Andrei Lepikhov continues a thread:
Recently, I looked into 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 question ‘how widespread is this pattern?’ After all, if you’re going to improve something, it helps to know how useful the improvement will be, right? Here, I want to give a sense of how this pattern leads to performance regressions and point out directions in the PostgreSQL optimiser that could make the situation easier.
Much of this is speculative in nature but the three proposed solution ideas are all interesting.