Re: Parallel Seq Scan

On Wed, Jan 28, 2015 at 10:40 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> The problem here, as I see it, is that we're flying blind. If there's
>> just one spindle, I think it's got to be right to read the relation
>> sequentially. But if there are multiple spindles, it might not be,
>> but it seems hard to predict what we should do. We don't know what
>> the RAID chunk size is or how many spindles there are, so any guess as
>> to how to chunk up the relation and divide up the work between workers
>> is just a shot in the dark.
>
> I thought the proposal to chunk on the basis of "each worker processes
> one 1GB-sized segment" should work all right. The kernel should see that
> as sequential reads of different files, issued by different processes;
> and if it can't figure out how to process that efficiently then it's a
> very sad excuse for a kernel.

I agree. But there's only value in doing something like that if we
have evidence that it improves anything. Such evidence is presently a
bit thin on the ground.

--
Robert Haas
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