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From Brian O'Neill <boneil...@gmail.com>
Subject Re: Cassandra to Oracle?
Date Sun, 22 Jan 2012 13:05:47 GMT

Eric,

Thinking even a little bit more about this...

We could go the distributed counter approach with additional column families to support the
ad hoc queries, but use triggers to implement it.  That would allow us to keep the client-side
code thin, but achieve the same result... without necessarily replicating to Oracle for the
attributes we can predict.

Maybe we'll take a look at that this week as well.

thanks again,
brian


On Jan 21, 2012, at 8:35 AM, Eric Czech wrote:

> Hi Brian,
> 
> We're trying to do the exact same thing and I find myself asking very similar questions.
> 
> Our solution though has been to find what kind of queries we need to satisfy on a preemptive
basis and leverage cassandra's built-in indexing features to build those result sets beforehand.
 The whole point here then is that our gain in cost efficiency comes from the fact that disk
space is really cheap and serving up result sets from disk is fast provided that those result
sets are pre-calculated and reasonable in size (even if we don't know all the values upfront).
 For example, when you're writing to your CF "X", you could also make writes to column family
"A" like this:
> 
> - write A[Z][Y] = 1
> where A = CF, Z = key, Y = column
> 
> Answering the question "select count(distinct Y) from X group by Z" then is as simple
as getting a list of rows for CF A and counting the distinct values of Y and grouping them
by Z on the client side.
> 
> Alternatively, there are much better ways to do this with composite keys/columns and
distributed counters but it's hard for me to tell what makes the most sense without knowing
more about your data / product requirements.
> 
> Either way, I feel your pain in getting things like this to work with Cassandra when
the domain of values for a particular key or column is unknown and secondary indexing doesn't
apply, but I'm positive there's a much cheaper way to make it work than paying for Oracle
if you have at least a decent idea about what kinds of queries you need to satisfy (which
it sounds like you do).  To Maxim's "death by index" point, you could certainly go overboard
with this concept and cross a pricing threshold with some other database technology, but I
can't imagine you're even close to being in that boat given how concise your query needs seem
to be.
> 
> If you're interested, I'd be happy to share how we do these things to save lots of money
over commercial databases and try to relate that to your use case, but if not, then I hope
at least some of that this useful for you.
> 
> Good luck either way!
> 
> On Fri, Jan 20, 2012 at 9:27 PM, Maxim Potekhin <potekhin@bnl.gov> wrote:
> I certainly agree with "difficult to predict". There is a Danish
> proverb, which goes "it's difficult to make predictions, especially
> about the future".
> 
> My point was that it's equally difficult with noSQL and RDBMS.
> The latter requires indexing to operate well, and that's a potential
> performance problem.
> 
> 
> On 1/20/2012 7:55 PM, Mohit Anchlia wrote:
> I think the problem stems when you have data in a column that you need
> to run adhoc query on which is not denormalized. In most cases it's
> difficult to predict the type of query that would be required.
> 
> Another way of solving this could be to index the fields in search engine.
> 
> On Fri, Jan 20, 2012 at 7:37 PM, Maxim Potekhin<potekhin@bnl.gov>  wrote:
> What makes you think that RDBMS will give you acceptable performance?
> 
> I guess you will try to index it to death (because otherwise the "ad hoc"
> queries won't work well if at all), and at this point you may be hit with a
> performance penalty.
> 
> It may be a good idea to interview users and build denormalized views in
> Cassandra, maybe on a separate "look-up" cluster. A few percent of users
> will be unhappy, but you'll find it hard to do better. I'm talking from my
> experience with an industrial strength RDBMS which doesn't scale very well
> for what you call "ad-hoc" queries.
> 
> Regards,
> Maxim
> 
> 
> 
> 
> 
> On 1/20/2012 9:28 AM, Brian O'Neill wrote:
> 
> I can't remember if I asked this question before, but....
> 
> We're using Cassandra as our transactional system, and building up quite a
> library of map/reduce jobs that perform data quality analysis, statistics,
> etc.
> (>  100 jobs now)
> 
> But... we are still struggling to provide an "ad-hoc" query mechanism for
> our users.
> 
> To fill that gap, I believe we still need to materialize our data in an
> RDBMS.
> 
> Anyone have any ideas?  Better ways to support ad-hoc queries?
> 
> Effectively, our users want to be able to select count(distinct Y) from X
> group by Z.
> Where Y and Z are arbitrary columns of rows in X.
> 
> We believe we can create column families with different key structures
> (using Y an Z as row keys), but some column names we don't know / can't
> predict ahead of time.
> 
> Are people doing bulk exports?
> Anyone trying to keep an RDBMS in synch in real-time?
> 
> -brian
> 
> --
> Brian ONeill
> Lead Architect, Health Market Science (http://healthmarketscience.com)
> mobile:215.588.6024
> blog: http://weblogs.java.net/blog/boneill42/
> blog: http://brianoneill.blogspot.com/
> 
> 
> 

-- 
Brian ONeill
Lead Architect, Health Market Science (http://healthmarketscience.com)
mobile:215.588.6024
blog: http://weblogs.java.net/blog/boneill42/
blog: http://brianoneill.blogspot.com/


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