Bitemporal - problem with correalated subquery?

Hi there, This is my first time posting in here and I'm hoping somebody can point out where I am going wrong?

I am currently trying to use Bitemporal tables. By this I mean a table with a valid times and transaction times. These
topicsare covered by Joe Celko and Richard Snodgrass in their respective books.
 

I have developed a simple schema to test the relevant constraints which are required to keep all the valid times and
transactiontimes in order and to make sure they don't overlap. This is shown below and is done using a similar schema
oftables for Customers, Properties and Prop_Owners as Richard Snodgrass does in his book.
 

Of course these constrains are not possible in Postgres, so I have made them as functions and then created triggers for
them.

Everything seems to be working except for my function/trigger that maintains the referential integrity between the
Prop_Ownerand Customers tables when there is a "gap" in the Customers valid time or transaction time.
 

I am using Postgres 8.1 on Suse10.2

vt = valid time
tt = transaction time

Here is the schema:

CREATE TABLE Customers
(customer_no INTEGER NOT NULL,customer_name CHAR(30) NOT NULL,vt_begin DATE DEFAULT CURRENT_DATE,vt_end DATE DEFAULT
DATE'9999-12-31',tt_start DATE DEFAULT CURRENT_DATE,tt_stop DATE DEFAULT DATE '9999-12-31',  CONSTRAINT
Cust_VTdates_correct   CHECK (vt_begin <= vt_end),  CONSTRAINT Cust_ttdates_correct    CHECK (tt_start <= tt_stop),
PRIMARYKEY (customer_no, vt_begin, vt_end, tt_start, tt_stop)
 
);


CREATE TABLE Properties
(prop_no INTEGER NOT NULL PRIMARY KEY,prop_name CHAR(20) NOT NULL
);


CREATE TABLE Prop_Owner
(prop_no INTEGER NOT NULL  REFERENCES Properties (prop_no),customer_no INTEGER NOT NULL,vt_begin DATE DEFAULT
CURRENT_DATE,vt_endDATE DEFAULT DATE '9999-12-31',tt_start DATE DEFAULT CURRENT_DATE,tt_stop DATE DEFAULT DATE
'9999-12-31', CONSTRAINT PropOwner_VTdates_correct    CHECK (vt_begin <= vt_end),  CONSTRAINT PropOwner_ttdates_correct
  CHECK (tt_start <= tt_stop),  PRIMARY KEY (prop_no, customer_no, vt_begin, vt_end, tt_start, tt_stop)
 
);


Here is the function/trigger I seem to be having trouble with (although there are others which maintain the integrity
ofthe data - meaning records cannot overlap):
 

CREATE OR REPLACE FUNCTION P_O_integrity() RETURNS TRIGGER AS
$$
DECLARE vald INTEGER;
BEGINSELECT 1 INTO valdWHERE NOT EXISTS (SELECT *  FROM Prop_Owner AS A
-- there was a row valid in <ReferencedTable> when A started  WHERE NOT EXISTS(SELECT * FROM Customers AS B WHERE
A.customer_no= B.customer_no   AND B.vt_begin <= A.vt_begin AND A.vt_begin < B.vt_end   AND B.tt_start <= A.tt_start
ANDA.tt_start < B.tt_stop)
 
-- there was a row valid in <ReferencedTable> when A ended  OR NOT EXISTS(SELECT * FROM Customers AS B WHERE
A.customer_no= B.customer_no   AND B.vt_begin < A.vt_end AND A.vt_end <= B.vt_end   AND B.tt_start < A.tt_stop AND
A.tt_stop<= B.tt_stop)
 
-- there are no gaps in <ReferencedTable> during A's period of validity  OR EXISTS(SELECT * FROM Customers AS B WHERE
A.customer_no= B.customer_no   AND ((A.vt_begin < B.vt_end AND B.vt_end < A.vt_end)                   OR (A.tt_start <
B.tt_stopAND B.tt_stop < A.tt_stop))   AND NOT EXISTS       (SELECT *    FROM Customers AS B2    WHERE B2.customer_no =
B.customer_no     AND ((B2.vt_begin <= B.vt_end AND B.vt_end < B2.vt_end)                         OR (B2.tt_start <=
B.tt_stopAND B.tt_stop < B2.tt_stop)))) );IF NOT FOUND THEN      RAISE EXCEPTION 'Referential integrity breached. No
coveringForeign Key';END IF;
 
