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

4 files changed, 18 insertions, 1 deletions

diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 334262b1dd8..7960cfe1a82 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1070,6 +1070,13 @@ typedef struct RangeTblEntry
 	List	   *joinrightcols;	/* right-side input column numbers */
 
 	/*
+	 * join_using_alias is an alias clause attached directly to JOIN/USING. It
+	 * is different from the alias field (below) in that it does not hide the
+	 * range variables of the tables being joined.
+	 */
+	Alias	   *join_using_alias;
+
+	/*
 	 * Fields valid for a function RTE (else NIL/zero):
 	 *
 	 * When funcordinality is true, the eref->colnames list includes an alias
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index d4ce0370884..f66e1449d86 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -1499,6 +1499,11 @@ typedef struct RangeTblRef
  * alias has a critical impact on semantics, because a join with an alias
  * restricts visibility of the tables/columns inside it.
  *
+ * join_using_alias is an Alias node representing the join correlation
+ * name that SQL:2016 and later allow to be attached to JOIN/USING.
+ * Its column alias list includes only the common column names from USING,
+ * and it does not restrict visibility of the join's input tables.
+ *
  * During parse analysis, an RTE is created for the Join, and its index
  * is filled into rtindex.  This RTE is present mainly so that Vars can
  * be created that refer to the outputs of the join.  The planner sometimes
@@ -1514,6 +1519,7 @@ typedef struct JoinExpr
 	Node	   *larg;			/* left subtree */
 	Node	   *rarg;			/* right subtree */
 	List	   *usingClause;	/* USING clause, if any (list of String) */
+	Alias	   *join_using_alias;	/* alias attached to USING clause, if any */
 	Node	   *quals;			/* qualifiers on join, if any */
 	Alias	   *alias;			/* user-written alias clause, if any */
 	int			rtindex;		/* RT index assigned for join, or 0 */
diff --git a/src/include/parser/parse_node.h b/src/include/parser/parse_node.h
index 70739bcd5b6..1500de2dd08 100644
--- a/src/include/parser/parse_node.h
+++ b/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.
  *
diff --git a/src/include/parser/parse_relation.h b/src/include/parser/parse_relation.h
index 5dbe5ba2e27..8336c2c5a29 100644
--- a/src/include/parser/parse_relation.h
+++ b/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,