On Tue, Aug 29, 2017 at 10:05 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Amit Kapila <amit.kapil...@gmail.com> writes:
>> On Tue, Aug 29, 2017 at 8:32 AM, Robert Haas <robertmh...@gmail.com> wrote:
>>> There's already ExecParallelReinitialize, which could be made to walk
>>> the nodes in addition to what it does already, but I don't understand
>>> exactly what else needs fixing.
>> Sure, but it is not advisable to reset state of all the nodes below
>> gather at that place, otherwise, it will be more or less like we are
>> forcing rescan of each node.  I think there we can reset the shared
>> parallel state of parallel-aware nodes (or anything related) and then
>> allow rescan to reset the master backend specific state for all nodes
>> beneath gather.
> Right, the idea is to make this happen separately from the "rescan"
> logic.  In general, it's a good idea for ExecReScanFoo to do as little
> as possible, so that you don't pay if a node is rescanned more than
> once before it's asked to do anything, or indeed if no rows are ever
> demanded from it at all.
> Attached is a WIP patch along this line.

The idea looks sane to me.

>  It's unfinished because
> I've not done the tedium of extending the FDW and CustomScan APIs
> to support this new type of per-node operation; but that part seems
> straightforward enough.  The core code is complete and survived
> light testing.

I have also played a bit with both of the patches together and didn't
found any problem.  In your second patch, I have a minor comment.

  ExecReScanGather(GatherState *node)
! /* Make sure any existing workers are gracefully shut down */

The above call doesn't ensure the shutdown. It just ensures that we
receive all messages from parallel workers.  Basically, it doesn't
call WaitForParallelWorkersToExit.

> I'm pretty happy with the results --- note in
> particular how we get rid of some very dubious coding in
> ExecReScanIndexScan and ExecReScanIndexOnlyScan.
> If you try the test case from a2b70c89c on this patch alone, you'll notice
> that instead of sometimes reporting too-small counts during the rescans,
> it pretty consistently reports too-large counts.  This is because we are
> now successfully resetting the shared state for the parallel seqscan, but
> we haven't done anything about the leader's HashAgg node deciding that it
> can re-use its old hashtable.  So on the first scan, the leader typically
> scans all or most of the table because of its startup time advantage, and
> saves those counts in its hashtable.  On later scans, the workers read all
> of the table while the leader decides it need do no scanning.  So we get
> counts that reflect all of the table (from the workers) plus whatever part
> of the table the leader read the first time.  So this by no means removes
> the need for my other patch.
> If no objections, I'll do the additional legwork and push.

No objections.

>  As before,
> I think we can probably get away without fixing 9.6, even though it's
> nominally got the same bug.


With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

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