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/backend/parser/analyze.c

@@ -1744,6 +1744,7 @@ transformSetOperationStmt(ParseState *pstate, SelectStmt *stmt)
 										NIL,
 										NIL,
 										NULL,
+										NULL,
 										false);
 
 	sv_namespace = pstate->p_namespace;

src/backend/parser/gram.y

@@ -509,7 +509,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <ival>	sub_type opt_materialized
 %type <value>	NumericOnly
 %type <list>	NumericOnly_list
-%type <alias>	alias_clause opt_alias_clause
+%type <alias>	alias_clause opt_alias_clause opt_alias_clause_for_join_using
 %type <list>	func_alias_clause
 %type <sortby>	sortby
 %type <ielem>	index_elem index_elem_options
@@ -12144,6 +12144,7 @@ joined_table:
 					n->larg = $1;
 					n->rarg = $4;
 					n->usingClause = NIL;
+					n->join_using_alias = NULL;
 					n->quals = NULL;
 					$$ = n;
 				}
@@ -12155,9 +12156,16 @@ joined_table:
 					n->larg = $1;
 					n->rarg = $4;
 					if ($5 != NULL && IsA($5, List))
-						n->usingClause = (List *) $5; /* USING clause */
+					{
+						 /* USING clause */
+						n->usingClause = linitial_node(List, castNode(List, $5));
+						n->join_using_alias = lsecond_node(Alias, castNode(List, $5));
+					}
 					else
-						n->quals = $5; /* ON clause */
+					{
+						/* ON clause */
+						n->quals = $5;
+					}
 					$$ = n;
 				}
 			| table_ref JOIN table_ref join_qual
@@ -12169,9 +12177,16 @@ joined_table:
 					n->larg = $1;
 					n->rarg = $3;
 					if ($4 != NULL && IsA($4, List))
-						n->usingClause = (List *) $4; /* USING clause */
+					{
+						/* USING clause */
+						n->usingClause = linitial_node(List, castNode(List, $4));
+						n->join_using_alias = lsecond_node(Alias, castNode(List, $4));
+					}
 					else
-						n->quals = $4; /* ON clause */
+					{
+						/* ON clause */
+						n->quals = $4;
+					}
 					$$ = n;
 				}
 			| table_ref NATURAL join_type JOIN table_ref
@@ -12182,6 +12197,7 @@ joined_table:
 					n->larg = $1;
 					n->rarg = $5;
 					n->usingClause = NIL; /* figure out which columns later... */
+					n->join_using_alias = NULL;
 					n->quals = NULL; /* fill later */
 					$$ = n;
 				}
@@ -12194,6 +12210,7 @@ joined_table:
 					n->larg = $1;
 					n->rarg = $4;
 					n->usingClause = NIL; /* figure out which columns later... */
+					n->join_using_alias = NULL;
 					n->quals = NULL; /* fill later */
 					$$ = n;
 				}
@@ -12228,6 +12245,22 @@ opt_alias_clause: alias_clause						{ $$ = $1; }
 			| /*EMPTY*/								{ $$ = NULL; }
 		;
 
+/*
+ * The alias clause after JOIN ... USING only accepts the AS ColId spelling,
+ * per SQL standard.  (The grammar could parse the other variants, but they
+ * don't seem to be useful, and it might lead to parser problems in the
+ * future.)
+ */
+opt_alias_clause_for_join_using:
+			AS ColId
+				{
+					$$ = makeNode(Alias);
+					$$->aliasname = $2;
+					/* the column name list will be inserted later */
+				}
+			| /*EMPTY*/								{ $$ = NULL; }
+		;
+
 /*
  * func_alias_clause can include both an Alias and a coldeflist, so we make it
  * return a 2-element list that gets disassembled by calling production.
@@ -12272,15 +12305,24 @@ opt_outer: OUTER_P
 
