What does setting SubclassFetchMode (Kodo OpenJPA) actually do?

StackOverflow https://stackoverflow.com/questions/2594546

  •  25-09-2019
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Question

What does setting SubClassFetchMode e.g. to EAGER_PARALLEL actually do?

Why would one want a subclass to have a different FetchMode?

The implementation spec for KODO and OpenJPA is notoriously bad and I can't figure it out.

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Solution

From http://download.oracle.com/docs/cd/E15523_01/apirefs.1111/e13946/ref_guide_professional_eager.html

"Setting your subclass fetch mode to parallel affects table-per-class and vertical inheritance hierarchies. Under parallel mode, Kodo issues separate selects for each subclass in a table-per-class inheritance hierarchy, rather than UNIONing all subclass tables together as in join mode. This applies to any operation on a table-per-class base class: query, by-id lookup, or relation traversal."

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