Gregory Stark wrote:
6) I still don't understand why you need unobserved_xids. We don't need this in normal running, an xid we don't know for certain is committed is exactly the same as a transaction we know is currently running or aborted. So why do you need it during HS?
In normal operation, any transaction that's in-progress has an entry in ProcArray. GetSnapshot() gathers the xids of all those in-progress transactions, so that they're seen as not-committed even when the they're later marked as committed in clog.
In HS, we might see the first WAL record of transaction 10 before we see the first WAL record of transaction 9. Without unobserved_xids, if you take a snapshot in the standby between those two WAL records, xid 10 is included in the snapshot, but 9 is not. If xact 9 later commits, it's marked in clog as committed, and it will suddenly appear as visible to the snapshot. To avoid that, when we replay the first WAL record of xact 10, we also add 9 to the unobserved xid array, so that it's included in snapshots.
So, you can think of the unobserved xids array as an extension of ProcArray. The entries are like light-weight PGPROC entries. In fact I proposed earlier to simply create dummy PGPROC entries instead.
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