OpenJPA
  1. OpenJPA
  2. OPENJPA-1920

N+1 select with entities that contain Maps (even with FetchType.EAGER)

    Details

    • Type: Improvement Improvement
    • Status: Open
    • Priority: Major Major
    • Resolution: Unresolved
    • Affects Version/s: 2.0.0
    • Fix Version/s: None
    • Component/s: kernel
    • Labels:
      None

      Description

      When I have an entity that contains a Map, e.g.:

      @Entity
      public class TestEntity

      { @ElementCollection(fetch = FetchType.EAGER) Map<String, String> strings = new HashMap<String, String>(); }

      And I select all existing entities
      String query = "SELECT z FROM " + TestEntity.class.getSimpleName()
      + " z";
      List<TestEntity> result = em.createQuery(query, TestEntity.class)
      .getResultList();

      n + 1 selects are performed which can be seen by using <property name="openjpa.Log" value="DefaultLevel=TRACE" />

      917 testPU TRACE [main] openjpa.Query - Executing query: SELECT z FROM TestEntity z
      918 testPU TRACE [main] openjpa.jdbc.SQL - <t 4837279, conn 6040101> executing prepstmnt 17507279 SELECT t0.id, t0.name FROM TestEntity t0
      918 testPU TRACE [main] openjpa.jdbc.SQL - <t 4837279, conn 6040101> [0 ms] spent
      919 testPU TRACE [main] openjpa.jdbc.SQLDiag - load field: 'strings' for oid=entities.TestEntity-1 class entities.TestEntity
      919 testPU TRACE [main] openjpa.jdbc.SQL - <t 4837279, conn 6040101> executing prepstmnt 7493991 SELECT t0.KEY0, t0.value FROM TestEntity_strings t0 WHERE t0.TESTENTITY_ID = ? [params=(long) 1]
      919 testPU TRACE [main] openjpa.jdbc.SQL - <t 4837279, conn 6040101> [0 ms] spent
      920 testPU TRACE [main] openjpa.jdbc.SQLDiag - load field: 'strings' for oid=entities.TestEntity-51 class entities.TestEntity
      920 testPU TRACE [main] openjpa.jdbc.SQL - <t 4837279, conn 6040101> executing prepstmnt 7200207 SELECT t0.KEY0, t0.value FROM TestEntity_strings t0 WHERE t0.TESTENTITY_ID = ? [params=(long) 51]

      etc.....

      It doesn't matter whether or not I use <property name="openjpa.jdbc.EagerFetchMode" value="parallel"/> or <property name="openjpa.jdbc.EagerFetchMode" value="join"/>.

      This is extremely inefficient when I wish to load a list of products that have their name set in multiple languages by using a Map from language to String.
      As a workaround I can turn the Map into a List and search the List myself for the right entry.

        Issue Links

          Activity

          Hide
          Vermeulen added a comment -

          Added OpenJPA unit tests that show the problem.
          Also added a test to the existing TestParallelEagerMap. This does NOT suffer from the N+1 select problem by using a MapKey annotation and a map value entity that has a map key column.

          So using a MapKey annotaiton may be a good workaround for people that encounter this problem.

          Also tried if the change to RelationFieldStrategy I mentioned in OPENJPA-2296 fixes the N+1 select problem, but it does not, so OPENJPA-2296 may have a different cause.

          Show
          Vermeulen added a comment - Added OpenJPA unit tests that show the problem. Also added a test to the existing TestParallelEagerMap. This does NOT suffer from the N+1 select problem by using a MapKey annotation and a map value entity that has a map key column. So using a MapKey annotaiton may be a good workaround for people that encounter this problem. Also tried if the change to RelationFieldStrategy I mentioned in OPENJPA-2296 fixes the N+1 select problem, but it does not, so OPENJPA-2296 may have a different cause.
          Hide
          Vermeulen added a comment - - edited

          I used the Eclipse debugger which I also used for the related OPENJPA-2296.

          When JDBCStoreManager.createEagerSelects considers the TestEntity.strings field for parallel select, the expression fms[i].supportsSelect(sel, Select.EAGER_PARALLEL, sm, this, fetch) returns 0 so it is ignored for efficient eager fetching.

          Does this mean that this kind of mapping does not support parallel fetch? Why not? Is this easy to add? Why doesn't it do join fetch instead?
          (EDIT: from the debugger I can see that this fetch mode is also considered inside JDBCStoreManager.createEagerSelects but somehow also rejected)

          Show
          Vermeulen added a comment - - edited I used the Eclipse debugger which I also used for the related OPENJPA-2296 . When JDBCStoreManager.createEagerSelects considers the TestEntity.strings field for parallel select, the expression fms [i] .supportsSelect(sel, Select.EAGER_PARALLEL, sm, this, fetch) returns 0 so it is ignored for efficient eager fetching. Does this mean that this kind of mapping does not support parallel fetch? Why not? Is this easy to add? Why doesn't it do join fetch instead? (EDIT: from the debugger I can see that this fetch mode is also considered inside JDBCStoreManager.createEagerSelects but somehow also rejected)
          Hide
          Rick Curtis added a comment -

          Ignore my previous comment, I wasn't running the correct scenario.

          Show
          Rick Curtis added a comment - Ignore my previous comment, I wasn't running the correct scenario.
          Hide
          Rick Curtis added a comment -

          Try using a @PersistentMap rather than a @ElementCollection to see if that resolves the performance issue.

          Show
          Rick Curtis added a comment - Try using a @PersistentMap rather than a @ElementCollection to see if that resolves the performance issue.

            People

            • Assignee:
              Unassigned
              Reporter:
              Vermeulen
            • Votes:
              2 Vote for this issue
              Watchers:
              3 Start watching this issue

              Dates

              • Created:
                Updated:

                Development