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 
Reviewed-by: Yugo NAGATA 
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 44629fb52f42d3130e4d48872b30a32e26af2a2a..4f6bf4c959c6cdd25c3da1690e6f9fcda5bf191d 100644 (file)
--- 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
index 10208484ee5bfaa89a2856eedc353230e0593426..9ee4559bd5a658daeade6803aa1a2c3bfd10148d 100644 (file)
--- 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.