Recently, I how common polymorphic associations actually are in relational databases — a performance-hostile pattern built around a discriminated foreign key that ORMs (Rails, Django, Hibernate), CRM platforms (Salesforce), and 1C generate automatically. The front page of a typical online store, or the activity feed of a CRM, is built by exactly this kind of query: a base table is LEFT JOIN-ed to every possible subtype through a (type, id) pair of columns.That earlier article answered the que...

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