Disallow USING clause when altering type of generated column

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 <[email protected]>
Reviewed-by: Yugo NAGATA <[email protected]>
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
index d3d9e2a89b442516a794e6ed91ba2e667427a5dd..78d529c22c022370379fd1cc628edce91b9f3937 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -10953,6 +10953,16 @@ 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
@@ -11029,11 +11039,12 @@ 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
index 242cb6fc553f60edaa10d1e3c1acac58d03dfcdd..1c06683c9d09bb680f1022bc8a182d13d25c04e3 100644 (file)
--- a/src/test/regress/expected/generated.out
+++ b/src/test/regress/expected/generated.out
@@ -747,7 +747,8 @@ 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
 -- It's possible to alter the column types this way: