Each row in the database has a tuple ID, a number visible as the system column named ctid in each row. You can use these to look up a row:
SELECT ctid,customerid FROM customers limit 3;
ctid | customerid
-------+------------
(0,1) | 1
(0,2) | 2
(0,3) | 3
EXPLAIN SELECT customerid FROM customers WHERE ctid='(0,1)';
QUERY PLAN
---------------------------------------------------------
Tid Scan on customers (cost=0.00..4.01 rows=1 width=4)
TID Cond: (ctid = '(0,1)'::tid)
These TID sequences cannot be relied upon as a stable way to access a particular row outside of a transaction, because common operations including UPDATE will change them. If you're referring to a row more than one in the same transaction, perhaps in a procedural ...
