Обсуждение: question on parallelism

On Thu, Jan 2, 2014 at 2:29 PM, Chris Travers <chris.travers@gmail.com> wrote:
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> On Wed, Jan 1, 2014 at 7:35 PM, Andrew McIntyre <amcintyre@m-m.com> wrote:
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>> does postgres have this capability? specifically local intrapartition?
>>
>> http://pic.dhe.ibm.com/infocenter/db2luw/v10r5/topic/com.ibm.db2.luw.admin.partition.doc/doc/c0004557.html
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> Nope.  Here's a quick breakdown of what is done in parallel and what is done
> sequentially in PostgreSQL.
>
> Separate queries run in parallel, and can piggy back on eachother's
> sequential scans.  In essence all of the work that is done in parallelism
> affects parallel queries.
>
> There is no intra-query parallelism however.  This means that partitioning
> doesn't generally help here (although sequential scans on the same partition
> by different queries can run in parallel with good results, a single query
> cannot run several scans on different partitions in parallel).
>
> If you need this sort of feature you are going to either need to go to a
> related product (like Postgres-XC) or you are going to need to have some
> other layer that can do the parallelism.
In the case of XC, it is worth noting that a query can take advantage
of parallelism on a sequential scan only roughly for sharded tables.
In the case of replicated table, query is usually simply pushed to one
singke remote node, making the scan occur on this single node.
Regards,
--
Michael