Florian Pflug <f...@phlo.org> wrote: > Oh, cool. I thought the opt-out only works for explicitly > DEFERRABLE transactions. Basically, if there is no serializable read-write transaction active which overlaps the read-only transaction and also overlaps a serializable transaction which wrote something and committed in time to be visible to the read-only transaction, then the read-only transaction's snapshot is "safe" and it can stop worrying about SSI logic. If these conditions happen to exist when a read-only transaction is starting, it never needs to set up for SSI; it can run just like a REPEATABLE READ transaction and still be safe from serialization anomalies. We make some effort to spot the transition to this state while a read-only transaction is running, allowing it to "drop out" of SSI while running. The fact that a read-only transaction can often skip some or all of the SSI overhead (beyond determining that opting out is safe) is why declaring transactions to be READ ONLY when possible is #1 on my list of performance considerations for SSI. -Kevin
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