[OPENJPA-2296] N+1 select problems when eager fetching OneToOne and ManyToOne fields

• Type: Bug
• Status: Open
• Priority: Critical
• Resolution: Unresolved
• Affects Version/s: 2.2.1
• Component/s: jdbc, kernel, performance
• Assignee: Unassigned
• Reporter: Vermeulen
• Created: 09/Nov/12 09:22
• Updated: 02/Sep/13 12:24

Description

EDIT:

• an N+1 select problem also always occurs when eagerly fetching a ManyToOne field that is bidirectional (see TestManyOneEagerSQL)
• The patch of "RelationFieldStrategy.selectEagerJoin" I suggested works for the unit tests but does not work in my real application so that may be a different N+1 select issue. (Somehow my fields are in the list of eager relations in OpenJPA's trace, but OpenJPA still uses a "load field" for them.)

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.

Issue Links

• is related to: OPENJPA-134 Extra unneeded SQL joins for OneToMany relationship with fetch type EAGER
• is related to: OPENJPA-1920 N+1 select with entities that contain Maps (even with FetchType.EAGER)