From f2bbcafc8fe7d1dbdf3c6fd49a34975fca9d804d Mon Sep 17 00:00:00 2001
From: "Paul A. Jungwirth" <pj@illuminatedcomputing.com>
Date: Tue, 12 May 2026 16:33:42 -0700
Subject: [PATCH v1] Forbid GENERATED columns in FOR PORTION OF

With VIRTUAL columns there is no column to assign to, and we shouldn't
assign directly to STORED columns either. (Once we have PERIODs, we will
allow a STORED column here, but we will assign to its start/end inputs.)

Discussion: https://postgr.es/m/agOOykf2HV26yVfU%40nathan
---
 src/backend/parser/analyze.c                 | 13 +++++++++
 src/test/regress/expected/for_portion_of.out | 30 ++++++++++++++++++++
 src/test/regress/sql/for_portion_of.sql      | 21 ++++++++++++++
 3 files changed, 64 insertions(+)

diff --git a/src/backend/parser/analyze.c b/src/backend/parser/analyze.c
index ffcf25a6be7..12b64b01e18 100644
--- a/src/backend/parser/analyze.c
+++ b/src/backend/parser/analyze.c
@@ -1354,6 +1354,19 @@ transformForPortionOfClause(ParseState *pstate,
 				 parser_errposition(pstate, forPortionOf->location)));
 	attr = TupleDescAttr(targetrel->rd_att, range_attno - 1);
 
+	/*
+	 * Reject generated columns. We can't write to a virtual generated column,
+	 * and a stored generated column should be written by its own expression.
+	 * XXX: We plan to implement PERIODs as stored generated columns, so later
+	 * we will loosen this restriction if the column belongs to a PERIOD.
+	 */
+	if (attr->attgenerated)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot use generated column \"%s\" in FOR PORTION OF",
+						forPortionOf->range_name),
+				 parser_errposition(pstate, forPortionOf->location)));
+
 	attbasetype = getBaseType(attr->atttypid);
 
 	rangeVar = makeVar(rtindex,
diff --git a/src/test/regress/expected/for_portion_of.out b/src/test/regress/expected/for_portion_of.out
index 0c0a205c44b..9979d816972 100644
--- a/src/test/regress/expected/for_portion_of.out
+++ b/src/test/regress/expected/for_portion_of.out
@@ -2152,4 +2152,34 @@ SELECT * FROM fpo_rule ORDER BY f1;
 (2 rows)
 
 DROP TABLE fpo_rule;
+-- UPDATE/DELETE FOR PORTION OF on a GENERATED VIRTUAL range column:
+CREATE TABLE fpo_gen_virtual (
+  a int,
+  b int4range GENERATED ALWAYS AS (int4range(a, a + 1)) VIRTUAL
+);
+INSERT INTO fpo_gen_virtual VALUES (1);
+DELETE FROM fpo_gen_virtual FOR PORTION OF b FROM 1 TO 2; -- fails
+ERROR:  cannot use generated column "b" in FOR PORTION OF
+LINE 1: DELETE FROM fpo_gen_virtual FOR PORTION OF b FROM 1 TO 2;
+                                                   ^
+UPDATE fpo_gen_virtual FOR PORTION OF b FROM 1 TO 2 SET a = 5; -- fails
+ERROR:  cannot use generated column "b" in FOR PORTION OF
+LINE 1: UPDATE fpo_gen_virtual FOR PORTION OF b FROM 1 TO 2 SET a = ...
+                                              ^
+DROP TABLE fpo_gen_virtual;
+-- UPDATE/DELETE FOR PORTION OF on a GENERATED STORED range column:
+CREATE TABLE fpo_gen_stored (
+  a int,
+  b int4range GENERATED ALWAYS AS (int4range(a, a + 1)) STORED
+);
+INSERT INTO fpo_gen_stored VALUES (1);
+DELETE FROM fpo_gen_stored FOR PORTION OF b FROM 1 TO 2; -- fails
+ERROR:  cannot use generated column "b" in FOR PORTION OF
+LINE 1: DELETE FROM fpo_gen_stored FOR PORTION OF b FROM 1 TO 2;
+                                                  ^
+UPDATE fpo_gen_stored FOR PORTION OF b FROM 1 TO 2 SET a = 5; -- fails
+ERROR:  cannot use generated column "b" in FOR PORTION OF
+LINE 1: UPDATE fpo_gen_stored FOR PORTION OF b FROM 1 TO 2 SET a = 5...
+                                             ^
+DROP TABLE fpo_gen_stored;
 RESET datestyle;
diff --git a/src/test/regress/sql/for_portion_of.sql b/src/test/regress/sql/for_portion_of.sql
index fd79a9b78e7..81bdeb8d13a 100644
--- a/src/test/regress/sql/for_portion_of.sql
+++ b/src/test/regress/sql/for_portion_of.sql
@@ -1398,4 +1398,25 @@ SELECT * FROM fpo_rule ORDER BY f1;
 
 DROP TABLE fpo_rule;
 
+-- UPDATE/DELETE FOR PORTION OF on a GENERATED VIRTUAL range column:
+CREATE TABLE fpo_gen_virtual (
+  a int,
+  b int4range GENERATED ALWAYS AS (int4range(a, a + 1)) VIRTUAL
+);
+INSERT INTO fpo_gen_virtual VALUES (1);
+DELETE FROM fpo_gen_virtual FOR PORTION OF b FROM 1 TO 2; -- fails
+UPDATE fpo_gen_virtual FOR PORTION OF b FROM 1 TO 2 SET a = 5; -- fails
+DROP TABLE fpo_gen_virtual;
+
+-- UPDATE/DELETE FOR PORTION OF on a GENERATED STORED range column:
+CREATE TABLE fpo_gen_stored (
+  a int,
+  b int4range GENERATED ALWAYS AS (int4range(a, a + 1)) STORED
+);
+INSERT INTO fpo_gen_stored VALUES (1);
+DELETE FROM fpo_gen_stored FOR PORTION OF b FROM 1 TO 2; -- fails
+UPDATE fpo_gen_stored FOR PORTION OF b FROM 1 TO 2 SET a = 5; -- fails
+DROP TABLE fpo_gen_stored;
+
+
 RESET datestyle;
-- 
2.47.3