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We have one pretty serious MVCC visibility bug with hot standby replicas. We incorrectly treat any transactions that are in progress in the primary, when the standby is started, as aborted. That can break MVCC for queries running concurrently in the standby. It can also lead to hint bits being set incorrectly, and that damage can last until the replica is restarted. The fundamental bug was that we treated any replica start as starting from a shut down server. The fix for that is straightforward: we need to set 'wasShutdown = false' in InitWalRecovery() (see changes in the postgres repo). However, that introduces a new problem: with wasShutdown = false, the standby will not open up for queries until it receives a running-xacts WAL record from the primary. That's correct, and that's how Postgres hot standby always works. But it's a problem for Neon, because: * It changes the historical behavior for existing users. Currently, the standby immediately opens up for queries, so if they now need to wait, we can breka existing use cases that were working fine (assuming you don't hit the MVCC issues). * The problem is much worse for Neon than it is for standalone PostgreSQL, because in Neon, we can start a replica from an arbitrary LSN. In standalone PostgreSQL, the replica always starts WAL replay from a checkpoint record, and the primary arranges things so that there is always a running-xacts record soon after each checkpoint record. You can still hit this issue with PostgreSQL if you have a transaction with lots of subtransactions running in the primary, but it's pretty rare in practice. To mitigate that, we introduce another way to collect the running-xacts information at startup, without waiting for the running-xacts WAL record: We can the CLOG for XIDs that haven't been marked as committed or aborted. It has limitations with subtransactions too, but should mitigate the problem for most users. See https://github.com/neondatabase/neon/issues/7236. Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>