Subtransaction SLRU overflow (the 64-subxid cliff)
Simple terms
A backend can hold 64 subtransactions in fast shared memory. Open the 65th and it overflows, and the part that catches people is who pays for it. Not that backend. From that point on, other sessions' visibility checks may have to go to disk for something that used to be a memory read, and the tax lands across the whole cluster. Nobody has to type SAVEPOINT for this, either. Every PL/pgSQL block with an EXCEPTION clause quietly opens a subtransaction of its own.
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A backend can cache only 64 active subtransaction XIDs. What exactly happens on the 65th, why does it silently slow down every OTHER session's visibility checks across the whole cluster (not just the backend that overflowed), and how would you prove and detect it in production? Where do subtransactions get created without anyone writing SAVEPOINT?
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