Re: Parallel Seq Scan

On Sat, Sep 26, 2015 at 8:38 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> QueryDesc's totaltime is for instrumentation information for plugin's
>> like pg_stat_statements and we need only the total buffer usage
>> of each worker to make it work as the other information is already
>> collected in master backend, so I think that should work as I have
>> written.
>
> I don't think that's right at all.  First, an extension can choose to
> look at any part of the Instrumentation, not just the buffer usage.
> Secondly, the buffer usage inside QueryDesc's totaltime isn't the same
> as the global pgBufferUsage.

Oh... but I'm wrong.  As long as our local pgBufferUsage gets update
correctly to incorporate the data from the other workers, the
InstrStopNode(queryDesc->totaltime) will suck in those statistics.
And the only other things getting updated are nTuples (which shouldn't
count anything that the workers did), firsttuple (similarly), and
counter (where the behavior is a bit more arguable, but just counting
the master's wall-clock time is at least defensible).  So now I think
you're right: this should be OK.

-- 
Robert Haas
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