	 */
 	if (stmt->relation->schemaname == NULL
 		&& stmt->relation->relpersistence != RELPERSISTENCE_TEMP)
-	{
-		Oid			namespaceid = RangeVarGetCreationNamespace(stmt->relation);
-
 		stmt->relation->schemaname = get_namespace_name(namespaceid);
-	}
 
 	/* Set up pstate and CreateStmtContext */
 	pstate = make_parsestate(NULL);

src/backend/tcop/utility.c

@@ -529,13 +529,6 @@ standard_ProcessUtility(Node *parsetree,
 												RELKIND_RELATION,
 												InvalidOid);
 
-						/*
-						 * If "IF NOT EXISTS" was specified and the relation
-						 * already exists, do nothing further.
-						 */
-						if (relOid == InvalidOid)
-							continue;
-
 						/*
 						 * Let AlterTableCreateToastTable decide if this one
 						 * needs a secondary relation too.
@@ -559,15 +552,8 @@ standard_ProcessUtility(Node *parsetree,
 						relOid = DefineRelation((CreateStmt *) stmt,
 												RELKIND_FOREIGN_TABLE,
 												InvalidOid);
-
-						/*
-						 * Unless "IF NOT EXISTS" was specified and the
-						 * relation already exists, create the
-						 * pg_foreign_table entry.
-						 */
-						if (relOid != InvalidOid)
-							CreateForeignTable((CreateForeignTableStmt *) stmt,
-											   relOid);
+						CreateForeignTable((CreateForeignTableStmt *) stmt,
+										   relOid);
 					}
 					else
 					{

src/include/catalog/heap.h

@@ -63,8 +63,7 @@ extern Oid heap_create_with_catalog(const char *relname,
 						 OnCommitAction oncommit,
 						 Datum reloptions,
 						 bool use_user_acl,
-						 bool allow_system_table_mods,
-						 bool if_not_exists);
+						 bool allow_system_table_mods);
 
 extern void heap_drop_with_catalog(Oid relid);
 

src/include/catalog/namespace.h

@@ -49,6 +49,7 @@ typedef struct OverrideSearchPath
 
 extern Oid	RangeVarGetRelid(const RangeVar *relation, bool failOK);
 extern Oid	RangeVarGetCreationNamespace(const RangeVar *newRelation);
+extern Oid	RangeVarGetAndCheckCreationNamespace(const RangeVar *newRelation);
 extern Oid	RelnameGetRelid(const char *relname);
 extern bool RelationIsVisible(Oid relid);