diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index 1f6306a7d356890b88f69c72a8d3842349fde876..7a1261d0fd2e213adaf35ccfd45f1042e5e84948 100644
--- a/src/backend/parser/parse_clause.c
+++ b/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);
diff --git a/src/backend/parser/parse_relation.c b/src/backend/parser/parse_relation.c
index 5f469135cbfe0d066953165b45c37fca0e593483..0052d21ad62fa63515e32982deaa64be919aea24 100644
--- a/src/backend/parser/parse_relation.c
+++ b/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.
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index d46ff26d82b46be00a8a87916630f69bdc949011..b59004ddf3cc33748e78ba7d579f2c641ab39a08 100644
--- a/src/test/regress/expected/join.out
+++ b/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
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 1a4dc03d18babd2a0b52af18ff201d7caf552d2a..e43b0fc0ffcebb93c613dd38a5a67026b46bb36d 100644
--- a/src/test/regress/sql/join.sql
+++ b/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;