Join between Oracle and non-Oracle Database

Question

We are working at a project with the follow architecture:
- 5 Databases (3 Oracle, 1 Sybase and 1 DB2);
- Session Bean like Session Facade Pattern;
- Entity Bean (BMP);
- OC4J (Oracle Container for J2EE);
The case is that we cannot make join between tables of different bases, for example, join with one table of Oracle and one other of DB2, therefore the customer does not possess tool midleware for this.
The idea that appeared was: we make a query in table XXX of Oracle and return a HashTable with its respective VO�s and Keys (PKs from the first query that will serve of base for the second query), later we have access the DB2 that one second collection would return, to inside make join of the application.
The problem is that with this solution, we will have fall in the performance. And we do not know if it is the best form.
Do you have any idea???
Thanks,
Eric Sander

Answer 1

Hi,
If you have different databases then the join has to be done in the VM (either by the appserver or by you).
If you want to do it yourself, I would try the following:
-write one DAO per business object (at least one per database)
-this DAO contains the specifics of its database
-return a hashtable of keys and VOs from one DAO (as you say you do?)
-in the other DAO, add a method "joinWith ( Hashtable foreignKeys )" that will return the join's result
-implement the method along these lines:
1. retrieve the candidate rows from the second DAO's tables (1 query execution)
2. put the keys in a new Hashtable, say "candidateKeys" (meaning 1 iteration over all results)
3. iterate over all elements of foreignKeys (1 iteration)
4. for each element, check if a key exists in "candidateKeys" (1 hash lookup, very fast)
5. add any such row to the result
This is called a hash join (Oracle does it internally if you do a join).
It is the fastest algorithm to do joins.
Best,
Guy
http://www.atomikos.com - JTA transaction manager