Fix dumping of outer joins with empty qual lists.

Normally, a JoinExpr would have empty "quals" only if it came from CROSS
JOIN syntax.  However, it's possible to get to this state by specifying
NATURAL JOIN between two tables with no common column names, and there
might be other ways too.  The code previously printed no ON clause if
"quals" was empty; that's right for CROSS JOIN but syntactically invalid
if it's some type of outer join.  Fix by printing ON TRUE in that case.

This got broken by commit 2ffa740be, which stopped using NATURAL JOIN
syntax in ruleutils output due to its brittleness in the face of
column renamings.  Back-patch to 9.3 where that commit appeared.

Per report from Tushar Ahuja.

Discussion: https://postgr.es/m/98b283cd-6dda-5d3f-f8ac-87db8c76a3da@enterprisedb.com

src/backend/utils/adt/ruleutils.c

@@ -10082,6 +10082,11 @@ get_from_clause_item(Node *jtnode, Query *query, deparse_context *context)
 			if (!PRETTY_PAREN(context))
 				appendStringInfoChar(buf, ')');
 		}
+		else if (j->jointype != JOIN_INNER)
+		{
+			/* If we didn't say CROSS JOIN above, we must provide an ON */
+			appendStringInfoString(buf, " ON TRUE");
+		}
 
 		if (!PRETTY_PAREN(context) || j->alias != NULL)
 			appendStringInfoChar(buf, ')');

src/test/regress/expected/create_view.out

@@ -1648,6 +1648,35 @@ select pg_get_viewdef('tt20v', true);
      CAST((1 + 2)::bigint AS bigint) i8(i8);
 (1 row)
 
+-- corner cases with empty join conditions
+create view tt21v as
+select * from tt5 natural inner join tt6;
+select pg_get_viewdef('tt21v', true);
+    pg_get_viewdef    
+----------------------
+  SELECT tt5.a,      +
+     tt5.b,          +
+     tt5.cc,         +
+     tt6.c,          +
+     tt6.d           +
+    FROM tt5         +
+      CROSS JOIN tt6;
+(1 row)
+
+create view tt22v as
+select * from tt5 natural left join tt6;
+select pg_get_viewdef('tt22v', true);
+       pg_get_viewdef        
+-----------------------------
+  SELECT tt5.a,             +
+     tt5.b,                 +
+     tt5.cc,                +
+     tt6.c,                 +
+     tt6.d                  +
+    FROM tt5                +
+      LEFT JOIN tt6 ON TRUE;
+(1 row)
+
 -- clean up all the random objects we made above
 set client_min_messages = warning;
 DROP SCHEMA temp_view_test CASCADE;

src/test/regress/sql/create_view.sql

@@ -559,6 +559,16 @@ select * from
   cast(1+2 as int8) as i8;
 select pg_get_viewdef('tt20v', true);
 
+-- corner cases with empty join conditions
+
+create view tt21v as
+select * from tt5 natural inner join tt6;
+select pg_get_viewdef('tt21v', true);
+
+create view tt22v as
+select * from tt5 natural left join tt6;
+select pg_get_viewdef('tt22v', true);
+
 -- clean up all the random objects we made above
 set client_min_messages = warning;
 DROP SCHEMA temp_view_test CASCADE;