Re: prefetching run amok
Re: prefetching run amok
- Subject: Re: prefetching run amok
- From: Arturo Pérez <email@hidden>
- Date: Fri, 18 Feb 2005 21:18:31 -0500
You're misinterpreting that 'fetch cancelled' thing. That always comes
out. I think it's the result of a rollback to force the cleanup of any
resources tied up by the JDBC/SQL connection.
I think there's something strange going on with your model or your
custom code that modifies the fetch specification.
The key to solving the problem would be to find out what where that
(1=1) where clause comes from. EOF wouldn't normally generate
something like that as it's completely unnecessary for fetching all the
rows of a table.
Have you tried to do what you want with a plain vanilla fetch
specification and prefetched relationships? Just as a comparison, not
necessarily to go forward with.
Well, my 2cents FWIW.
On Feb 18, 2005, at 9:03 PM, Zak Burke wrote:
On closer inspection, I discovered the problem is only happening where
I'm using fetchSpec.setFetchLimit(n). Perhaps JDBC (or the SQL
standard) doesn't have a "limit" function to restrict the number of
rows returned, and EOF implements this by canceling the query after
retrieving the rows it wants without passing on that fact to the other
prefetch queries.
The SQL log is instructive here:
[2005-02-18 17:48:40 EST] <WorkerThread11> evaluateExpression:
<com.webobjects.jdbcadaptor.MySQLPlugIn$MySQLExpression: "SELECT
t0.authors, t0.channel_id, t0.contact_id, t0.created, t0.created_by,
t0.date_live, t0.date_published, t0.did, t0.is_featured, t0.name,
t0.original_source_id, t0.source_id, t0.status_id, t0.summary,
t0.updated, t0.updated_by, t0.url_id FROM news_document t0 ORDER BY
t0.date_live DESC">
[2005-02-18 17:48:45 EST] <WorkerThread11> fetch canceled
[2005-02-18 17:48:45 EST] <WorkerThread11> 3 row(s) processed
...
[2005-02-18 17:48:47 EST] <WorkerThread11> evaluateExpression:
<com.webobjects.jdbcadaptor.MySQLPlugIn$MySQLExpression:
"SELECT DISTINCT t0.contact_id, t0.created, t0.created_by,
t0.url_description, t0.updated, t0.updated_by, t0.url, t0.url_id FROM
url t0 WHERE (1=1)">
[2005-02-18 17:48:53 EST] <WorkerThread11> 121805 row(s) processed
I guess I need to find a way to explicitly retrieve the rows I want,
rather than letting EOF help me.
zak.
Zak Burke wrote on 2/18/05 4:48 PM:
I am trying to use prefetching but it's not behaving in the way I
expect, or the way I want. I have a two tables, STORY and URL, with a
to-one relationship defined from STORY to URL. Other tables have
to-one relationships with URL as well, so it has a lot (100k+) of
rows.
When I retrieve a set of stories, I always want the stories' related
URL items as well so I use
EOFetchSpecification.setPrefetchingRelationshipKeyPaths to add an
array containing this field to my fetch.
This works, kind of. Instead of individual queries for each URL item,
there's only one query (good), but it selects *every* row in URL
rather than just those which are related to the STORY rows in the
current query (bad). When I run a simple search, I'm retrieving 15
STORY rows and 112,000 URL rows. Yikes.
The query EOF generates is this:
SELECT DISTINCT t0.contact_id, t0.created,
t0.created_by, t0.url_description, t0.updated,
t0.updated_by, t0.url, t0.url_id
FROM url t0
WHERE (1=1)
I expected a where clause that ORed together the selected
STORY.URL_ID fields.
Is there way to restrict prefetching to only grab the rows related to
the current query?
zak.
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--arturo
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