Apache JDO Project - Query closures for JDO 2.2

Author

Query closures for JDO 2.2

Craig L Russell

2007-12-01, 1:11 pm

Wesley Biggs

2007-12-01, 7:11 pm

+1. I like the "perform" term; it reinforces the abstraction of
"select these into Java, then perform this action" -- i.e. you get
side effects.

Wes

P.S. Your code example needs 1 + :percent instead of 1.06.

On 01/12/2007, Craig L Russell <Craig.Russell@sun.com> wrote:
> As you know, I'm not fond of the non-object style of query update,
> and there are cases where you want to do a bulk update efficiently in
> the datastore, but SQL just doesn't capture the object mapping that
> is used in the rest of the application.
>
> So what about defining a query closure, that is, declare a set of
> statements that is executed for each qualifying instance that
> satisfies the filter.
>
> This could execute in memory for memory collections, or in the
> datastore for non-instantiated collections. For portability we need
> to define the closure in terms that can be mapped directly to SQL.
>
> For example,
>
> PERFORM salary *= 1. + :percent; lastSalaryAdjustment = :date FROM
> Employee WHERE rating == :rating
>
> Query q = pm.newQuery(Employee, "rating > :rating");
> q.setPerform("salary *= 1.06; lastSalaryAdjustment = :date");
> q.execute(.06, new Date(), 7);
>
>
> Craig Russell
> Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
> 408 276-5638 mailto:Craig.Russell@sun.com
> P.S. A good JDO? O, Gasp!
>
>
>

Michael Bouschen

2007-12-09, 7:11 pm

Hi,

during the JDO TCK conference call last Friday a question came up about
possible restrictions on such a perform block. SQL does not allow
multiple tables in the FROM clause of an UPDATE statement. So should we
put restrictions on JDOQL to allow a 1-1 mapping to SQL? Issues to be
considered: update of fields mapped to a secondary table, relationship
updates and use of variables to update related instances. However,
multiple SQL statements and/or subqueries might be needed anyway for
inheritance and existential queries (meaning relationship navigation in
the WHERE clause).

Another question is about the term "perform". It reinforces the idea
that you select a number of Java instances and perform some block action
on them. However, people might expect to use the word "update" for such
an operation.

What do you think?

Regards Michael

> +1: bulk update is efficient feature.
>
> 2007/12/1, Wesley Biggs <wesley@cacas.org>:
>
>
>
>

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Erik Bengtson

2007-12-10, 1:11 pm

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Christiaan

2007-12-11, 7:11 pm

Hi,
since you are calling this "perform" would there also be a possibility to
somehow incorporate my copy by query feature request?
http://www.nabble.com/Feature-reque...to13855888.html

As a side and practical note, I am working in a domain where many objects
are involved, so for performance and memory issues it would really benefit
from these "bulk" operations. The delete-by-query was a real must have in
that regard. However, in the field I am working, I doubt whether
update-by-query would be of the same added value for two reasons:
1) updating in our domain often involves some complex logic, eg. values
should be updated based on values in other (children) objects and other
factors. Could this be expressed in the closure? (and if it is, wouldn't
that be too much going against the OO paradigm)
2) our experience is that these update operations can be done really quick
within reasonable memory usage due to fetchgroups (you only load what you
need) and fetchsize (having the right value really speeds up iterating).

On the other hand, as mentioned in the copy-by-query request, copying can be
a very time and memory consuming task, since both original and copy need to
be completely in memory, while most of the time there is no need to load
them into memory.

kind regards,
Christiaan

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