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https://issues.apache.org/jira/browse/OPENJPA-2296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13493917#comment-13493917
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Vermeulen commented on OPENJPA-2296:
------------------------------------
Moving up the call stack I see that the eager select type used for fetching the
field entities.Company.employee is indeeds set to join
by RelationFieldStrategy.selectEagerJoin. I think it is logical that a toOne
uses a join instead of parallel select. However somehow further on this causes
JDBCStoreManager.select to not consider parallel mode anymore for any of the
relation fields of Employee. This can be seen where it calls createEagerSelects
using the line
eager = Math.min(eager, fetch.getEagerFetchMode())
which results in JOIN, not parallel. Somehow this also doesn't result in an
efficient SQL with join but gives the N+1 select.
I suspect that this problem may have been introduced when someone fixed another
bug. The RelationFieldStrategy.selectEagerJoin that restricts the eager select
mode to join has this comment:
// limit the eager mode to single on recursive eager fetching b/c
// at this point the select has been modified and an attempt to
// clone it for a to-many eager select can result in a clone that
// produces invalid SQL
I am now giving up my debugging because I really have insufficient knowledge of
OpenJPA to know how to proceed from this without introducing new bugs. But I
think I have provided more than enough information for an active OpenJPA
committer to proceed from here to solve the problem for which I would be very
grateful.
> N+1 select in eager fetching relation fields of toOne relations
> ---------------------------------------------------------------
>
> Key: OPENJPA-2296
> URL: https://issues.apache.org/jira/browse/OPENJPA-2296
> Project: OpenJPA
> Issue Type: Bug
> Components: jdbc, kernel, performance
> Affects Versions: 2.2.1
> Reporter: Vermeulen
> Priority: Critical
>
> We have very serious performance problems in fetching our JPA entities caused
> by N+1 select problems. I suspect that these are caused by a small bug in
> OpenJPA's eager fetching implementation that should not be too hard to fix.
> This is especially bad in our relatively complex reservation entity where we
> need to eagerly fetch customer, contacts, orderlines, products etc. It takes
> over 3 seconds to fetch a single reservation from a local db. The problem
> seems to occur at multiple places in this object graph. The cause of the
> problem is probably the following issue where eager fetching of relations of
> toOne relations use neither a parallel or join select strategy but uses
> separate "load field"s of EACH field of EACH toOne relation in the object
> graph. I am not the only one having this issue, see also mailing list
> http://openjpa.208410.n2.nabble.com/Fetchgroups-recursion-problem-td3874382.html#a7581598
> Suppose we have entities Company 1..* Employee 1..* Project
> According to Chapter 8. Eager Fetching of the manual: "Continuing our
> example, if the Employee class had a list of Projects in one of the fetch
> groups being loaded, OpenJPA would execute a single additional select in
> parallel to load the projects of all employees of the matching companies."
> This is indeed what happens when both relations are eager (or are marked lazy
> but are in the fetchplan). The number of queries needed to fetch a Company
> does not depend on the number of employees or projects:
> 8913 testPU TRACE [main] openjpa.Query - Executing query: SELECT z FROM
> Company z
> 19885 testPU TRACE [main] openjpa.jdbc.SQLDiag - Eager relations:
> [entities.Employee.projects, entities.Company.employees]
> 19889 testPU TRACE [main] openjpa.jdbc.SQL - <t 12833564, conn 5535786>
> executing prepstmnt 20965449 SELECT t0.id, t0.name FROM Company t0
> 19891 testPU TRACE [main] openjpa.jdbc.SQL - <t 12833564, conn 5535786> [2
> ms] spent
> 20237 testPU TRACE [main] openjpa.jdbc.SQLDiag - Eager relations:
> [entities.Employee.projects, entities.Company.employees]
> 20237 testPU TRACE [main] openjpa.jdbc.SQL - <t 12833564, conn 5535786>
> executing prepstmnt 21924553 SELECT t0.id, t1.id, t1.name FROM Company t0
> INNER JOIN Employee t1 ON t0.id = t1.COMPANY_ID ORDER BY t0.id ASC
> 20248 testPU TRACE [main] openjpa.jdbc.SQL - <t 12833564, conn 5535786>
> [10 ms] spent
> 21308 testPU TRACE [main] openjpa.jdbc.SQLDiag - Eager relations:
> [entities.Employee.projects, entities.Company.employees]
> 21309 testPU TRACE [main] openjpa.jdbc.SQL - <t 12833564, conn 5535786>
> executing prepstmnt 13123665 SELECT DISTINCT t1.id, t2.id, t2.name FROM
> Company t0 INNER JOIN Employee t1 ON t0.id = t1.COMPANY_ID INNER JOIN Project
> t2 ON t1.id = t2.EMPLOYEE_ID ORDER BY t1.id ASC
> 21376 testPU TRACE [main] openjpa.jdbc.SQL - <t 12833564, conn 5535786>
> [67 ms] spent
> 22247 testPU TRACE [main] openjpa.jdbc.JDBC - <t 12833564, conn 0> [0 ms]
> close
> However when the relation between Company and Employee is replaced with a
> OneToOne or ManyToOne relation, this efficient eager fetching fails and
> OpenJPA resorts to single object loads for the projects field of each
> Employee:
> ....
> 41580 testPU TRACE [main] openjpa.Query - Executing query: SELECT z FROM
> Company z
> 49121 testPU TRACE [main] openjpa.jdbc.SQLDiag - Eager relations:
> [entities.Company.employee]
> 49121 testPU TRACE [main] openjpa.jdbc.SQL - <t 22418487, conn 10229202>
> executing prepstmnt 17563180 SELECT t0.id, t1.id, t1.name, t0.name FROM
> Company t0 LEFT OUTER JOIN Employee t1 ON t0.id = t1.COMPANY_ID
> 49142 testPU TRACE [main] openjpa.jdbc.SQL - <t 22418487, conn 10229202>
> [21 ms] spent
> 61282 testPU TRACE [main] openjpa.jdbc.SQLDiag - load field: 'projects'
> for oid=51 class entities.Employee
> 62593 testPU TRACE [main] openjpa.jdbc.SQL - <t 22418487, conn 10229202>
> executing prepstmnt 27638389 SELECT t0.id, t0.name FROM Project t0 WHERE
> t0.EMPLOYEE_ID = ? [params=?]
> 62594 testPU TRACE [main] openjpa.jdbc.SQL - <t 22418487, conn 10229202>
> [1 ms] spent
> 64554 testPU TRACE [main] openjpa.jdbc.SQLDiag - load field: 'projects'
> for oid=52 class entities.Employee
> 67645 testPU TRACE [main] openjpa.jdbc.SQL - <t 22418487, conn 10229202>
> executing prepstmnt 18914646 SELECT t0.id, t0.name FROM Project t0 WHERE
> t0.EMPLOYEE_ID = ? [params=?]
> ....
> In the first (efficient) case OpenJPA logs
> openjpa.jdbc.SQLDiag - Eager relations: [entities.Employee.projects,
> entities.Company.employees]
>
> In the second (inefficient) case OpenJPA logs
> openjpa.jdbc.SQLDiag - Eager relations: [entities.Company.employee]
> This issue may be related to
> https://issues.apache.org/jira/browse/OPENJPA-1920 which also uses single
> "load field"s.
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