Fix incorrect printing of queries with duplicated join names.

Given a query in which multiple JOIN nodes used the same alias
(which'd necessarily be in different sub-SELECTs), ruleutils.c
would assign the JOIN nodes distinct aliases for clarity ...
but then it forgot to print the modified aliases when dumping
the JOIN nodes themselves.  This results in a dump/reload hazard
for views, because the emitted query is flat-out incorrect:
Vars will be printed with table names that have no referent.

This has been wrong for a long time, so back-patch to all supported
branches.

Philip Dubé

Discussion: https://postgr.es/m/CY4PR2101MB080246F2955FF58A6ED1FEAC98140@CY4PR2101MB0802.namprd21.prod.outlook.com

src/backend/utils/adt/ruleutils.c

@@ -10245,8 +10245,16 @@ get_from_clause_item(Node *jtnode, Query *query, deparse_context *context)
 		/* Yes, it's correct to put alias after the right paren ... */
 		if (j->alias != NULL)
 		{
+			/*
+			 * Note that it's correct to emit an alias clause if and only if
+			 * there was one originally.  Otherwise we'd be converting a named
+			 * join to unnamed or vice versa, which creates semantic
+			 * subtleties we don't want.  However, we might print a different
+			 * alias name than was there originally.
+			 */
 			appendStringInfo(buf, " %s",
-							 quote_identifier(j->alias->aliasname));
+							 quote_identifier(get_rtable_name(j->rtindex,
+															  context)));
 			get_column_alias_list(colinfo, context);
 		}
 	}

src/test/regress/expected/create_view.out

@@ -769,6 +769,41 @@ View definition:
            FROM temp_view_test.tx1 tx1_1
           WHERE tx1.y1 = tx1_1.f1));
 
+-- Test aliasing of joins
+create view view_of_joins as
+select * from
+  (select * from (tbl1 cross join tbl2) same) ss,
+  (tbl3 cross join tbl4) same;
+\d+ view_of_joins
+                    View "testviewschm2.view_of_joins"
+ Column |  Type   | Collation | Nullable | Default | Storage | Description 
+--------+---------+-----------+----------+---------+---------+-------------
+ a      | integer |           |          |         | plain   | 
+ b      | integer |           |          |         | plain   | 
+ c      | integer |           |          |         | plain   | 
+ d      | integer |           |          |         | plain   | 
+ e      | integer |           |          |         | plain   | 
+ f      | integer |           |          |         | plain   | 
+ g      | integer |           |          |         | plain   | 
+ h      | integer |           |          |         | plain   | 
+View definition:
+ SELECT ss.a,
+    ss.b,
+    ss.c,
+    ss.d,
+    same.e,
+    same.f,
+    same.g,
+    same.h
+   FROM ( SELECT same_1.a,
+            same_1.b,
+            same_1.c,
+            same_1.d
+           FROM (tbl1
+             CROSS JOIN tbl2) same_1) ss,
+    (tbl3
+     CROSS JOIN tbl4) same;
+
 -- Test view decompilation in the face of column addition/deletion/renaming
 create table tt2 (a int, b int, c int);
 create table tt3 (ax int8, b int2, c numeric);
@@ -1721,4 +1756,4 @@ select pg_get_ruledef(oid, true) from pg_rewrite
 DROP SCHEMA temp_view_test CASCADE;
 NOTICE:  drop cascades to 27 other objects
 DROP SCHEMA testviewschm2 CASCADE;
-NOTICE:  drop cascades to 62 other objects
+NOTICE:  drop cascades to 63 other objects

src/test/regress/sql/create_view.sql

@@ -319,6 +319,15 @@ ALTER TABLE tmp1 RENAME TO tx1;
 \d+ aliased_view_3
 \d+ aliased_view_4
 
+-- Test aliasing of joins
+
+create view view_of_joins as
+select * from
+  (select * from (tbl1 cross join tbl2) same) ss,
+  (tbl3 cross join tbl4) same;
+
+\d+ view_of_joins
+
 -- Test view decompilation in the face of column addition/deletion/renaming
 
 create table tt2 (a int, b int, c int);