But this makes me wonder how it works when there are concurrent requests. What if a second thread requests data that is being written to memory by the first thread? Shouldn't it also wait for both the write intent record and completion record having been flushed to disk? Otherwise you could end up with a query that returns data that after a crash won't exist anymore.
It's not the write ahead log that prevents that scenario, it's transaction isolation. And note that the more permissive isolation levels offered by Postgres, for example, do allow that failure mode to occur.
If thats the hypothesis, it would be good to see some numbers or proof of concept. The real world performance impact seems not that obvious to predict here.
