Re: Parallel Seq Scan

On Wed, Oct 14, 2015 at 12:30 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> - I continue to think GetParallelShmToc is the wrong approach.
>> Instead, each time ExecParallelInitializeDSM or
>> ExecParallelInitializeDSM calls a nodetype-specific initialized
>> function (as described in the previous point), have it pass d->pcxt as
>> an argument. The node can get the toc from there if it needs it. I
>> suppose it could store a pointer to the toc in its scanstate if it
>> needs it, but it really shouldn't. Instead, it should store a pointer
>> to, say, the ParallelHeapScanDesc in the scanstate.
>
> How will this idea work for worker backend. Basically in worker
> if want something like this to work, toc has to be passed via
> QueryDesc to Estate and then we can retrieve ParallelHeapScanDesc
> during PartialSeqScan initialization (ExecInitPartialSeqScan).
> Do you have something else in mind?

Good question. I think when the worker starts up it should call yet
another planstate-walker, e.g. ExecParallelInitializeWorker, which can
call nodetype-specific functions for parallel-aware nodes and give
each of them a chance to access the toc and store a pointer to their
parallel shared state (ParallelHeapScanDesc in this case) in their
scanstate. I think this should get called from ParallelQueryMain
after ExecutorStart and before ExecutorRun:
ExecParallelInitializeWorker(queryDesc->planstate, toc).

--
Robert Haas
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