Postgres discrepency in trigger execution speed?

Question

I have a trigger that executes a function on table insert or update. It looks like this:

CREATE OR REPLACE FUNCTION func_fk_location_area()
RETURNS "trigger" AS $$
BEGIN
    IF EXISTS (
        -- there was a row valid in area when location started
        SELECT * FROM location
        WHERE NOT EXISTS (
            SELECT * FROM area
             WHERE area.key=location.key
               AND area.id=location.area_id
               AND (  (area.tr_from<=location.tr_from AND area.tr_until>location.tr_from) OR
                      (area.tr_from=location.tr_from AND area.tr_until=location.tr_from)))
    ) OR EXISTS (
        -- there was a row valid in area when location ended
        SELECT * FROM location
        WHERE NOT EXISTS (
            SELECT * FROM area
             WHERE area.key=location.key
               AND area.id=location.area_id
               AND (  (area.tr_from<location.tr_until AND area.tr_until>=location.tr_until) OR
                      (area.tr_from=location.tr_until AND area.tr_until=location.tr_until)))
    )
    THEN
        RAISE EXCEPTION 'FK location_area integrity violation.';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;
CREATE TRIGGER trigger_fk_area_location AFTER DELETE OR UPDATE ON area
    FOR EACH ROW EXECUTE PROCEDURE func_fk_location_area();
CREATE TRIGGER trigger_fk_location_area AFTER INSERT OR UPDATE ON location
    FOR EACH ROW EXECUTE PROCEDURE func_fk_location_area();

When I insert a row, it seems to run very slowly. Using explain analyze I determined that this trigger was taking nearly 400ms to complete.

 Result  (cost=0.00..0.03 rows=1 width=0) (actual time=0.026..0.029 rows=1 loops=1)
 Trigger for constraint location_fkey_tr_by: time=0.063 calls=1
 Trigger trigger_fk_location_area: time=361.878 calls=1
 Trigger trigger_update_objects_location: time=355.033 calls=1
 Total runtime: 717.229 ms
(5 rows)

However, if I run the two lots of SQL in the function, they each only take 3 or 4ms to run!

FIRST PART:

mydb=# explain analyze
mydb-#             SELECT * FROM location
mydb-#             WHERE NOT EXISTS (
mydb(#                 SELECT * FROM area
mydb(#                  WHERE area.key=location.key
mydb(#                    AND area.id=location.area_id
mydb(#                    AND (  (area.tr_from<location.tr_until AND area.tr_until>=location.tr_until) OR
mydb(#                           (area.tr_from=location.tr_until AND area.tr_until=location.tr_until)));

 Hash Anti Join  (cost=14.68..146.84 rows=1754 width=126) (actual time=5.512..5.512 rows=0 loops=1)
   Hash Cond: ((location.key = area.key) AND (location.area_id = area.id))
   Join Filter: (((area.tr_from < location.tr_until) AND (area.tr_until >= location.tr_until)) OR ((area.tr_from = location.tr_until) AND (area.tr_until = locat
ion.tr_until)))
   ->  Seq Scan on location  (cost=0.00..79.91 rows=2391 width=126) (actual time=0.005..1.016 rows=2393 loops=1)
   ->  Hash  (cost=8.87..8.87 rows=387 width=37) (actual time=0.497..0.497 rows=387 loops=1)
         ->  Seq Scan on area  (cost=0.00..8.87 rows=387 width=37) (actual time=0.004..0.250 rows=387 loops=1)
 Total runtime: 5.562 ms
(7 rows)

SECOND PART:

mydb=# explain analyze
mydb-#             SELECT * FROM location
mydb-#             WHERE NOT EXISTS (
mydb(#                 SELECT * FROM area
mydb(#                  WHERE area.key=location.key
mydb(#                    AND area.id=location.area_id
mydb(#                    AND (  (area.tr_from<location.tr_until AND area.tr_until>=location.tr_until) OR
mydb(#                           (area.tr_from=location.tr_until AND area.tr_until=location.tr_until)));

 Hash Anti Join  (cost=14.68..146.84 rows=1754 width=126) (actual time=5.666..5.666 rows=0 loops=1)
   Hash Cond: ((location.key = area.key) AND (location.area_id = area.id))
   Join Filter: (((area.tr_from < location.tr_until) AND (area.tr_until >= location.tr_until)) OR ((area.tr_from = location.tr_until) AND (area.tr_until = locat
ion.tr_until)))
   ->  Seq Scan on location  (cost=0.00..79.91 rows=2391 width=126) (actual time=0.005..1.072 rows=2393 loops=1)
   ->  Hash  (cost=8.87..8.87 rows=387 width=37) (actual time=0.509..0.509 rows=387 loops=1)
         ->  Seq Scan on area  (cost=0.00..8.87 rows=387 width=37) (actual time=0.007..0.239 rows=387 loops=1)
 Total runtime: 5.725 ms
(7 rows)

This makes no sense to me.

Any thoughts?

Thanks.

Answer 1

You're setting up the trigger to run for each row, and then inside the trigger function you're doing another select on the whole table. Do one or the other. (Try changing FOR EACH ROW to FOR EACH STATEMENT.)