--- a/schema/gemma.sql	Wed Mar 11 10:59:34 2020 +0100
+++ b/schema/gemma.sql	Wed Mar 11 11:24:20 2020 +0100
@@ -71,22 +71,30 @@
 
 -- Trigger function to be used as a constraint trigger to enforce uniqueness
 -- of geometries in the column with its name given as an argument to the
--- trigger function
+-- trigger function. If additional column names are given,
+-- the group of given columns is tested for equality.
 CREATE OR REPLACE FUNCTION prevent_st_equals() RETURNS trigger AS
 $$
 DECLARE
     new_geom geometry;
+    tg_arg text;
+    filters text;
     has_equal boolean;
 BEGIN
     EXECUTE format('SELECT CAST($1.%I AS geometry)', TG_ARGV[0])
         INTO new_geom
         USING NEW;
+    FOREACH tg_arg IN ARRAY TG_ARGV[1:] LOOP
+        -- Test each additional argument for equality
+        filters = format('%s AND %2$I = $2.%2$I', filters, tg_arg);
+    END LOOP;
     EXECUTE format(
-            'SELECT bool_or(ST_Equals($1, CAST(%I AS geometry))) FROM %I.%I '
-                'WHERE id <> $2',
-            TG_ARGV[0], TG_TABLE_SCHEMA, TG_TABLE_NAME)
+            'SELECT EXISTS(SELECT 1 FROM %I.%I '
+                'WHERE id <> $2.id AND ST_Equals($1, CAST(%I AS geometry))'
+                '%s)',
+             TG_TABLE_SCHEMA, TG_TABLE_NAME, TG_ARGV[0], filters)
         INTO has_equal
-        USING new_geom, NEW.id;
+        USING new_geom, NEW;
     IF has_equal THEN
         RAISE EXCEPTION
             'new row for relation "%" violates constraint trigger "%"',