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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