Re: [Postgres-xc-developers] Postgres-xc coordinator operation

[Abbas B. <abb...@en...>]

On Wed, Jun 3, 2015 at 4:11 AM, Manikandan Soundarapandian <ma...@vt...>
wrote:

> Hi,
>
> I am a graduate student working on my research in parallel databases. I
> would like to know how the postgres-xc coordinator works. I understand that
> the datanodes run the query in parallel and the results are collected by
> the coordinator which runs any more computation that is required or just
> provides the output to the client that requested the query. I would like to
> know whether the coordinator does this data collection from datanodes in a
> sequential fashion?
>

The coordinator uses multiplexed IO using select on all fds of datanodes.
For more details please see pgxc_node_receive function in pgxcnode.c. The
loop for reading data on all set fds is sequential, but the coordinator
does not wait for data from the datanode to which the coordinator had sent
the query first.


> For example, lets consider we want to run the query on table *table_x*
> which is hash distributed among 10 datanodes,
> *select count(*) from table_x;*
> Each datanode will run the query and give their local counts and the
> coordinator has to collect the individual counts and come up with the final
> count before sending the output. Is the data collection process at the
> coordinator done in a sequential fashion? I am actually looking to
> introduce some kind of parallelism in this data collection if it is
> sequential and do performance studies. Please clarify.
>

To improve performance of any system, first study the bottleneck, and
target to widen that. Have you done any study of Postgres-XC to find where
the performance bottleneck is?


>
> --
> Thanks
> Mani
> Department of Computer Science
> Virginia Tech
>
>
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