From ccc7c3ad86b67cd85805824d40782fa5b92b1fc5 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 6 Jan 2022 16:46:46 -0500
Subject: [PATCH] Prevent altering partitioned table's rowtype, if it's used
 elsewhere.

We disallow altering a column datatype within a regular table,
if the table's rowtype is used as a column type elsewhere,
because we lack code to go around and rewrite the other tables.
This restriction should apply to partitioned tables as well, but it
was not checked because ATRewriteTables and ATPrepAlterColumnType
were not on the same page about who should do it for which relkinds.

Per bug #17351 from Alexander Lakhin.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/17351-6db1870f3f4f612a@postgresql.org
---
 src/backend/commands/tablecmds.c          |  7 +++----
 src/test/regress/expected/alter_table.out | 23 ++++++++++++++++++-----
 src/test/regress/sql/alter_table.sql      | 19 +++++++++++++++----
 3 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 5705674492..5942905cf5 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -10852,12 +10852,11 @@ ATPrepAlterColumnType(List **wqueue,
 				 errmsg("\"%s\" is not a table",
 						RelationGetRelationName(rel))));
 
-	if (tab->relkind == RELKIND_COMPOSITE_TYPE ||
-		tab->relkind == RELKIND_FOREIGN_TABLE)
+	if (!RELKIND_HAS_STORAGE(tab->relkind))
 	{
 		/*
-		 * For composite types, do this check now.  Tables will check it later
-		 * when the table is being rewritten.
+		 * For relations without storage, do this check now.  Regular tables
+		 * will check it later when the table is being rewritten.
 		 */
 		find_composite_type_dependencies(rel->rd_rel->reltype, rel, NULL);
 	}
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 065ac77730..5c462d8e66 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -2050,11 +2050,24 @@ select * from another;
 (3 rows)
 
 drop table another;
--- table's row type
-create table tab1 (a int, b text);
-create table tab2 (x int, y tab1);
-alter table tab1 alter column b type varchar; -- fails
-ERROR:  cannot alter table "tab1" because column "tab2.y" uses its row type
+-- We disallow changing table's row type if it's used for storage
+create table at_tab1 (a int, b text);
+create table at_tab2 (x int, y at_tab1);
+alter table at_tab1 alter column b type varchar; -- fails
+ERROR:  cannot alter table "at_tab1" because column "at_tab2.y" uses its row type
+drop table at_tab2;
+-- Use of row type in an expression is defended differently
+create table at_tab2 (x int, y text, check((x,y)::at_tab1 = (1,'42')::at_tab1));
+alter table at_tab1 alter column b type varchar; -- allowed, but ...
+insert into at_tab2 values(1,'42'); -- ... this will fail
+ERROR:  ROW() column has type text instead of type character varying
+drop table at_tab1, at_tab2;
+-- Check it for a partitioned table, too
+create table at_tab1 (a int, b text) partition by list(a);
+create table at_tab2 (x int, y at_tab1);
+alter table at_tab1 alter column b type varchar; -- fails
+ERROR:  cannot alter table "at_tab1" because column "at_tab2.y" uses its row type
+drop table at_tab1, at_tab2;
 -- Alter column type that's part of a partitioned index
 create table at_partitioned (a int, b text) partition by range (a);
 create table at_part_1 partition of at_partitioned for values from (0) to (1000);
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index fa8308a1e1..5467683290 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1411,10 +1411,21 @@ select * from another;
 
 drop table another;
 
--- table's row type
-create table tab1 (a int, b text);
-create table tab2 (x int, y tab1);
-alter table tab1 alter column b type varchar; -- fails
+-- We disallow changing table's row type if it's used for storage
+create table at_tab1 (a int, b text);
+create table at_tab2 (x int, y at_tab1);
+alter table at_tab1 alter column b type varchar; -- fails
+drop table at_tab2;
+-- Use of row type in an expression is defended differently
+create table at_tab2 (x int, y text, check((x,y)::at_tab1 = (1,'42')::at_tab1));
+alter table at_tab1 alter column b type varchar; -- allowed, but ...
+insert into at_tab2 values(1,'42'); -- ... this will fail
+drop table at_tab1, at_tab2;
+-- Check it for a partitioned table, too
+create table at_tab1 (a int, b text) partition by list(a);
+create table at_tab2 (x int, y at_tab1);
+alter table at_tab1 alter column b type varchar; -- fails
+drop table at_tab1, at_tab2;
 
 -- Alter column type that's part of a partitioned index
 create table at_partitioned (a int, b text) partition by range (a);