v2-0001-Disallow-USING-clause-when-altering-type-of-gener.patch

From 4ae50b2ee8602b39749bcc074f91ff0a0112848e Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Thu, 22 Aug 2024 09:04:13 +0200
Subject: [PATCH v2] 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.

Discussion: https://www.postgresql.org/message-id/flat/c7083982-69f4-4b14-8315-f9ddb20b9834%40eisentraut.org
---
 src/backend/commands/tablecmds.c        | 13 ++++++++++++-
 src/test/regress/expected/generated.out |  3 ++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 7a36db6af6d..e10e539152b 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -12736,6 +12736,16 @@ 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_TABLE_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
@@ -12813,10 +12823,11 @@ ATPrepAlterColumnType(List **wqueue,
 						 errmsg("column \"%s\" cannot be cast automatically to type %s",
 								colName, format_type_be(targettype)),
 				/* translator: USING is SQL, don't translate it */
+						 !attTup->attgenerated ?
 						 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 44058db7c1d..499072e14ca 100644
--- a/src/test/regress/expected/generated.out
+++ b/src/test/regress/expected/generated.out
@@ -1026,7 +1026,8 @@ 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.
-- 
2.46.0