 /* JOIN qualification clauses
  * Possibilities are:
- *	USING ( column list ) allows only unqualified column names,
+ *	USING ( column list ) [ AS alias ]
+ *						  allows only unqualified column names,
  *						  which must match between tables.
  *	ON expr allows more general qualifications.
  *
- * We return USING as a List node, while an ON-expr will not be a List.
+ * We return USING as a two-element List (the first item being a sub-List
+ * of the common column names, and the second either an Alias item or NULL).
+ * An ON-expr will not be a List, so it can be told apart that way.
  */
 
-join_qual:	USING '(' name_list ')'					{ $$ = (Node *) $3; }
-			| ON a_expr								{ $$ = $2; }
+join_qual: USING '(' name_list ')' opt_alias_clause_for_join_using
+				{
+					$$ = (Node *) list_make2($3, $5);
+				}
+			| ON a_expr
+				{
+					$$ = $2;
+				}
 		;
 
 

src/backend/parser/parse_clause.c

@@ -1265,6 +1265,13 @@ transformFromClauseItem(ParseState *pstate, Node *n,
 			j->usingClause = rlist;
 		}
 
+		/*
+		 * If a USING clause alias was specified, save the USING columns as
+		 * its column list.
+		 */
+		if (j->join_using_alias)
+			j->join_using_alias->colnames = j->usingClause;
+
 		/*
 		 * Now transform the join qualifications, if any.
 		 */
@@ -1460,6 +1467,7 @@ transformFromClauseItem(ParseState *pstate, Node *n,
 										   res_colvars,
 										   l_colnos,
 										   r_colnos,
+										   j->join_using_alias,
 										   j->alias,
 										   true);
 
@@ -1493,6 +1501,30 @@ transformFromClauseItem(ParseState *pstate, Node *n,
 		pstate->p_joinexprs = lappend(pstate->p_joinexprs, j);
 		Assert(list_length(pstate->p_joinexprs) == j->rtindex);
 
+		/*
+		 * If the join has a USING alias, build a ParseNamespaceItem for that
+		 * and add it to the list of nsitems in the join's input.
+		 */
+		if (j->join_using_alias)
+		{
+			ParseNamespaceItem *jnsitem;
+
+			jnsitem = (ParseNamespaceItem *) palloc(sizeof(ParseNamespaceItem));
+			jnsitem->p_names = j->join_using_alias;
+			jnsitem->p_rte = nsitem->p_rte;
+			jnsitem->p_rtindex = nsitem->p_rtindex;
+			/* no need to copy the first N columns, just use res_nscolumns */
+			jnsitem->p_nscolumns = res_nscolumns;
+			/* set default visibility flags; might get changed later */
+			jnsitem->p_rel_visible = true;
+			jnsitem->p_cols_visible = true;
+			jnsitem->p_lateral_only = false;
+			jnsitem->p_lateral_ok = true;
+			/* Per SQL, we must check for alias conflicts */
+			checkNameSpaceConflicts(pstate, list_make1(jnsitem), my_namespace);
+			my_namespace = lappend(my_namespace, jnsitem);
+		}
+
 		/*
 		 * Prepare returned namespace list.  If the JOIN has an alias then it
 		 * hides the contained RTEs completely; otherwise, the contained RTEs

