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From Aaron Morton <aa...@thelastpickle.com>
Subject Re: is the select result grouped by the value of the partition key?
Date Thu, 12 Sep 2013 02:19:06 GMT
> GROUP BY "feature",
I would not think of it like that, this is about physical order of rows.  

> since it seems really important yet does not seem to be mentioned in the
> CQL reference documentation.
It's baked in, this is how the data is organised on the row. 

http://www.datastax.com/dev/blog/thrift-to-cql3
We often say the PRIMARY KEY is the PARTITION KEY and the GROUPING COLUMNS
http://www.datastax.com/documentation/cql/3.0/webhelp/index.html#cql/cql_reference/create_table_r.html

See also http://thelastpickle.com/blog/2013/01/11/primary-keys-in-cql.html

> Is it something we can bet the farm and farmer's family on?
Sure. 

> The kinds of scenarios where I am wondering if it's possible for partition-key groups
> to get intermingled are :
All instances of the table entity with the same value(s) for the PARTITION KEY portion of
the PRIMARY KEY existing in the same storage engine row. 

>   .   what if the node containing primary copy of a row is down
There is no primary copy of a row. 

>   .   what if there is a heavy stream of UPDATE activity from applications which
>       connect to all nodes,   causing different nodes to have different versions of replicas
of same row?
That's fine with me. 
It's only an issue when the data is read, and at that point the Consistency Level determines
what we do. 

Hope that helps. 


-----------------
Aaron Morton
New Zealand
@aaronmorton

Co-Founder & Principal Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com

On 12/09/2013, at 7:43 AM, John Lumby <johnlumby@hotmail.com> wrote:

> I would like to make quite sure about this implicit GROUP BY "feature",
> 
> since it seems really important yet does not seem to be mentioned in the
> CQL reference documentation.
> 
> 
> 
> Aaron,   you said "yes"  --   is that "yes,  always,   in all scenarios no matter what"
> 
> or "yes usually"?      Is it something we can bet the farm and farmer's family on?
> 
> 
> 
> The kinds of scenarios where I am wondering if it's possible for partition-key groups
> to get intermingled are :
> 
> 
> 
>   .   what if the node containing primary copy of a row is down
>                 and 
> cassandra fetches this row from a replica on a different node
>                (e.g.  with CONSISTENCY ONE)
> 
>   .   what if there is a heavy stream of UPDATE activity from applications which
>       connect to all nodes,   causing different nodes to have different versions of replicas
of same row?
> 
> 
> 
> Can you point me to some place in the cassandra source code where this grouping is ensured?
> 
> 
> 
> Many thanks,
> 
> John Lumby 		 	   		  


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