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commit    5867ee0056a2db40160926e1a4684d435a099ef4
Author:   Peter Eisentraut <peter@eisentraut.org>
Date:     Thu Aug 29 06:38:29 2024 +0000
First released in: 18.0, 12.21, 13.17, 14.14, 15.9, 16.5, 17.0

    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 <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


 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 8fd656bd3a8..b9ccbd49054 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -12333,6 +12333,16 @@ 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
@@ -12409,11 +12419,12 @@ 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 0f623f71192..58814203884 100644
--- a/src/test/regress/expected/generated.out
+++ b/src/test/regress/expected/generated.out
@@ -888,7 +888,8 @@ 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.


[parent: b39c5272c1d2]