View on GitHub BIRDS

BIdirectional transformation for Relational view update Datalog-based Strategies

Trigger generation

BIRDS compiles a Datalog program of view definition and update strategy into an SQL program that creates the corresponding view with associated triggers in PostgreSQL. Without recursions in the Datalog program, BIRDS automatically translates each derived (IDB) relation (source deltas, view, …) into an equivalent SQL query. The SQL query of the view definition is used to create the view. Meanwhile, the SQL queries of the source deltas are used in a trigger function that implements the view update strategy.

Suppose a view v is defined over base tables s1, s2, …, sn. The first generated SQL statement is to define the view as follows:

CREATE OR REPLACE VIEW public.v <SQL-query-defining-the-view>;

The created view v is a virtual view. After that, BIRDS creates a trigger to initialize some temporary tables and triggers:

CREATE OR REPLACE FUNCTION public.v_materialization() RETURNS TRIGGER LANGUAGE plpgsql
SECURITY DEFINER AS $$
  DECLARE
  ...
  BEGIN
    CREATE TEMPORARY TABLE __temp__Δ_ins_v ( LIKE public.v INCLUDING ALL ) WITH OIDS ON COMMIT DROP;
    CREATE CONSTRAINT TRIGGER __temp__v_trigger_delta_action
    AFTER INSERT OR UPDATE OR DELETE ON 
        __temp__Δ_ins_v DEFERRABLE INITIALLY DEFERRED 
        FOR EACH ROW EXECUTE PROCEDURE public.v_delta_action();

    CREATE TEMPORARY TABLE __temp__Δ_del_v ( LIKE public.v INCLUDING ALL ) WITH OIDS ON COMMIT DROP;
    CREATE CONSTRAINT TRIGGER __temp__v_trigger_delta_action
    AFTER INSERT OR UPDATE OR DELETE ON 
        __temp__Δ_del_v DEFERRABLE INITIALLY DEFERRED 
        FOR EACH ROW EXECUTE PROCEDURE public.v_delta_action();
  EXCEPTION
    ...
  END;
$$;

DROP TRIGGER IF EXISTS v_trigger_materialization ON public.v;
CREATE TRIGGER v_trigger_materialization
    BEFORE INSERT OR UPDATE OR DELETE ON
      public.v FOR EACH STATEMENT EXECUTE PROCEDURE public.v_materialization();

Next, BIRDS creates a trigger that handles any SQL DML statements on the view v to derive the view deltas:

CREATE OR REPLACE FUNCTION public.v_update() RETURNS TRIGGER LANGUAGE plpgsql
SECURITY DEFINER AS $$
  DECLARE
  ...
  BEGIN
    IF TG_OP = 'INSERT' THEN
      DELETE FROM __temp__Δ_del_v WHERE ROW(X) = NEW;
      INSERT INTO __temp__Δ_ins_v SELECT (NEW).*; 
    ELSIF TG_OP = 'UPDATE' THEN
      DELETE FROM __temp__Δ_ins_v WHERE ROW(X) = OLD;
      INSERT INTO __temp__Δ_del_v SELECT (OLD).*;
      DELETE FROM __temp__Δ_del_v WHERE ROW(X) = NEW;
      INSERT INTO __temp__Δ_ins_v SELECT (NEW).*; 
    ELSIF TG_OP = 'DELETE' THEN
      DELETE FROM __temp__Δ_ins_v WHERE ROW(X) = OLD;
      INSERT INTO __temp__Δ_del_v SELECT (OLD).*;
    END IF;
    RETURN NULL;
  EXCEPTION
    ...
  END;
$$;

DROP TRIGGER IF EXISTS v_trigger_update ON public.v;
CREATE TRIGGER v_trigger_update
    INSTEAD OF INSERT OR UPDATE OR DELETE ON
      public.v FOR EACH ROW EXECUTE PROCEDURE public.v_update();

Finally, BIRDS creates a trigger function that implements the view update strategy. The trigger checks all the constraints, then computes the source deltas and applies them to the source tables.

CREATE OR REPLACE FUNCTION public.v_delta_action() RETURNS TRIGGER LANGUAGE plpgsql 
SECURITY DEFINER AS $$
DECLARE
... 
BEGIN
    IF EXISTS (SELECT WHERE <SQL-of-the-view-constraints> )
    THEN 
        RAISE check_violation USING MESSAGE = 'Invalid view update: constraints on the view are violated';
    END IF;
    IF EXISTS (SELECT WHERE <SQL-of-the-source-constraints> )
    THEN 
        RAISE check_violation USING MESSAGE = 'Invalid view update: constraints on the source relations are violated';
    END IF;

    CREATE TEMPORARY TABLE __dummy__delta__delete__s1 WITH OIDS ON COMMIT DROP AS 
    SELECT ... FROM (...) ;
    ...
    CREATE TEMPORARY TABLE __dummy__delta__delete__sn WITH OIDS ON COMMIT DROP AS 
    SELECT ... FROM (...) ;

    CREATE TEMPORARY TABLE __dummy__delta__insert__s1 WITH OIDS ON COMMIT DROP AS 
    SELECT ... FROM (...);
    ...
    CREATE TEMPORARY TABLE __dummy__delta__insert__sn WITH OIDS ON COMMIT DROP AS 
    SELECT ... FROM (...);

    FOR temprec IN ( SELECT * FROM __dummy__delta__delete__s1) LOOP 
            DELETE FROM public.s1 WHERE ROW(X) IS NOT DISTINCT FROM  temprec;
            END LOOP;
    DROP TABLE __dummy__delta__delete__s1;
    ...
    FOR temprec IN ( SELECT * FROM __dummy__delta__delete__sn) LOOP 
            DELETE FROM public.s1 WHERE ROW(X) IS NOT DISTINCT FROM  temprec;
            END LOOP;
    DROP TABLE __dummy__delta__delete__sn;

    INSERT INTO public.s1 SELECT * FROM  __dummy__delta__insert__s1; 
    ...
    INSERT INTO public.s1 SELECT * FROM  __dummy__delta__insert__sn; 
    DROP TABLE __dummy__delta__insert__s1;

    RETURN NULL;
EXCEPTION
    ...
END;
$$;