Re-allow duplicate aliases within aliased JOINs.

Although the SQL spec forbids duplicate table aliases, historically
we've allowed queries like
    SELECT ... FROM tab1 x CROSS JOIN (tab2 x CROSS JOIN tab3 y) z
on the grounds that the aliased join (z) hides the aliases within it,
therefore there is no conflict between the two RTEs named "x".  The
LATERAL patch broke this, on the misguided basis that "x" could be
ambiguous if tab3 were a LATERAL subquery.  To avoid breaking existing
queries, it's better to allow this situation and complain only if
tab3 actually does contain an ambiguous reference.  We need only remove
the check that was throwing an error, because the column lookup code
is already prepared to handle ambiguous references.  Per bug #8444.

src/backend/parser/parse_clause.c

@@ -720,14 +720,15 @@ transformFromClauseItem(ParseState *pstate, Node *n,
 		 * we always push them into the namespace, but mark them as not
 		 * lateral_ok if the jointype is wrong.
 		 *
+		 * Notice that we don't require the merged namespace list to be
+		 * conflict-free.  See the comments for scanNameSpaceForRefname().
+		 *
 		 * NB: this coding relies on the fact that list_concat is not
 		 * destructive to its second argument.
 		 */
 		lateral_ok = (j->jointype == JOIN_INNER || j->jointype == JOIN_LEFT);
 		setNamespaceLateralState(l_namespace, true, lateral_ok);
 
-		checkNameSpaceConflicts(pstate, pstate->p_namespace, l_namespace);
-
 		sv_namespace_length = list_length(pstate->p_namespace);
 		pstate->p_namespace = list_concat(pstate->p_namespace, l_namespace);
 

src/backend/parser/parse_relation.c

@@ -130,6 +130,18 @@ refnameRangeTblEntry(ParseState *pstate,
  * Search the query's table namespace for an RTE matching the
  * given unqualified refname.  Return the RTE if a unique match, or NULL
  * if no match.  Raise error if multiple matches.
+ *
+ * Note: it might seem that we shouldn't have to worry about the possibility
+ * of multiple matches; after all, the SQL standard disallows duplicate table
+ * aliases within a given SELECT level.  Historically, however, Postgres has
+ * been laxer than that.  For example, we allow
+ *		SELECT ... FROM tab1 x CROSS JOIN (tab2 x CROSS JOIN tab3 y) z
+ * on the grounds that the aliased join (z) hides the aliases within it,
+ * therefore there is no conflict between the two RTEs named "x".  However,
+ * if tab3 is a LATERAL subquery, then from within the subquery both "x"es
+ * are visible.  Rather than rejecting queries that used to work, we allow
+ * this situation, and complain only if there's actually an ambiguous
+ * reference to "x".
  */
 static RangeTblEntry *
 scanNameSpaceForRefname(ParseState *pstate, const char *refname, int location)
@@ -174,8 +186,7 @@ scanNameSpaceForRefname(ParseState *pstate, const char *refname, int location)
 /*
  * Search the query's table namespace for a relation RTE matching the
  * given relation OID.	Return the RTE if a unique match, or NULL
- * if no match.  Raise error if multiple matches (which shouldn't
- * happen if the namespace was checked correctly when it was created).
+ * if no match.  Raise error if multiple matches.
  *
  * See the comments for refnameRangeTblEntry to understand why this
  * acts the way it does.

src/test/regress/expected/join.out

@@ -3093,6 +3093,24 @@ SELECT * FROM
 (5 rows)
 
 rollback;
+-- bug #8444: we've historically allowed duplicate aliases within aliased JOINs
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y) j on q1 = f1; -- error
+ERROR:  column reference "f1" is ambiguous
+LINE 2: ..._tbl x join (int4_tbl x cross join int4_tbl y) j on q1 = f1;
+                                                                    ^
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y) j on q1 = y.f1; -- error
+ERROR:  invalid reference to FROM-clause entry for table "y"
+LINE 2: ...bl x join (int4_tbl x cross join int4_tbl y) j on q1 = y.f1;
+                                                                  ^
+HINT:  There is an entry for table "y", but it cannot be referenced from this part of the query.
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y(ff)) j on q1 = f1; -- ok
+ q1 | q2 | f1 | ff 
+----+----+----+----
+(0 rows)
+
 --
 -- Test LATERAL
 --
@@ -3947,6 +3965,12 @@ ERROR:  invalid reference to FROM-clause entry for table "a"
 LINE 1: ...m int4_tbl a full join lateral generate_series(0, a.f1) g on...
                                                              ^
 DETAIL:  The combining JOIN type must be INNER or LEFT for a LATERAL reference.
+-- check we complain about ambiguous table references
+select * from
+  int8_tbl x cross join (int4_tbl x cross join lateral (select x.f1) ss);
+ERROR:  table reference "x" is ambiguous
+LINE 2: ...cross join (int4_tbl x cross join lateral (select x.f1) ss);
+                                                             ^
 -- LATERAL can be used to put an aggregate into the FROM clause of its query
 select 1 from tenk1 a, lateral (select max(a.unique1) from int4_tbl b) ss;
 ERROR:  aggregate functions are not allowed in FROM clause of their own query level

src/test/regress/sql/join.sql

@@ -892,6 +892,15 @@ SELECT * FROM
 
 rollback;
 
+-- bug #8444: we've historically allowed duplicate aliases within aliased JOINs
+
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y) j on q1 = f1; -- error
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y) j on q1 = y.f1; -- error
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y(ff)) j on q1 = f1; -- ok
+
 --
 -- Test LATERAL
 --
@@ -1079,5 +1088,8 @@ select f1,g from int4_tbl a cross join (select a.f1 as g) ss;
 -- SQL:2008 says the left table is in scope but illegal to access here
 select f1,g from int4_tbl a right join lateral generate_series(0, a.f1) g on true;
 select f1,g from int4_tbl a full join lateral generate_series(0, a.f1) g on true;
+-- check we complain about ambiguous table references
+select * from
+  int8_tbl x cross join (int4_tbl x cross join lateral (select x.f1) ss);
 -- LATERAL can be used to put an aggregate into the FROM clause of its query
 select 1 from tenk1 a, lateral (select max(a.unique1) from int4_tbl b) ss;