Thread: how is a query passed between a coordinator and a datanode

Hello, 

I'm trying to figure out how a query and its result is passed between a coordinator and a datanode. I know there are
manymessages passed between them to finish a query.
 


I did a test against the coordinator by adding a row to a table and the sql was, insert into hg1(id, name)
values(1,'tom').
 

I found a command 'P' was sent from the coordinator to a datanode and there was a remote statement as following,


stmt_name=p_1_25af_f
query_string=Remote Subplan
plan_string={REMOTESTMT :commandType 3 :hasReturning false ...}


My questions are,1-does the coordinator use the remote statement to tell a datanode what to do? If so, how is the plan
stringcreated by the coordinator and how is the plan_string parsed by the datanode?
 
2-if there are multiple rows in the result of the query, how are the rows of data passed from the datanode to the
coordinator?Does the datanode just send all the rows of data to the coordinator? or the coordinator get each row of
databy sending a query?
 

Thank you very much!
Rui Hai

On Tue, Jun 23, 2015 at 5:07 AM, Rui Hai Jiang <ruihaijiang@msn.com> wrote:
> I'm trying to figure out how a query and its result is passed between a coordinator and a datanode. I know there are
manymessages passed between them to finish a query. 
>
>
> I did a test against the coordinator by adding a row to a table and the sql was, insert into hg1(id, name)
values(1,'tom').
>
> I found a command 'P' was sent from the coordinator to a datanode and there was a remote statement as following,
>
>
> stmt_name=p_1_25af_f
> query_string=Remote Subplan
> plan_string={REMOTESTMT :commandType 3 :hasReturning false ...}
>
>
> My questions are,
>  1-does the coordinator use the remote statement to tell a datanode what to do? If so, how is the plan string created
bythe coordinator and how is the plan_string parsed by the datanode? 
>
>  2-if there are multiple rows in the result of the query, how are the rows of data passed from the datanode to the
coordinator?Does the datanode just send all the rows of data to the coordinator? or the coordinator get each row of
databy sending a query? 
>
>
>  Thank you very much!

This is probably an appropriate question for postgres-xl-developers,
but not pgsql-hackers.  Those concepts do not exist in PostgreSQL,
only Postgres-XC or one of its proliferating set of forks.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company