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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