src/backend/parser/parse_expr.c

@@ -2512,26 +2512,61 @@ static Node *
 transformWholeRowRef(ParseState *pstate, ParseNamespaceItem *nsitem,
 					 int sublevels_up, int location)
 {
-	Var		   *result;
-
 	/*
-	 * Build the appropriate referencing node.  Note that if the RTE is a
-	 * function returning scalar, we create just a plain reference to the
-	 * function value, not a composite containing a single column.  This is
-	 * pretty inconsistent at first sight, but it's what we've done
-	 * historically.  One argument for it is that "rel" and "rel.*" mean the
-	 * same thing for composite relations, so why not for scalar functions...
+	 * Build the appropriate referencing node.  Normally this can be a
+	 * whole-row Var, but if the nsitem is a JOIN USING alias then it contains
+	 * only a subset of the columns of the underlying join RTE, so that will
+	 * not work.  Instead we immediately expand the reference into a RowExpr.
+	 * Since the JOIN USING's common columns are fully determined at this
+	 * point, there seems no harm in expanding it now rather than during
+	 * planning.
+	 *
+	 * Note that if the RTE is a function returning scalar, we create just a
+	 * plain reference to the function value, not a composite containing a
+	 * single column.  This is pretty inconsistent at first sight, but it's
+	 * what we've done historically.  One argument for it is that "rel" and
+	 * "rel.*" mean the same thing for composite relations, so why not for
+	 * scalar functions...
 	 */
-	result = makeWholeRowVar(nsitem->p_rte, nsitem->p_rtindex,
-							 sublevels_up, true);
+	if (nsitem->p_names == nsitem->p_rte->eref)
+	{
+		Var		   *result;
 
-	/* location is not filled in by makeWholeRowVar */
-	result->location = location;
+		result = makeWholeRowVar(nsitem->p_rte, nsitem->p_rtindex,
+								 sublevels_up, true);
 
-	/* mark relation as requiring whole-row SELECT access */
-	markVarForSelectPriv(pstate, result);
+		/* location is not filled in by makeWholeRowVar */
+		result->location = location;
 
-	return (Node *) result;
+		/* mark relation as requiring whole-row SELECT access */
+		markVarForSelectPriv(pstate, result);
+
+		return (Node *) result;
+	}
+	else
+	{
+		RowExpr    *rowexpr;
+		List	   *fields;
+
+		/*
+		 * We want only as many columns as are listed in p_names->colnames,
+		 * and we should use those names not whatever possibly-aliased names
+		 * are in the RTE.  We needn't worry about marking the RTE for SELECT
+		 * access, as the common columns are surely so marked already.
+		 */
+		expandRTE(nsitem->p_rte, nsitem->p_rtindex,
+				  sublevels_up, location, false,
+				  NULL, &fields);
+		rowexpr = makeNode(RowExpr);
+		rowexpr->args = list_truncate(fields,
+									  list_length(nsitem->p_names->colnames));
+		rowexpr->row_typeid = RECORDOID;
+		rowexpr->row_format = COERCE_IMPLICIT_CAST;
+		rowexpr->colnames = copyObject(nsitem->p_names->colnames);
+		rowexpr->location = location;
+
+		return (Node *) rowexpr;
+	}
 }
 
 /*

src/backend/parser/parse_relation.c

@@ -753,6 +753,12 @@ scanNSItemForColumn(ParseState *pstate, ParseNamespaceItem *nsitem,
  *	  else return InvalidAttrNumber.
  *	  If the name proves ambiguous within this RTE, raise error.
  *
+ * Actually, we only search the names listed in "eref".  This can be either
+ * rte->eref, in which case we are indeed searching all the column names,
+ * or for a join it can be rte->join_using_alias, in which case we are only
+ * considering the common column names (which are the first N columns of the
+ * join, so everything works).
+ *
  * pstate and location are passed only for error-reporting purposes.
  *
  * Side effect: if fuzzystate is non-NULL, check non-system columns
@@ -2134,6 +2140,7 @@ addRangeTableEntryForJoin(ParseState *pstate,
 						  List *aliasvars,
 						  List *leftcols,
 						  List *rightcols,
+						  Alias *join_using_alias,
 						  Alias *alias,
 						  bool inFromCl)
 {
@@ -2162,6 +2169,7 @@ addRangeTableEntryForJoin(ParseState *pstate,
 	rte->joinaliasvars = aliasvars;
 	rte->joinleftcols = leftcols;
 	rte->joinrightcols = rightcols;
+	rte->join_using_alias = join_using_alias;
 	rte->alias = alias;
 
 	eref = alias ? copyObject(alias) : makeAlias("unnamed_join", NIL);