Allow aggregate functions to be VARIADIC.

There's no inherent reason why an aggregate function can't be variadic
(even VARIADIC ANY) if its transition function can handle the case.
Indeed, this patch to add the feature touches none of the planner or
executor, and little of the parser; the main missing stuff was DDL and
pg_dump support.

It is true that variadic aggregates can create the same sort of ambiguity
about parameters versus ORDER BY keys that was complained of when we
(briefly) had both one- and two-argument forms of string_agg().  However,
the policy formed in response to that discussion only said that we'd not
create any built-in aggregates with varying numbers of arguments, not that
we shouldn't allow users to do it.  So the logical extension of that is
we can allow users to make variadic aggregates as long as we're wary about
shipping any such in core.

In passing, this patch allows aggregate function arguments to be named, to
the extent of remembering the names in pg_proc and dumping them in pg_dump.
You can't yet call an aggregate using named-parameter notation.  That seems
like a likely future extension, but it'll take some work, and it's not what
this patch is really about.  Likewise, there's still some work needed to
make window functions handle VARIADIC fully, but I left that for another
day.

initdb forced because of new aggvariadic field in Aggref parse nodes.

1 files changed, 10 insertions, 6 deletions

diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 1f02c9a5757..2bd24c89c87 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -385,7 +385,7 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 	}
 
 	/*
-	 * When function is called an explicit VARIADIC labeled parameter,
+	 * When function is called with an explicit VARIADIC labeled parameter,
 	 * and the declared_arg_type is "any", then sanity check the actual
 	 * parameter type now - it must be an array.
 	 */
@@ -425,8 +425,9 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 		aggref->aggtype = rettype;
 		/* aggcollid and inputcollid will be set by parse_collate.c */
 		/* args, aggorder, aggdistinct will be set by transformAggregateCall */
-		aggref->aggstar = agg_star;
 		aggref->aggfilter = agg_filter;
+		aggref->aggstar = agg_star;
+		aggref->aggvariadic = func_variadic;
 		/* agglevelsup will be set by transformAggregateCall */
 		aggref->location = location;
 
@@ -448,10 +449,13 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 					 parser_errposition(pstate, location)));
 
 		/*
-		 * Currently it's not possible to define an aggregate with named
-		 * arguments, so this case should be impossible.  Check anyway because
-		 * the planner and executor wouldn't cope with NamedArgExprs in an
-		 * Aggref node.
+		 * We might want to support named arguments later, but disallow it for
+		 * now.  We'd need to figure out the parsed representation (should the
+		 * NamedArgExprs go above or below the TargetEntry nodes?) and then
+		 * teach the planner to reorder the list properly.	Or maybe we could
+		 * make transformAggregateCall do that?  However, if you'd also like
+		 * to allow default arguments for aggregates, we'd need to do it in
+		 * planning to avoid semantic problems.
 		 */
 		if (argnames != NIL)
 			ereport(ERROR,