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From "Nick Johnson (JIRA)" <j...@apache.org>
Subject [jira] Created: (OPENJPA-450) Suboptimal Query strategy for JPA queries that return multiple Entities
Date Fri, 23 Nov 2007 23:41:43 GMT
Suboptimal Query strategy for JPA queries that return multiple Entities
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                 Key: OPENJPA-450
                 URL: https://issues.apache.org/jira/browse/OPENJPA-450
             Project: OpenJPA
          Issue Type: Improvement
          Components: query
    Affects Versions: 1.0.1
            Reporter: Nick Johnson
            Priority: Minor


When a Query returns instances of  multiple Entity classes, the query strategy is suboptimal.
 First it issues a query to select all the primary keys, and then additional queries are issued
for every one of those primary keys.  If exactly one type of Entity is in the SELECT clause,
a much more efficient, single query is constructed.

I'll attach a test case that demonstrates this.  Watch what happens when this:
		Query q = em.createQuery("Select message, article from Message message, Article article
 "
is changed to this:
		Query q = em.createQuery("Select message from Message message, Article article  "

in terms of which SQL statements are executed.

FYI,  there's more detail on the users list with the subject, "how to avoid large number of
queries" and Patrick Linskey and I have emailed back and forth a bit about it.  I'm opening
this as a nicer way of keeping track of the issue.

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