Error out for ALTER TABLE ... SET ACCESS METHOD DEFAULT (#7803)

PG17 introduced ALTER TABLE ... SET ACCESS METHOD DEFAULT

This PR introduces and enforces an error check preventing ALTER TABLE
... SET ACCESS METHOD DEFAULT on both Citus local tables (added via
citus_add_local_table_to_metadata) and distributed/partitioned
distributed tables. The regression tests now demonstrate that each table
type raises an error advising users to explicitly specify an access
method, rather than relying on DEFAULT. This ensures consistent behavior
across local and distributed environments in Citus.

The reason why we currently don't support this is that we can't simply
propagate the command as it is, because the default table access method
may be different across Citus cluster nodes.

Relevant PG commit:
https://github.com/postgres/postgres/commit/d61a6cad6

src/backend/distributed/commands/table.c

@@ -3666,6 +3666,24 @@ ErrorIfUnsupportedAlterTableStmt(AlterTableStmt *alterTableStatement)
 
 #if PG_VERSION_NUM >= PG_VERSION_15
 			case AT_SetAccessMethod:
+			{
+				/*
+				 * If command->name == NULL, that means the user is trying to use
+				 * ALTER TABLE ... SET ACCESS METHOD DEFAULT
+				 * which we don't support currently.
+				 */
+				if (command->name == NULL)
+				{
+					ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+									errmsg(
+										"DEFAULT option in ALTER TABLE ... SET ACCESS METHOD "
+										"is currently unsupported."),
+									errhint(
+										"You can rerun the command by explicitly writing the access method name.")));
+				}
+				break;
+			}
+
 #endif
 			case AT_SetNotNull:
 			case AT_ReplicaIdentity:

src/test/regress/expected/pg17.out

@@ -1333,6 +1333,49 @@ SELECT b, c FROM forcetest WHERE a = 6;
 
 \pset null ''
 -- End of Testing FORCE_NOT_NULL and FORCE_NULL options
+-- Test for ALTER TABLE SET ACCESS METHOD DEFAULT
+-- Step 1: Local table setup (non-distributed)
+CREATE TABLE test_local_table (id int);
+SELECT citus_add_local_table_to_metadata('test_local_table');
+ citus_add_local_table_to_metadata
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Step 2: Attempt to set access method to DEFAULT on a Citus local table (should fail)
+ALTER TABLE test_local_table SET ACCESS METHOD DEFAULT;
+ERROR:  DEFAULT option in ALTER TABLE ... SET ACCESS METHOD is currently unsupported.
+HINT:  You can rerun the command by explicitly writing the access method name.
+-- Step 3: Setup: create and distribute a table
+CREATE TABLE test_alter_access_method (id int);
+SELECT create_distributed_table('test_alter_access_method', 'id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Step 4: Attempt to set access method to DEFAULT on a distributed table (should fail with your custom error)
+ALTER TABLE test_alter_access_method SET ACCESS METHOD DEFAULT;
+ERROR:  DEFAULT option in ALTER TABLE ... SET ACCESS METHOD is currently unsupported.
+HINT:  You can rerun the command by explicitly writing the access method name.
+-- Step 5: Create and distribute a partitioned table
+CREATE TABLE test_partitioned_alter (id int, val text) PARTITION BY RANGE (id);
+CREATE TABLE test_partitioned_alter_part1 PARTITION OF test_partitioned_alter FOR VALUES FROM (1) TO (100);
+SELECT create_distributed_table('test_partitioned_alter', 'id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Step 6: Attempt to set access method to DEFAULT on a partitioned, distributed table (should fail)
+ALTER TABLE test_partitioned_alter SET ACCESS METHOD DEFAULT;
+ERROR:  DEFAULT option in ALTER TABLE ... SET ACCESS METHOD is currently unsupported.
+HINT:  You can rerun the command by explicitly writing the access method name.
+-- Cleanup
+DROP TABLE test_local_table CASCADE;
+DROP TABLE test_alter_access_method CASCADE;
+DROP TABLE test_partitioned_alter CASCADE;
+-- End of Test for ALTER TABLE SET ACCESS METHOD DEFAULT
 \set VERBOSITY terse
 SET client_min_messages TO WARNING;
 DROP SCHEMA pg17 CASCADE;

src/test/regress/sql/pg17.sql

@@ -711,6 +711,36 @@ SELECT b, c FROM forcetest WHERE a = 6;
 
 -- End of Testing FORCE_NOT_NULL and FORCE_NULL options
 
+-- Test for ALTER TABLE SET ACCESS METHOD DEFAULT
+-- Step 1: Local table setup (non-distributed)
+CREATE TABLE test_local_table (id int);
+
+SELECT citus_add_local_table_to_metadata('test_local_table');
+
+-- Step 2: Attempt to set access method to DEFAULT on a Citus local table (should fail)
+ALTER TABLE test_local_table SET ACCESS METHOD DEFAULT;
+
+-- Step 3: Setup: create and distribute a table
+CREATE TABLE test_alter_access_method (id int);
+SELECT create_distributed_table('test_alter_access_method', 'id');
+
+-- Step 4: Attempt to set access method to DEFAULT on a distributed table (should fail with your custom error)
+ALTER TABLE test_alter_access_method SET ACCESS METHOD DEFAULT;
+
+-- Step 5: Create and distribute a partitioned table
+CREATE TABLE test_partitioned_alter (id int, val text) PARTITION BY RANGE (id);
+CREATE TABLE test_partitioned_alter_part1 PARTITION OF test_partitioned_alter FOR VALUES FROM (1) TO (100);
+SELECT create_distributed_table('test_partitioned_alter', 'id');
+
+-- Step 6: Attempt to set access method to DEFAULT on a partitioned, distributed table (should fail)
+ALTER TABLE test_partitioned_alter SET ACCESS METHOD DEFAULT;
+
+-- Cleanup
+DROP TABLE test_local_table CASCADE;
+DROP TABLE test_alter_access_method CASCADE;
+DROP TABLE test_partitioned_alter CASCADE;
+-- End of Test for ALTER TABLE SET ACCESS METHOD DEFAULT
+
 \set VERBOSITY terse
 SET client_min_messages TO WARNING;
 DROP SCHEMA pg17 CASCADE;