On Tue, May 18, 2021 at 11:27 AM Pengchengliu <pengcheng...@tju.edu.cn> wrote: > > Hi Greg, > > Actually I am very confused about ActiveSnapshot and TransactionSnapshot. > I don't know why main process send ActiveSnapshot and TransactionSnapshot > separately. And what is exact difference between them? > If you know that, could you explain that for me? It will be very > appreciated.
In the context of a parallel-worker, I am a little confused too, so I can't explain it either. It is not really explained in the file "src\backend\access\transam\README.parallel", it only mentions the following as part of the state that needs to be copied to each worker: - The transaction snapshot. - The active snapshot, which might be different from the transaction snapshot. So they might be different, but exactly when and why? When I debugged a typical parallel-SELECT case, I found that prior to plan execution, GetTransactionSnapshot() was called and its return value was stored in both the QueryDesc and the estate (es_snapshot), which was then pushed on the ActiveSnapshot stack. So by the time InitializeParallelDSM() was called, the (top) ActiveSnapshot was the last snapshot returned from GetTransactionSnapshot(). So why InitializeParallelDSM() calls GetTransactionSnapshot() again is not clear to me (because isn't then the ActiveSnapshot a potentially earlier snapshot? - which it shouldn't be, AFAIK. And also, it's then different to the non-parallel case). > Before we know them exactly, I think we should not modify the > TransactionSnapshot to ActiveSnapshot in main process. If it is, the main > process should send ActiveSnapshot only. I think it would be worth you trying my suggested change (if you have a development environment, which I assume you have). Sure, IF it was deemed a proper solution, you'd only send the one snapshot, and adjust accordingly in ParallelWorkerMain(), but we need not worry about that in order to test it. Regards, Greg Nancarrow Fujitsu Australia