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From Tyler Hobbs <ty...@datastax.com>
Subject Re: Understanding SELECT * paging/ordering
Date Fri, 18 Mar 2016 23:17:25 GMT
On Fri, Mar 18, 2016 at 4:58 PM, Dan Checkoway <dcheckoway@gmail.com> wrote:

> Say I have a table with 50M rows in a keyspace with RF=3 in a cluster of
> 15 nodes (single local data center).  When I do "SELECT * FROM table" and
> page through those results (with a fetch size of say 1000), I'd like to
> understand better how that paging works.
>
> Specifically, what determines the order in which which rows are returned?
>

Results are returned in token order (murmur3 hash of the partition key),
and within a single partition, rows are ordered by the clustering key.


>   And what's happening under the hood...i.e. is the coordinator fetching
> pages of 1000 from each node, passing some sort of paging state to each
> node, and the coordinator merges the per-node sorted result sets?
>

The coordinator sequentially[1] queries each token range until it has
enough rows to meet the page size.  When the next page is fetched, it
resumes this process, but starts at the last-used token (which is in the
paging state that the driver passes to the coordinator) rather than the
start of the ring.


> I'm also curious how consistency level comes into play.  i.e. if I use ONE
> vs. QUORUM vs. ALL, how that impacts where the results come from and how
> they're ordered, merged, and who knows what else I don't know...  :-)
>

The only difference between ONE and QUORUM is that the coordinator will
query multiple replicas for each token range and perform the standard
conflict resolution.

[1] In reality, based on estimates of how many token ranges it will need to
query in order to meet the page size, it will query multiple token ranges
in parallel.  See CASSANDRA-1337 for details.

-- 
Tyler Hobbs
DataStax <http://datastax.com/>

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