In general wider rows take a bit longer to read, however different access patterns have different performance. I did some tests here http://www.slideshare.net/aaronmorton/cassandra-sf-2012-technical-deep-dive-query-performance and http://thelastpickle.com/2011/07/04/Cassandra-Query-Plans/

I would suggest 1MM cols is fine, if you get to 10MM cols per row you probably have gone too far. Remember the byte size of the row is also important; larger rows churn memory more and take longer to compact / repair.

Hope that helps. 

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Aaron Morton
Freelance Developer
@aaronmorton

On 8/09/2012, at 11:05 AM, Data Craftsman ľ <database.craftsman@gmail.com> wrote:

Hello experts.

Should I limit the number of rows per Composite Primary Key's leading column?

I think it falls into the same wide row good practice for number of
columns per row for CQL 2.0, e.g. 10M or less.

Any comments will be appreciated.

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Thanks,

Charlie (@mujiang) ľ
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Data Architect Developer ԰DBA
http://mujiang.blogspot.com