release-20.2: sql: ban renaming table to a new database unless both schemas are public

pkg/sql/logictest/testdata/logic_test/rename_table

@@ -261,3 +261,59 @@ query TTTI
 SHOW TABLES
 ----
 public  kv  table  0
+
+# Test that tables can't be renamed to a different database unless both the
+# old and new schemas are in the public schema.
+subtest rename_table_across_dbs
+
+statement ok
+CREATE DATABASE olddb
+
+statement ok
+CREATE DATABASE newdb
+
+statement ok
+USE olddb
+
+statement ok
+CREATE SCHEMA oldsc
+
+statement ok
+USE newdb
+
+statement ok
+CREATE SCHEMA newsc
+
+statement ok
+CREATE TABLE olddb.public.tbl1();
+
+statement ok
+CREATE TABLE olddb.oldsc.tbl2();
+
+statement error pgcode 42602 cannot change database of table
+ALTER TABLE olddb.public.tbl1 RENAME TO newdb.newsc.tbl1
+
+statement error pgcode 42602 cannot change database of table
+ALTER TABLE olddb.oldsc.tbl2 RENAME TO newdb.public.tbl2
+
+statement error pgcode 42602 cannot change database of table
+ALTER TABLE olddb.oldsc.tbl2 RENAME TO newdb.newsc.tbl2
+
+# Try different but equivalent names.
+
+statement error pgcode 42602 cannot change database of table
+ALTER TABLE olddb.tbl1 RENAME TO newdb.newsc.tbl1
+
+statement error pgcode 42602 cannot change database of table
+ALTER TABLE olddb.oldsc.tbl2 RENAME TO newdb.tbl2
+
+# Using the public schemas should still work.
+
+statement ok
+ALTER TABLE olddb.public.tbl1 RENAME TO newdb.public.tbl1
+
+statement ok
+DROP DATABASE olddb CASCADE
+
+statement ok
+DROP DATABASE newdb CASCADE

pkg/sql/rename_table.go

@@ -144,6 +144,15 @@ func (n *renameTableNode) startExec(params runParams) error {
 		)
 	}
 
+	// Disable renaming objects between databases unless both the source and
+	// target schemas are the public schema. This preserves backward compatibility
+	// for the behavior prior to user-defined schemas.
+	if oldTn.Catalog() != newTn.Catalog() &&
+		(oldTn.Schema() != string(tree.PublicSchemaName) || newTn.Schema() != string(tree.PublicSchemaName)) {
+		return pgerror.Newf(pgcode.InvalidName,
+			"cannot change database of table unless both the old and new schemas are the public schema in each database")
+	}
+
 	// oldTn and newTn are already normalized, so we can compare directly here.
 	if oldTn.Catalog() == newTn.Catalog() &&
 		oldTn.Schema() == newTn.Schema() &&
@@ -167,6 +176,10 @@ func (n *renameTableNode) startExec(params runParams) error {
 		return err
 	}
 
+	// The parent schema ID is never modified here because changing the schema of
+	// a table within the same database is disallowed, and changing the database
+	// of a table is only allowed if both the source and target schemas are the
+	// public schema.
 	tableDesc.SetName(newTn.Table())
 	tableDesc.ParentID = targetDbDesc.GetID()