--- a/schema/gemma.sql	Thu Nov 14 14:36:41 2019 +0100
+++ b/schema/gemma.sql	Thu Nov 14 14:53:15 2019 +0100
@@ -56,6 +56,19 @@
     END;
 $$;
 
+--
+-- Trigger functions to be used in constraint triggers
+--
+/* The constraint triggers are subject to a possible race condition
+   due to PostgreSQL MVCC: Trigger functions do not see uncommited changes
+   of other transactions running in parallel.
+
+   Since we serialize imports at application level, the race condition
+   should not happen, though. In case serialization at application level
+   should be removed, database side locking/serialization would have
+   to be applied.
+*/
+
 -- 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