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From Sylvain Lebresne <>
Subject Re: select count(1) and OOM
Date Fri, 05 Jul 2013 18:02:03 GMT
> I was playing with a single-node Cassandra installation when discovered
> that a request like [SELECT COUNT(*) FROM CF] seems to load the entire
> dataset of CF into RAM.

This is the case (the whole CF will be loaded in memory). And it's
currently a know limitation of Cassandra 1.2. This will be fix in Cassandra
2.0 but require some ground work (made in that is too complex
to backport in 1.2. So avoid those count queries for now unless you know
the data set is small.

> As far as I understand, a counting request works roughly the same way as
> [SELECT * FROM] with only difference that it doesn't return any data back.
> Is my reasoning correct?

That part is pretty much correct. If you do SELECT * FROM CF (without any
WHERE clause that is), it will also load the whole CF in memory and I would
bet that this OOM as well (if the count(*) OOM).


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