Disallow using alter_table udfs with tables having any identity cols (#4635)

pg_get_tableschemadef_string doesn't know how to deparse identity
columns so we cannot reflect those columns when creating table
from scratch. For this reason, we don't allow using alter_table udfs
with tables having any identity cols.

src/backend/distributed/commands/alter_table.c

@@ -909,6 +909,20 @@ CreateTableConversion(TableConversionParameters *params)
 		ereport(ERROR, (errmsg("cannot complete operation "
 							   "because no such table exists")));
 	}
+
+	TupleDesc relationDesc = RelationGetDescr(relation);
+	if (RelationUsesIdentityColumns(relationDesc))
+	{
+		/*
+		 * pg_get_tableschemadef_string doesn't know how to deparse identity
+		 * columns so we cannot reflect those columns when creating table
+		 * from scratch. For this reason, error out here.
+		 */
+		ereport(ERROR, (errmsg("cannot complete command because relation "
+							   "%s has identity column",
+							   generate_qualified_relation_name(con->relationId)),
+						errhint("Drop the identity columns and re-try the command")));
+	}
 	relation_close(relation, NoLock);
 	con->distributionKey =
 		BuildDistributionKeyFromColumnName(relation, con->distributionColumn);

src/test/regress/expected/alter_table_set_access_method.out

@@ -676,6 +676,11 @@ SELECT relname, relkind
  v_ref   | v
 (6 rows)
 
+CREATE TABLE identity_cols_test (a int, b int generated by default as identity (increment by 42));
+-- errors out since we don't support alter_table.* udfs with tables having any identity columns
+SELECT alter_table_set_access_method('identity_cols_test', 'columnar');
+ERROR:  cannot complete command because relation alter_table_set_access_method.identity_cols_test has identity column
+HINT:  Drop the identity columns and re-try the command
 SET client_min_messages TO WARNING;
 DROP SCHEMA alter_table_set_access_method CASCADE;
 SELECT 1 FROM master_remove_node('localhost', :master_port);

src/test/regress/sql/alter_table_set_access_method.sql

@@ -209,6 +209,10 @@ SELECT relname, relkind
 	)
 	ORDER BY relname ASC;
 
+CREATE TABLE identity_cols_test (a int, b int generated by default as identity (increment by 42));
+-- errors out since we don't support alter_table.* udfs with tables having any identity columns
+SELECT alter_table_set_access_method('identity_cols_test', 'columnar');
+
 SET client_min_messages TO WARNING;
 DROP SCHEMA alter_table_set_access_method CASCADE;
 SELECT 1 FROM master_remove_node('localhost', :master_port);