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Re: [eclipselink-users] N + 1 SELECT: not seeing it on an EAGER load
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I believe the confusion comes from the fact you are using a 1:M
relationship. Think of it as a 1:1 and it should be more clear.
EclipseLink needs to use only a single query for any relationship, so
1:M and 1:1 work the same with respect to batching.
N refers to the number of results from the first query, while the 1 is
the first query itself. So if a query for A returns 10 results, you
will see 11 queries - 1 for the As, and 10 for Bs (as EclipseLink builds
each A, it will query for that A's Bs).
With batching, those 10 queries can be pushed into a single query to
return all Bs (instead of just the Bs for the currently being built A).
With Joining, that extra query gets pushed into the original query.
You are only returning a single A, so see 2 queries.
On 13/06/2013 2:10 PM, Laird Nelson wrote:
On Thu, Jun 13, 2013 at 10:49 AM, Christopher Delahunt
<christopher.delahunt@xxxxxxxxxx
<mailto:christopher.delahunt@xxxxxxxxxx>> wrote:
The find query only return [sic] a single value, an A with ID=1 and
so only needed a single additional query for its associated Bs.
OK; I see. From reading
http://java-persistence-performance.blogspot.com/2010/08/batch-fetching-optimizing-object-graph.html,
specifically the section entitled "N + 1 queries problem", it looked
like EclipseLink would somehow issue a single query per child. For
example, in the example in that article the employee's phone numbers are
each retrieved by ID, not by WHERE (employeeId = ?). I guess that's not
actually how it works?
What I hear you saying instead is that for any parent (A), by default
all children of a @OneToMany relationship (Bs) will be selected in one
shot for that one parent/A instance. What will NOT be the case is that
if I select lots of As, all of the relevant Bs will be selected in one
shot, UNLESS I turn on batch fetching.
To put it yet another way:
* Without batch fetching: if I get one A that logically has 10 Bs,
I'll have two queries: one for the A, and one for all Bs that relate
to it. (Or that's what I'm observing, and what I had a question
about; I would have thought I'd have 11 queries, not 2.)
* Without batch fetching: if I get two As, each of which logically has
10 Bs, I'll have four queries: one each for each A, and one each for
each A's Bs. (Again, I thought I'd see 22 queries.)
* With batch fetching: if I get ten As, each of which logically has 10
Bs, I'll still get only two queries (not worrying about batch size
at the moment): one for the As, and one for all relevant Bs.
My question can then be better asked: why, in my first bullet point
above, do I see two queries and not 11?
Thanks,
Best,
Laird
--
http://about.me/lairdnelson
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