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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: Big list'o questions.....
Date Mon, 03 Mar 2008 07:43:41 GMT
Hi Frits,

On Mar 3, 2008, at 1:37 AM, Frits Jalvingh wrote:
>> The later. This is not configurable in Cayenne. I strongly suspect
>> that PreparedStatement caching is never a bottleneck in the case of
>> update (and in any event, if we issue a set of different UPDATE
>> queries for each entity, each variation can be cached just as well).
>> If somebody can prove me wrong with some profiling data, we can
>> reconsider the update approach.
> Ok. It will be a bottleneck though for tasks doing lots of repetitive
> work: preparing a statement in Oracle easily takes 100x (!!) as much
> time as executing it. And sending lots of different statements to the
> DataSource's cache does not work- it caches only some of them to  
> prevent
> open cursor troubles so if you send it too many it does not work.
> Finally, sending lots of statements to Oracle means the database's SQL
> cache (SGA) which holds the database's cached execution plans will  
> loose
> efficiency. This will add up to quite large costs when the application
> is used by many people.

I may actually try it myself one day. The JDBC part of such test  
should be fairly easy to recreate. I think this also depends on the  
database. As for very large updates within a single transaction, I  
don't think the situation is that bad. Cayenne is using JDBC batching  
(that happens to actually make most difference on Oracle), and there  
is a fixed number of query permutations. Say if you have 1000 Artist  
objects, with 500 artists having their name updated, and 500 -  
dateOfBirth. So we have only two PreparedStatement batches to execute.  
Also I suspect Oracle will behave differently from other DB's.

Anyways, this requires research, and I am glad you brought it up. In  
fact I wish that your strategy of a full row update would end up being  
faster across the board as this would simplify the update code  
significantly. Although with large data sets other factors will play  
out, such as network latency, etc.

> This means I would not use Cayenne for any kind of work that handles
> large amounts of objects; something that is not advisable anyway for  
> the
> current generation of ORM's.

As Ari and Malcolm pointed out, this is too general of a statement.  
And as you pointed out, many scenarios can be optimized within the ORM  
paradigm ;-)

> Crystal clear ;-) This means you have no "previous" state in memory to
> restore the objects to pristine state?

Not true. The previously committed state is available. The algorithm  
just follows a cheaper path during rollback.

> Ok. Just to be clear: would commitChanges() commit all of the changes
> including the changes of all parents into the database?

Yes.

>> You can achieve that with JPA-like lifecycle callbacks (that are
>> available in Cayenne outside JPA) - POST_UPDATE, POST_PERSIST.
> By "refreshing" the objects in such a callback? That might be doable
> then; I would like a declarative way better though because I would  
> need
> to code every separate record's generated fields into such a  
> handler...

Cayenne mapping *API* allows to tag arbitrary columns that are DB- 
generated as such, but since the backend only supports PK retrieval  
via 'getGeneratedKeys', the Modeler gives an impression that this is  
only supported for a PK. So a single generic catch-all callback should  
be possible (in fact I'd think that we should do it on Cayenne end,  
even if this is solved as UPDATE/SELECT).


>> Hmm... does it get passed over JDBC back to the client, and if so, do
>> you have a JDBC example? (I know that even the standard callback via
>> 'Statement.getGeneratedKeys()' still doesn't work with many drivers).
> Sure, I used it in a proof-of-concept ORM that I built to check  
> whether
> it was possible to build something more reasonable than Hibernate or
> JPA. It does depend on the database, but the ones I use (Oracle,
> Postgresql) both support it albeit in slightly different ways.
>
> For Oracle you need to use a CallableStatement instead of a
> PreparedStatement and create a statement like:
>
> begin
>  update xxx set ..... where .... returning [columnname],
> [columnname]...;
> end;
>
> Before calling CallableStatement.execute() you must call
> CallableStatement.registerOutput() with the SQLType of each returned
> column.
>
> After calling you can retrieve the values using
> CallableStatement.getXXX(). This is very fast for Oracle; way faster
> than Hibernate's "update-then-reselect". I also used it to support
> native Oracle primary keys using sequences; you can generate inserts  
> the
> same way using the sequence in the values list, like:
>
> begin
>  insert into xxx(id, a, b...) values(my_sequence.nextval, ?, ?, ...)
>    returning id;
> end
>
> which is again faster than selecting the sequence in a separate
> statement.
>
> Postgresql supports something much alike but uses a PreparedStatement,
> no begin/end, and to retrieve the data you need to call getResultSet()
> on the preparedstatement; the resultset contains the data from the
> returning clause. I did not check the performance of this although the
> Postgresql documentation leads me to believe it is a single roundtrip.
>
> The sad thing is that this makes database adapter classes rather  
> complex
> because lots of things need to be delegated there to support these  
> quite
> different ways of supporting native sequences and quick returns of
> updated data. The advantage is that performance is way better.
>
> All of this, plus using bytecode instead of reflection makes my
> proof-of-concept code easily outperform Hibernate. If only completing
> (and maintaining!!) it wasn't such a lot of work ;-)....
>
> If you need a concrete example I can cook one up.

I got the idea. May actually be worth exploring.

Andrus


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