Vermeulen created OPENJPA-2296:
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Summary: 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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