RE: RE: [Objectbridge-developers] XML used in test.ojb.server, et c

[Dixon-Peugh, D. <Dav...@tp...>]

Are you asking for the underlying objects of the OQL?  Or something a bit
more sophisticated.
 
The underlying objects are likely just going to be nodes in a parse tree,
and not really have a lot of stuff on them.
 
The way I understand ObJectBridge will handle its OQL stuff, is that it will
just take the queries, rewrite them into
SQL and then execute it on the RDBMS. . .
 
Oh, now I get it.  Since data can be pulled from multiple stores at one
time, it becomes necessary to do some
DB type work in memory.  Being able to compose a query which runs against
many DBs is what you are after.
 
So, for the OQL support we will need:
 
1:    Parse OQL into Parse Tree
 
2:    Rewrite the Parse Tree into one or more statements per datasource,
plus one or more statements
        which can be handled in memory by OJB.
 
3:    Execute each of the statements on the remote datasource, collecting
data into OJB.
 
4:    Execute the OJB specific operations to join the data
 
5:    Return the results in the ODMG type requested.
 
------------
 
So, what you are looking for are steps 2 - 5, correct?  (Step 1 is probably
pretty easy.  I've seen a couple
of JavaCC parsers for OQL out there.)
 
Some things we can note here:
    OJB does not necessarily have to implement all operations of OQL.  So,
theoretically, we could rewrite
        a "BETWEEN" that has to happen as a "<" "AND" ">" structure.  (Even
though it would probably
        be better to write "BETWEEN" in there, as <&> will take longer.
 
    OJB will probably need to index things as they come in.  Fortunately,
OJB also has access to the
        execution plan, and can build the apropriate index (without relying
on someone to find it for us.)
 
    OJB will need to be able to translate OQL into SQL of the apropriate
form.
-----------
 
All in all, it looks like a tall order.  It can be done however.
 
Merant did the same type of thing for their "Reflections" ODBC driver.
(Only for RDBMSes though.)
 
David (D-P)
 
 
 
 
 
        

-----Original Message-----
From: obj...@li...
[mailto:obj...@li...]On Behalf Of
David Forslund
Sent: Thursday, January 11, 2001 9:58 AM
To: Mahler Thomas
Cc: objectbridge
Subject: RE: RE: [Objectbridge-developers] XML used in test.ojb.server, etc


At 08:37 AM 1/11/2001 +0100, Mahler Thomas wrote:


Between is harder and not really needed.  "LIKE" is usually sufficient,
although GREATER_THAN and LESS_THAN or almost as important.  What we do is
aggregate the constraints with "AND" and "OR".  I'm not sure how to express
the AND and OR with a query by example.  



 

 I don't see a way to have BETWEEN, >, < and conjunction of clauses with AND
or OR in a query by example solution. Of course we could start with an adhoc
solution but I think it will be better to directly go for OQL, as we need to
implement it anyway to be ODMG compliant. 


We don't see a need for "between"   Simply "<",">", and "AND" provides that.
I don't see how to use AND's and OR's in a QBE, either.   But to implement
an OQL solution, you need to have underlying Java objects. These need to be
implemented first and then parse OQL to map to them.  We would be interested
in the underlying Java objects.





Would this be a helpful feature for you? I can start working on such a
functionality within the next days. 



In our application on sourceforge (OpenEMed), we have a persistent
abstraction layer that is implemented over a number of different kinds of
persistent store (ODBMS, and RDBMS).  The RDBMS part uses Software Tree's
JDX product that works very well.  We would like to use an open source
product, if at all possible, however.  Castor is a likely candidate, but
still is evolving rapidly. 

  

I'll have a look at your stuff to get an impression. 

 




Don't be overwhelmed by the size of our application.  Look in the tools
directory in the Database package to see what we are up to. 
 

 I had problems to get the big tarball (> 14M) due to problems with my
companies proxy servers, will try it later.


You can browse the CVS repository for OpenEMed/src/tools/gov/lanl/Database
to see the relevant files.

Thanks,



David W. Forslund                               dw...@la...    
Computer and Computational Sciences
http://www.acl.lanl.gov/~dwf <http://www.acl.lanl.gov/~dwf> 
Los Alamos National Laboratory          Los Alamos, NM 87545            
505-665-1907                                    FAX: 505-665-4939