Re: [Postgres-xc-general] modifying values at the datanode

[Ashutosh B. <ash...@en...>]

On Sat, Oct 5, 2013 at 7:44 PM, Sandeep Gupta <gup...@gm...>wrote:

> Thanks Michael. I understand. The only issue is that we have an update
> query as
>
> update T set T.a = -1 from A where A.x = T.x
>
>
> Both A and T and distributed by x column. The problem is that coordinator
> first does the join and then
> calls update several times at each datanode. This is turning out to be too
> slow. Would have
> been better if the entire query was shipped to the datanodes.
>
>
Right now there is no way to ship a DML with more than one relation
involved there. But that's something, I have been thinking about. If you
have developer resources and can produce a patch. I can help.


> Thanks.
> Sandeep
>
>
>
> On Sat, Oct 5, 2013 at 6:27 AM, Michael Paquier <mic...@gm...
> > wrote:
>
>> On Sat, Oct 5, 2013 at 2:58 AM, Sandeep Gupta <gup...@gm...>
>> wrote:
>> >  I understand that the datanodes are read only and that updates/insert
>> can
>> > happen at coordinator.
>> You got it.
>>
>> > Also, it does not allow modification of column over which the records
>> are distributed.
>> Hum no, 1.1 allows ALTER TABLE that you can use to change the
>> distribution type of a table.
>>
>> > However, in case I know what I am doing, it there anyway possible to
>> modify
>> > the values directly at datanodes.
>> > The modifications are not over column over which distribution happens.
>> If you mean by connecting directly to the Datanodes, no. You would
>> break data consistency if table is replicated by the way by doing
>> that. Let the Coordinator planner do the job and choose the remote
>> nodes for you.
>>
>> There have been discussion to merge Coordinators and Datanodes
>> together though. This would allow what you say, with a simpler cluster
>> design.
>> --
>> Michael
>>
>
>
>
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-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company