Support "variadic" functions, which can accept a variable number of arguments

so long as all the trailing arguments are of the same (non-array) type.
The function receives them as a single array argument (which is why they
have to all be the same type).

It might be useful to extend this facility to aggregates, but this patch
doesn't do that.

This patch imposes a noticeable slowdown on function lookup --- a follow-on
patch will fix that by adding a redundant column to pg_proc.

Pavel Stehule

src/test/regress/sql/polymorphism.sql

@@ -426,3 +426,46 @@ create aggregate build_group(int8, integer) (
   SFUNC = add_group,
   STYPE = int8[]
 );
+
+-- test variadic polymorphic functions
+
+create function myleast(variadic anyarray) returns anyelement as $$
+  select min($1[i]) from generate_subscripts($1,1) g(i)
+$$ language sql immutable strict;
+
+select myleast(10, 1, 20, 33);
+select myleast(1.1, 0.22, 0.55);
+select myleast('z'::text);
+select myleast(); -- fail
+
+-- test with variadic call parameter
+select myleast(variadic array[1,2,3,4,-1]);
+select myleast(variadic array[1.1, -5.5]);
+
+--test with empty variadic call parameter
+select myleast(variadic array[]::int[]);
+
+-- an example with some ordinary arguments too
+create function concat(text, variadic anyarray) returns text as $$
+  select array_to_string($2, $1);
+$$ language sql immutable strict;
+
+select concat('%', 1, 2, 3, 4, 5);
+select concat('|', 'a'::text, 'b', 'c');
+select concat('|', variadic array[1,2,33]);
+select concat('|', variadic array[]::int[]);
+
+drop function concat(text, anyarray);
+
+-- mix variadic with anyelement
+create function formarray(anyelement, variadic anyarray) returns anyarray as $$
+  select array_prepend($1, $2);
+$$ language sql immutable strict;
+
+select formarray(1,2,3,4,5);
+select formarray(1.1, variadic array[1.2,55.5]);
+select formarray(1.1, array[1.2,55.5]); -- fail without variadic
+select formarray(1, 'x'::text); -- fail, type mismatch
+select formarray(1, variadic array['x'::text]); -- fail, type mismatch
+
+drop function formarray(anyelement, variadic anyarray);