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Tagged ids for table-per-class

If SmallPublisher extends Publisher, modeled as two small_publishers and publishers tables, what should the tagged id be of smallPublisher.id?

  1. Same tag across all subtypes
    • Pro: can avoid teaching sameEntity that p:1 === sp:1
    • Con: cannot know, when creating a tag for a FK, which type to use; does the publisher_id=1 point to a LargePublisher or SmallPublisher? Should the key be sp:1 or lp:1? We can't know w/o probing the type.
    • Con: Having p:1 doesn't guarantee the constructor is Publisher, it could be SmallPublisher
  2. Per-subtype tags
    • Pro: Neat to just glance at an id and know the subtype
    • Pro: Keeps tag -> correct constructor convention
    • Con: Foreign keys would need to support either sp:1 or lp:1 runtime tag checks
    • Con: Complicates sameEntity for tag values of p:1 and sp:1
  3. Subdivide the keyspace by type, odds are small publishers, even are large publishers
    • Con: Only works when knowing the number of subtypes up-front. Recoding would be impossible.
  4. Add an extra kind column to the base table that is the subtype+id like kind=sp:1. FKs go the kind.