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Re: Re[2]: [JBoss-user] Page by Page iteration
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From: <Dav...@na...> - 2003-05-06 12:47:29
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Julien,
The JBossCMP book seems to indicate that this is not the case w/ commit
option B:
"JBossCMP assumes that there is more than one user of the database but
keeps the
context information about entities between transactions. This context
information is used
for optimizing loading of the entity. [20]This is the default commit
option."
is that wrong, or am I just misunderstanding it? (I would have assumed that
the "context" info mentioned there was the list cache data).
If so, why would on_load be any better than on_find -- just because you may
load less data depending on when you stop iterating through the collection?
julien> you can associate a read-ahead on_load with the size of the
julien> limit parameter.
But then you need to find a way to ensure that the on_load page-size in the
deployment descriptor and the limit paramether your code passes in stay in
sync. Can LIMIT take a literal?
And wouldn't on_find work just as well in this case (since you're certain
you're going to read all the returned entries unless an exception occurs).
Dave
julien viet
<ju...@jb...> To: "Dav...@na..." <jbo...@li...>
Sent by: cc:
jbo...@li...ur Subject: Re[2]: [JBoss-user] Page by Page iteration
ceforge.net
05/06/2003 07:41 AM
Please respond to
jboss-user
Dnc> Can't you use read-ahead w/ on_load strategy? I.e., in
jbosscmp-jdbc.xml,
Dnc> have something like:
this will not work because you cannot cache a collection
between transactions.
what you should try is the use of JBossQL limit offset :
SELECT OBJECT(o) FROM object AS o ORDER BY o.id OFFSET ?1 LIMIT ?2
this will give you what you want. this will work efficiently
on relatively small size collection < 10000.
you can associate a read-ahead on_load with the size of the limit
parameter.
julien
Dnc> Can't you use read-ahead w/ on_load strategy? I.e., in
jbosscmp-jdbc.xml,
Dnc> have something like:
Dnc> <entity>
Dnc> <ejb-name>MyEJB</ejb-name>
Dnc> <query>
Dnc> <query-method>
Dnc> <method-name>findAll_paged</method-name>
Dnc> <method-params/>
Dnc> </query-method>
Dnc> <jboss-ql><![CDATA[SELECT OBJECT(o) FROM my_table o ORDER
BY
o.somefield]]>></jboss-ql>
Dnc> <read-ahead>
Dnc> <strategy>on_load</strategy>
Dnc> <page-size>20</page-size>
Dnc> </read-ahead>
Dnc> </query>
Dnc> </entity>
Dnc> Then your finder will pre-load the data for the first 20 rows into the
Dnc> pre-load cache, and I believe that when you iterate to #21 it'll grab
the
Dnc> data for the next 20.
Dnc> I'm not sure how this works when you throw CMR into the mix.
Dnc> And of course, it's not portable.
Dnc> Dave
Dnc> "Brian McSweeney"
Dnc> <bri...@au... To:
<jbo...@li...>
Dnc> > cc:
Dnc> Sent by: Subject:
[JBoss-user] Page by Page iteration
Dnc> jbo...@li...ur
Dnc> ceforge.net
Dnc> 05/06/2003 05:51 AM
Dnc> Please respond to
Dnc> jboss-user
Dnc> Hi everyone,
Dnc> I'm looking for a little advice. I'm implementing a struts-jboss web
app
Dnc> which
Dnc> uses page-by-page data iteration. On the ejb side I'm using session
facades
Dnc> with cmp & cmr entity beans. However, in order to get the relevant
data
Dnc> back
Dnc> to iterate over, I currently have to utilise findAll, to retrieve a
Dnc> collection of
Dnc> entity beans.
Dnc> This is surely not good - as the data gets large, I'm getting back
Dnc> thousands
Dnc> of ejbs, just to display the first 20 or 30 on the first page. I
presume
Dnc> the
Dnc> correct way to do this with raw sql would be something like:
Dnc> 1) Get a count of all the relevant rows
Dnc> 2) Select the first 20
Dnc> 3) For each next page, select 20 offsetting at the relevant point.
Dnc> Am I correct in this being the proper approach?
Dnc> However, I haven't used raw sql with ejbs and I'm not sure how to.
Dnc> Advice here please.
Dnc> Finally, if I use raw sql, are stored procedures the correct way to
go.
Dnc> I've been told that they are considerably faster.
Dnc> thanks for all your help,
Dnc> Brian
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--
Best regards,
julien mailto:ju...@jb...
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