Allow an alias to be attached to a JOIN ... USING

This allows something like

    SELECT ... FROM t1 JOIN t2 USING (a, b, c) AS x

where x has the columns a, b, c and unlike a regular alias it does not
hide the range variables of the tables being joined t1 and t2.

Per SQL:2016 feature F404 "Range variable for common column names".

Reviewed-by: Vik Fearing <vik.fearing@2ndquadrant.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/454638cf-d563-ab76-a585-2564428062af@2ndquadrant.com

src/include/parser/parse_node.h

@@ -228,7 +228,10 @@ struct ParseState
  * An element of a namespace list.
  *
  * p_names contains the table name and column names exposed by this nsitem.
- * (Currently, it's always equal to p_rte->eref.)
+ * (Typically it's equal to p_rte->eref, but for a JOIN USING alias it's
+ * equal to p_rte->join_using_alias.  Since the USING columns will be the
+ * join's first N columns, the net effect is just that we expose only those
+ * join columns via this nsitem.)
  *
  * p_rte and p_rtindex link to the underlying rangetable entry.
  *

src/include/parser/parse_relation.h

@@ -88,6 +88,7 @@ extern ParseNamespaceItem *addRangeTableEntryForJoin(ParseState *pstate,
 													 List *aliasvars,
 													 List *leftcols,
 													 List *rightcols,
+													 Alias *joinalias,
 													 Alias *alias,
 													 bool inFromCl);
 extern ParseNamespaceItem *addRangeTableEntryForCTE(ParseState *pstate,