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From Bill de hÓra <b...@dehora.net>
Subject Re: Indexes and hard disk
Date Sat, 12 Feb 2011 15:15:53 GMT
On Sat, 2011-02-12 at 14:03 +0100, Filip Nguyen wrote:


> Why the secondary indexes are even present in Cassandra? I thought the
> point is that development in Cassandra is query driven, that when you
> want to search and fetch for example by birth date you should create
> new ColumnFamilly...



Yes and no. Systems like Cassandra are designed such that you should
write the data out as you want to read it in (because writes are cheap).
However most systems will want to access data via a few other criteria.
For example a blogging system that supports tags will need to list your
blog entries by date and by tag equally efficiently . As you say, you
can spin up a new ColumnFamilly for that, but it's such a common need
that Cassandra 0.7 supports it directly and saves developers having to
manage indexes by hand  (under the hood, a 0.7 index is a 'private' CF).
This for me is one of the features that really sets Cassandra apart  -
scaling and indexing data at the same time is hard, and very few systems
do both well.

Bill 

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