RETURN NULL;
END;
$$
LANGUAGE plpgsql;


CREATE TRIGGER P_O_integrity
AFTER INSERT OR UPDATE OR DELETE  ON Prop_OwnerFOR EACH ROW EXECUTE PROCEDURE P_O_integrity();



It is this trigger/function (P_O_integrity) that does not work properly. Specifically it is the following part:

-- there are no gaps in <ReferencedTable> during A's period of validity  OR EXISTS(SELECT * FROM Customers AS B WHERE
A.customer_no= B.customer_no   AND ((A.vt_begin < B.vt_end AND B.vt_end < A.vt_end)                   OR (A.tt_start <
B.tt_stopAND B.tt_stop < A.tt_stop))   AND NOT EXISTS       (SELECT *    FROM Customers AS B2    WHERE B2.customer_no =
B.customer_no     AND ((B2.vt_begin <= B.vt_end AND B.vt_end < B2.vt_end)                         OR (B2.tt_start <=
B.tt_stopAND B.tt_stop < B2.tt_stop))))
 


This can be rewritten as follows:

SELECT customer_no
FROM Prop_Owner AS A
WHERE EXISTS (SELECT customer_no  FROM Customers AS B  WHERE A.customer_no = B.customer_no    AND ((A.vt_begin <
B.vt_endAND B.vt_end < A.vt_end)             OR (A.tt_start < B.tt_stop AND B.tt_stop < A.tt_stop))    AND NOT EXISTS
  (SELECT customer_no       FROM Customers AS B2       WHERE B2.customer_no = B.customer_no         AND ((B2.vt_begin
<=B.vt_end AND B.vt_end < B2.vt_end)                  OR (B2.tt_start <= B.tt_stop AND B.tt_stop < B2.tt_stop))));
 

And if run on the data below, should pull out customer_no's '2' and '3'.
But does not seem to select any of the rows in which there are gaps in Customers during the validity of Prop_Owner??

The data I used is as follows:
Customers:
customer_no |customer_name |  vt_begin  |   vt_end   |  tt_start  |  tt_stop
-------------+--------------------------------+------------+------------+------------+------------          1 | keith
  | 2006-01-01 | 9999-12-31 | 2006-01-01 | 2006-12-31          1 | keith      | 2006-01-01 | 2006-12-31 | 2006-12-31 |
9999-12-31         1 | keith      | 2006-12-31 | 9999-12-31 | 2006-12-31 | 2007-12-31          1 | keith      |
2006-12-31| 2007-12-31 | 2007-12-31 | 9999-12-31          2 | simon    | 2004-01-01 | 9999-12-31 | 2004-01-01 |
2004-12-01         2 | simon    | 2004-01-01 | 2004-12-31 | 2004-12-01 | 9999-12-31 <=          2 | simon    |
2004-12-31| 9999-12-31 | 2004-12-15 | 9999-12-31 <=          3 | john       | 2000-01-01 | 9999-12-31 | 2000-01-01 |
2001-01-01         3 | john       | 2000-01-01 | 2001-01-01 | 2001-01-01 | 9999-12-31 <=          3 | john       |
2002-01-01| 9999-12-31 | 2002-01-01 | 9999-12-31 <=
 

The arrows indicate where there are gaps.

Properties:prop_no |      prop_name
---------+----------------------      1 | house      2 | flat      3 | penthouse

Prop_Owner:
prop_no | customer_no |  vt_begin  |   vt_end   |  tt_start  |  tt_stop
---------+-------------+------------+------------+------------+------------      1 |           1 | 2006-02-01 |
9999-12-31| 2006-02-01 | 2006-12-01      2 |           2 | 2004-02-01 | 9999-12-31 | 2004-01-01 | 2004-12-25      3 |
       3 | 2000-02-01 | 9999-12-31 | 2000-01-01 | 9999-12-31
 

The 2nd and 3rd lines should have not been able to be inserted with the trigger as they "bridge" records with gaps in
theCustomers table.
 

I hope this makes sense and that someone can explain why the last part of P_O_integrity is not selecting the records
coveredwith gaps correctly.
 
It seems to be returning B2 from the NOT EXISTS correlated sub query rather than B?!?

Thanks for any help you can give.
Keith