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https://issues.apache.org/jira/browse/CAY-2909?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Nikita Timofeev reassigned CAY-2909:
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Assignee: Nikita Timofeev
> Vertical Inheritence: Unnecessary Disjoint-By-Id Prefetch Query
> ---------------------------------------------------------------
>
> Key: CAY-2909
> URL: https://issues.apache.org/jira/browse/CAY-2909
> Project: Cayenne
> Issue Type: Bug
> Affects Versions: 5.0-M1
> Reporter: Jadon Hansell
> Assignee: Nikita Timofeev
> Priority: Major
> Fix For: 5.0-M2
>
> Attachments: unnecessary-prefetch-fix.patch
>
>
> Using the entities from the {{inheritance-vertical}} test map:
> {{IvBase}} has a child entity {{IvImpl}}. Say we have a one-to-many
> relationship from {{IvImpl}} to {{IvOther}} called {{other}}. The important
> part is that the foreign key is on the child entity's table:
> {{IV_IMPL.OTHER_ID}}. The db relationship path would start from {{IV_BASE}}
> and be {{impl.other}}.
> The reverse relationship would be many-to-one from {{IvOther}} to {{IvImpl}}
> called {{relatedImpls}} (or similar). The db relationship path would start
> from {{IV_OTHER}} and be {{relatedImpls.base}}.
> Because it is a flattened relationship,
> {{ObjRelationship.isSourceIndependentFromTargetChange()}} returns {{true}}
> for the {{IvImpl.other}} relationship. This causes
> {{HierarchicalObjectResolver.processDisjointByIdNode}} to add the reverse
> relationship to the query, and for the {{PrefetchPrcessorTreeBuilder}} to
> choose the {{JoinedIdParentAttachmentStrategy}}. On most flattened paths this
> works well.
> In this case, since {{IvImpl}} is a child entity, the data row for the root
> entity of the query already contains the path {{impl.OTHER_ID}}, so we
> already have the id and this extra query for the reverse relationship is
> unnecessary.
> I've attached a (very) experimental patch that fixes this issue on master.
> I'm not super familiar with this part of the codebase, so I've probably
> overlooked some edge cases. It introduces an {{isFkOnChildEntity}} method on
> {{ObjRelationship}} that allows me to check for this edge case and avoid
> querying the reverse relationship. I then updated
> {{ResultScanParentAttachmentStrategy}} to handle this case by adding the
> appropriate prefix ({{"impl."}}) to get the id off of the {{DataRow}}.
> I'm not sure how to write a breaking test for an unnecessary extra query, but
> when I manually tested it in my own environment it reduced the execution time
> of some queries from ~10s to ~100ms.
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