Add num_nulls() and num_nonnulls() to count NULL arguments.

An example use-case is "CHECK(num_nonnulls(a,b,c) = 1)" to assert that
exactly one of a,b,c isn't NULL.  The functions are variadic, so they
can also be pressed into service to count the number of null or nonnull
elements in an array.

Marko Tiikkaja, reviewed by Pavel Stehule

src/backend/utils/adt/misc.c

@@ -44,6 +44,127 @@
 #define atooid(x)  ((Oid) strtoul((x), NULL, 10))
 
 
+/*
+ * Common subroutine for num_nulls() and num_nonnulls().
+ * Returns TRUE if successful, FALSE if function should return NULL.
+ * If successful, total argument count and number of nulls are
+ * returned into *nargs and *nulls.
+ */
+static bool
+count_nulls(FunctionCallInfo fcinfo,
+			int32 *nargs, int32 *nulls)
+{
+	int32		count = 0;
+	int			i;
+
+	/* Did we get a VARIADIC array argument, or separate arguments? */
+	if (get_fn_expr_variadic(fcinfo->flinfo))
+	{
+		ArrayType  *arr;
+		int			ndims,
+					nitems,
+				   *dims;
+		bits8	   *bitmap;
+
+		Assert(PG_NARGS() == 1);
+
+		/*
+		 * If we get a null as VARIADIC array argument, we can't say anything
+		 * useful about the number of elements, so return NULL.  This behavior
+		 * is consistent with other variadic functions - see concat_internal.
+		 */
+		if (PG_ARGISNULL(0))
+			return false;
+
+		/*
+		 * Non-null argument had better be an array.  We assume that any call
+		 * context that could let get_fn_expr_variadic return true will have
+		 * checked that a VARIADIC-labeled parameter actually is an array.  So
+		 * it should be okay to just Assert that it's an array rather than
+		 * doing a full-fledged error check.
+		 */
+		Assert(OidIsValid(get_base_element_type(get_fn_expr_argtype(fcinfo->flinfo, 0))));
+
+		/* OK, safe to fetch the array value */
+		arr = PG_GETARG_ARRAYTYPE_P(0);
+
+		/* Count the array elements */
+		ndims = ARR_NDIM(arr);
+		dims = ARR_DIMS(arr);
+		nitems = ArrayGetNItems(ndims, dims);
+
+		/* Count those that are NULL */
+		bitmap = ARR_NULLBITMAP(arr);
+		if (bitmap)
+		{
+			int			bitmask = 1;
+
+			for (i = 0; i < nitems; i++)
+			{
+				if ((*bitmap & bitmask) == 0)
+					count++;
+
+				bitmask <<= 1;
+				if (bitmask == 0x100)
+				{
+					bitmap++;
+					bitmask = 1;
+				}
+			}
+		}
+
+		*nargs = nitems;
+		*nulls = count;
+	}
+	else
+	{
+		/* Separate arguments, so just count 'em */
+		for (i = 0; i < PG_NARGS(); i++)
+		{
+			if (PG_ARGISNULL(i))
+				count++;
+		}
+
+		*nargs = PG_NARGS();
+		*nulls = count;
+	}
+
+	return true;
+}
+
+/*
+ * num_nulls()
+ *	Count the number of NULL arguments
+ */
+Datum
+pg_num_nulls(PG_FUNCTION_ARGS)
+{
+	int32		nargs,
+				nulls;
+
+	if (!count_nulls(fcinfo, &nargs, &nulls))
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT32(nulls);
+}
+
+/*
+ * num_nonnulls()
+ *	Count the number of non-NULL arguments
+ */
+Datum
+pg_num_nonnulls(PG_FUNCTION_ARGS)
+{
+	int32		nargs,
+				nulls;
+
+	if (!count_nulls(fcinfo, &nargs, &nulls))
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT32(nargs - nulls);
+}
+
+
 /*
  * current_database()
  *	Expose the current database to the user