disallow set type using (expr)

Signed-off-by: Aykut Bozkurt <aykut.bozkurt@snowflake.com>

Author: sfc-gh-abozkurt

pg_lake_table/src/ddl/alter_table.c

@@ -872,14 +872,29 @@ ErrorIfUnsupportedAlterWritablePgLakeTableStmt(AlterTableStmt *alterStmt,
 			if (cmd->subtype == AT_AlterColumnType &&
 				tableType == PG_LAKE_ICEBERG_TABLE_TYPE)
 			{
-				ereport(ERROR,
-						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-						 errmsg("ALTER TABLE %s command not supported for "
-								"%s tables", cmdTypeStr, tableTypeStr),
-						 errdetail("Allowed type promotions for Iceberg tables are: "
-								   "int -> bigint, float -> double, and "
-								   "decimal(P,S) -> decimal(P',S) where P' > P "
-								   "(wider precision, same scale).")));
+				ColumnDef  *def = (ColumnDef *) cmd->def;
+
+				if (def->raw_default != NULL)
+				{
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("ALTER TABLE %s command not supported for "
+									"%s tables", cmdTypeStr, tableTypeStr),
+							 errdetail("USING requires rewriting data files, "
+									   "but Iceberg schema evolution is a "
+									   "metadata-only operation.")));
+				}
+				else
+				{
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("ALTER TABLE %s command not supported for "
+									"%s tables", cmdTypeStr, tableTypeStr),
+							 errdetail("Allowed type promotions for Iceberg tables "
+									   "are: int -> bigint, float -> double, and "
+									   "decimal(P,S) -> decimal(P',S) where P' > P "
+									   "(wider precision, same scale).")));
+				}
 			}
 			else
 			{
@@ -1108,6 +1123,13 @@ AllowedAlterColumnTypeForIceberg(Node *arg, Oid relationId)
 	ColumnDef  *def = (ColumnDef *) cmd->def;
 	TypeName   *newTypeName = def->typeName;
 
+	/*
+	 * Reject USING clause. USING requires rewriting data files, but Iceberg
+	 * schema evolution is metadata-only so data files are never rewritten.
+	 */
+	if (def->raw_default != NULL)
+		return false;
+
 	/* resolve the new type OID and typmod */
 	int32		newTypMod = 0;
 	Oid			newTypeOid = InvalidOid;

pg_lake_table/tests/pytests/test_iceberg_ddl.py

@@ -1832,6 +1832,48 @@ def test_alter_column_type_disallowed_promotions(pg_conn, s3, with_default_locat
     pg_conn.commit()
 
 
+def test_alter_column_type_using_clause_rejected(pg_conn, s3, with_default_location):
+    """Test that USING clause is rejected for Iceberg type promotions.
+
+    Iceberg schema evolution is metadata-only; data files are never rewritten,
+    so arbitrary USING expressions are not supported.
+    """
+    run_command("CREATE SCHEMA test_alter_using;", pg_conn)
+    run_command(
+        "CREATE TABLE test_alter_using.tbl (a int, b real) USING iceberg;",
+        pg_conn,
+    )
+    run_command("INSERT INTO test_alter_using.tbl VALUES (1, 1.5);", pg_conn)
+    pg_conn.commit()
+
+    # USING with an otherwise-allowed promotion (int -> bigint)
+    error = run_command(
+        "ALTER TABLE test_alter_using.tbl ALTER COLUMN a TYPE bigint USING a::bigint;",
+        pg_conn,
+        raise_error=False,
+    )
+    assert "command not supported for pg_lake_iceberg tables" in str(error)
+    assert "USING requires rewriting data files" in str(error)
+    pg_conn.rollback()
+
+    # USING with an expression on an allowed promotion (float -> double)
+    error = run_command(
+        "ALTER TABLE test_alter_using.tbl ALTER COLUMN b TYPE double precision USING b * 2;",
+        pg_conn,
+        raise_error=False,
+    )
+    assert "command not supported for pg_lake_iceberg tables" in str(error)
+    assert "USING requires rewriting data files" in str(error)
+    pg_conn.rollback()
+
+    # Verify the table is intact (nothing changed)
+    result = run_query("SELECT a, b FROM test_alter_using.tbl;", pg_conn)
+    assert result == [[1, 1.5]]
+
+    run_command("DROP SCHEMA test_alter_using CASCADE;", pg_conn)
+    pg_conn.commit()
+
+
 def test_alter_column_type_same_type_noop(pg_conn, s3, with_default_location):
     """Test that changing to the same type is allowed (no-op)"""
     run_command("CREATE SCHEMA test_alter_type_noop;", pg_conn)