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They are probably using it as dumb key value storage in a way, and probably found it easier to shard + existing experience.

I'm not sure PostgreSQL sharding ist there yet

Also, uh, it's a pun on haystack... as DHH pointed out somewhere in the thread.

i expect that on their bug tracker, issues have been renamed "needles" so that each issue they work on, is a "needle in the hey stack"