Disallow USING clause when altering type of generated column

petere · petere · commit cf49a606c416 · 2024-08-29T09:01:02.000+02:00
This does not make sense. It would write the output of the USING clause into the converted column, which would violate the generation expression. This adds a check to error out if this is specified. There was a test for this, but that test errored out for a different reason, so it was not effective. Reported-by: Jian He <jian.universality@gmail.com> Reviewed-by: Yugo NAGATA <nagata@sraoss.co.jp> Discussion: https://www.postgresql.org/message-id/flat/c7083982-69f4-4b14-8315-f9ddb20b9834%40eisentraut.org
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
@@ -12307,6 +12307,16 @@ ATPrepAlterColumnType(List **wqueue,
 				 errmsg("cannot alter system column \"%s\"",
 						colName)));
 
+	/*
+	 * Cannot specify USING when altering type of a generated column, because
+	 * that would violate the generation expression.
+	 */
+	if (attTup->attgenerated && def->cooked_default)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_COLUMN_DEFINITION),
+				 errmsg("cannot specify USING when altering type of generated column"),
+				 errdetail("Column \"%s\" is a generated column.", colName)));
+
 	/*
 	 * Don't alter inherited columns.  At outer level, there had better not be
 	 * any inherited definition; when recursing, we assume this was checked at
@@ -12383,11 +12393,12 @@ ATPrepAlterColumnType(List **wqueue,
 						(errcode(ERRCODE_DATATYPE_MISMATCH),
 						 errmsg("column \"%s\" cannot be cast automatically to type %s",
 								colName, format_type_be(targettype)),
+						 !attTup->attgenerated ?
 				/* translator: USING is SQL, don't translate it */
 						 errhint("You might need to specify \"USING %s::%s\".",
 								 quote_identifier(colName),
 								 format_type_with_typemod(targettype,
-														  targettypmod))));
+														  targettypmod)) : 0));
 		}
 
 		/* Fix collations after all else */
diff --git a/src/test/regress/expected/generated.out b/src/test/regress/expected/generated.out
@@ -836,7 +836,8 @@ SELECT * FROM gtest27;
 (2 rows)
 
 ALTER TABLE gtest27 ALTER COLUMN x TYPE boolean USING x <> 0;  -- error
-ERROR:  generation expression for column "x" cannot be cast automatically to type boolean
+ERROR:  cannot specify USING when altering type of generated column
+DETAIL:  Column "x" is a generated column.
 ALTER TABLE gtest27 ALTER COLUMN x DROP DEFAULT;  -- error
 ERROR:  column "x" of relation "gtest27" is a generated column
 HINT:  Use ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION instead.
