Reject attempts to alter composite types used in indexes.

find_composite_type_dependencies() ignored indexes, which is a poor
decision because an expression index could have a stored column of
a composite (or other container) type even when the underlying table
does not.  Teach it to detect such cases and error out.  We have to
work a bit harder than for other relations because the pg_depend entry
won't identify the specific index column of concern, but it's not much
new code.

This does not address bug #17872's original complaint that dropping
a column in such a type might lead to violations of the uniqueness
property that a unique index is supposed to ensure.  That seems of
much less concern to me because it won't lead to crashes.

Per bug #17872 from Alexander Lakhin.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/17872-d0fbb799dc3fd85d@postgresql.org

src/backend/commands/tablecmds.c

@@ -6508,6 +6508,7 @@ find_composite_type_dependencies(Oid typeOid, Relation origRelation,
 	{
 		Form_pg_depend pg_depend = (Form_pg_depend) GETSTRUCT(depTup);
 		Relation	rel;
+		TupleDesc	tupleDesc;
 		Form_pg_attribute att;
 
 		/* Check for directly dependent types */
@@ -6524,18 +6525,57 @@ find_composite_type_dependencies(Oid typeOid, Relation origRelation,
 			continue;
 		}
 
-		/* Else, ignore dependees that aren't user columns of relations */
-		/* (we assume system columns are never of interesting types) */
-		if (pg_depend->classid != RelationRelationId ||
-			pg_depend->objsubid <= 0)
+		/* Else, ignore dependees that aren't relations */
+		if (pg_depend->classid != RelationRelationId)
 			continue;
 
 		rel = relation_open(pg_depend->objid, AccessShareLock);
-		att = TupleDescAttr(rel->rd_att, pg_depend->objsubid - 1);
+		tupleDesc = RelationGetDescr(rel);
 
-		if (rel->rd_rel->relkind == RELKIND_RELATION ||
-			rel->rd_rel->relkind == RELKIND_MATVIEW ||
-			rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+		/*
+		 * If objsubid identifies a specific column, refer to that in error
+		 * messages.  Otherwise, search to see if there's a user column of the
+		 * type.  (We assume system columns are never of interesting types.)
+		 * The search is needed because an index containing an expression
+		 * column of the target type will just be recorded as a whole-relation
+		 * dependency.  If we do not find a column of the type, the dependency
+		 * must indicate that the type is transiently referenced in an index
+		 * expression but not stored on disk, which we assume is OK, just as
+		 * we do for references in views.  (It could also be that the target
+		 * type is embedded in some container type that is stored in an index
+		 * column, but the previous recursion should catch such cases.)
+		 */
+		if (pg_depend->objsubid > 0 && pg_depend->objsubid <= tupleDesc->natts)
+			att = TupleDescAttr(tupleDesc, pg_depend->objsubid - 1);
+		else
+		{
+			att = NULL;
+			for (int attno = 1; attno <= tupleDesc->natts; attno++)
+			{
+				att = TupleDescAttr(tupleDesc, attno - 1);
+				if (att->atttypid == typeOid && !att->attisdropped)
+					break;
+				att = NULL;
+			}
+			if (att == NULL)
+			{
+				/* No such column, so assume OK */
+				relation_close(rel, AccessShareLock);
+				continue;
+			}
+		}
+
+		/*
+		 * We definitely should reject if the relation has storage.  If it's
+		 * partitioned, then perhaps we don't have to reject: if there are
+		 * partitions then we'll fail when we find one, else there is no
+		 * stored data to worry about.  However, it's possible that the type
+		 * change would affect conclusions about whether the type is sortable
+		 * or hashable and thus (if it's a partitioning column) break the
+		 * partitioning rule.  For now, reject for partitioned rels too.
+		 */
+		if (RELKIND_HAS_STORAGE(rel->rd_rel->relkind) ||
+			RELKIND_HAS_PARTITIONS(rel->rd_rel->relkind))
 		{
 			if (origTypeName)
 				ereport(ERROR,

src/test/regress/expected/alter_table.out

@@ -3097,6 +3097,13 @@ CREATE TYPE test_type1 AS (a int, b text);
 CREATE TABLE test_tbl1 (x int, y test_type1);
 ALTER TYPE test_type1 ALTER ATTRIBUTE b TYPE varchar; -- fails
 ERROR:  cannot alter type "test_type1" because column "test_tbl1.y" uses it
+DROP TABLE test_tbl1;
+CREATE TABLE test_tbl1 (x int, y text);
+CREATE INDEX test_tbl1_idx ON test_tbl1((row(x,y)::test_type1));
+ALTER TYPE test_type1 ALTER ATTRIBUTE b TYPE varchar; -- fails
+ERROR:  cannot alter type "test_type1" because column "test_tbl1_idx.row" uses it
+DROP TABLE test_tbl1;
+DROP TYPE test_type1;
 CREATE TYPE test_type2 AS (a int, b text);
 CREATE TABLE test_tbl2 OF test_type2;
 CREATE TABLE test_tbl2_subclass () INHERITS (test_tbl2);
@@ -3208,7 +3215,8 @@ Typed table of type: test_type2
  c      | text    |           |          | 
 Inherits: test_tbl2
 
-DROP TABLE test_tbl2_subclass;
+DROP TABLE test_tbl2_subclass, test_tbl2;
+DROP TYPE test_type2;
 CREATE TYPE test_typex AS (a int, b text);
 CREATE TABLE test_tblx (x int, y test_typex check ((y).a > 0));
 ALTER TYPE test_typex DROP ATTRIBUTE a; -- fails

src/test/regress/sql/alter_table.sql

@@ -1983,6 +1983,14 @@ CREATE TYPE test_type1 AS (a int, b text);
 CREATE TABLE test_tbl1 (x int, y test_type1);
 ALTER TYPE test_type1 ALTER ATTRIBUTE b TYPE varchar; -- fails
 
+DROP TABLE test_tbl1;
+CREATE TABLE test_tbl1 (x int, y text);
+CREATE INDEX test_tbl1_idx ON test_tbl1((row(x,y)::test_type1));
+ALTER TYPE test_type1 ALTER ATTRIBUTE b TYPE varchar; -- fails
+
+DROP TABLE test_tbl1;
+DROP TYPE test_type1;
+
 CREATE TYPE test_type2 AS (a int, b text);
 CREATE TABLE test_tbl2 OF test_type2;
 CREATE TABLE test_tbl2_subclass () INHERITS (test_tbl2);
@@ -2010,7 +2018,8 @@ ALTER TYPE test_type2 RENAME ATTRIBUTE a TO aa CASCADE;
 \d test_tbl2
 \d test_tbl2_subclass
 
-DROP TABLE test_tbl2_subclass;
+DROP TABLE test_tbl2_subclass, test_tbl2;
+DROP TYPE test_type2;
 
 CREATE TYPE test_typex AS (a int, b text);
 CREATE TABLE test_tblx (x int, y test_typex check ((y).a > 0));