Error out for ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION (#7814)

PG17 added support for
ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION.
Relevant PG commit: https://github.com/postgres/postgres/commit/5d06e99a3

We currently don't support propagating this command for Citus tables.
It is added to future work.

This PR disallows `ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION` on
all Citus table types (local, distributed, and partitioned distributed)
by adding an error check in `ErrorIfUnsupportedAlterTableStmt`. A new
regression test verifies that each table type fails with a consistent
error message when attempting to set an expression.

src/backend/distributed/commands/table.c

@@ -3664,6 +3664,17 @@ ErrorIfUnsupportedAlterTableStmt(AlterTableStmt *alterTableStatement)
 				break;
 			}
 
+#if PG_VERSION_NUM >= PG_VERSION_17
+			case AT_SetExpression:
+			{
+				ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+								errmsg(
+									"ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION commands "
+									"are currently unsupported.")));
+				break;
+			}
+
+#endif
 #if PG_VERSION_NUM >= PG_VERSION_15
 			case AT_SetAccessMethod:
 			{

src/test/regress/expected/pg17.out

@@ -1376,6 +1376,47 @@ 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
+-- Test for ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION
+-- Step 1: Local table setup (non-distributed)
+CREATE TABLE test_local_table_expr (id int, col int);
+SELECT citus_add_local_table_to_metadata('test_local_table_expr');
+ citus_add_local_table_to_metadata
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Step 2: Attempt to set expression on a Citus local table (should fail)
+ALTER TABLE test_local_table_expr ALTER COLUMN col SET EXPRESSION AS (id * 4);
+ERROR:  ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION commands are currently unsupported.
+-- Step 3: Create and distribute a table
+CREATE TABLE test_distributed_table_expr (id int, col int);
+SELECT create_distributed_table('test_distributed_table_expr', 'id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Step 4: Attempt to set expression on a distributed table (should fail)
+ALTER TABLE test_distributed_table_expr ALTER COLUMN col SET EXPRESSION AS (id * 4);
+ERROR:  ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION commands are currently unsupported.
+-- Step 5: Create and distribute a partitioned table
+CREATE TABLE test_partitioned_expr (id int, val text) PARTITION BY RANGE (id);
+CREATE TABLE test_partitioned_expr_part1 PARTITION OF test_partitioned_expr
+  FOR VALUES FROM (1) TO (100);
+SELECT create_distributed_table('test_partitioned_expr', 'id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Step 6: Attempt to set expression on a partitioned, distributed table (should fail)
+ALTER TABLE test_partitioned_expr ALTER COLUMN val SET EXPRESSION AS (id * 4);
+ERROR:  ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION commands are currently unsupported.
+-- Cleanup
+DROP TABLE test_local_table_expr CASCADE;
+DROP TABLE test_distributed_table_expr CASCADE;
+DROP TABLE test_partitioned_expr CASCADE;
+-- End of Test for ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION
 \set VERBOSITY terse
 SET client_min_messages TO WARNING;
 DROP SCHEMA pg17 CASCADE;

src/test/regress/sql/pg17.sql

@@ -741,6 +741,38 @@ DROP TABLE test_alter_access_method CASCADE;
 DROP TABLE test_partitioned_alter CASCADE;
 -- End of Test for ALTER TABLE SET ACCESS METHOD DEFAULT
 
+-- Test for ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION
+
+-- Step 1: Local table setup (non-distributed)
+CREATE TABLE test_local_table_expr (id int, col int);
+
+SELECT citus_add_local_table_to_metadata('test_local_table_expr');
+
+-- Step 2: Attempt to set expression on a Citus local table (should fail)
+ALTER TABLE test_local_table_expr ALTER COLUMN col SET EXPRESSION AS (id * 4);
+
+-- Step 3: Create and distribute a table
+CREATE TABLE test_distributed_table_expr (id int, col int);
+SELECT create_distributed_table('test_distributed_table_expr', 'id');
+
+-- Step 4: Attempt to set expression on a distributed table (should fail)
+ALTER TABLE test_distributed_table_expr ALTER COLUMN col SET EXPRESSION AS (id * 4);
+
+-- Step 5: Create and distribute a partitioned table
+CREATE TABLE test_partitioned_expr (id int, val text) PARTITION BY RANGE (id);
+CREATE TABLE test_partitioned_expr_part1 PARTITION OF test_partitioned_expr
+  FOR VALUES FROM (1) TO (100);
+SELECT create_distributed_table('test_partitioned_expr', 'id');
+
+-- Step 6: Attempt to set expression on a partitioned, distributed table (should fail)
+ALTER TABLE test_partitioned_expr ALTER COLUMN val SET EXPRESSION AS (id * 4);
+
+-- Cleanup
+DROP TABLE test_local_table_expr CASCADE;
+DROP TABLE test_distributed_table_expr CASCADE;
+DROP TABLE test_partitioned_expr CASCADE;
+-- End of Test for ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION
+
 \set VERBOSITY terse
 SET client_min_messages TO WARNING;
 DROP SCHEMA pg17 CASCADE;