Change SELECT to PERFORM in function and drop triggers first

layers/waterway/merge_waterway.sql

@@ -1,3 +1,5 @@
+DROP TRIGGER IF EXISTS trigger_flag ON osm_waterway_linestring;
+DROP TRIGGER IF EXISTS trigger_refresh ON waterway.updates;
 
 -- We merge the waterways by name like the highways
 -- This helps to drop not important rivers (since they do not have a name)
@@ -74,9 +76,6 @@ CREATE OR REPLACE FUNCTION waterway.refresh() RETURNS trigger AS
   $BODY$
 language plpgsql;
 
-DROP TRIGGER IF EXISTS trigger_flag ON osm_waterway_linestring;
-DROP TRIGGER IF EXISTS trigger_refresh ON waterway.updates;
-
 CREATE TRIGGER trigger_flag
     AFTER INSERT OR UPDATE OR DELETE ON osm_waterway_linestring
     FOR EACH STATEMENT