add paging to get_count
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Key: CASSANDRA-2894
URL: https://issues.apache.org/jira/browse/CASSANDRA-2894
Project: Cassandra
Issue Type: Improvement
Components: API
Reporter: Jonathan Ellis
Priority: Minor
Fix For: 1.0
It is non-intuitive that get_count materializes the entire slice-to-count on
the coordinator node (to perform read repair and > CL.ONE consistency). Even
experienced users have been known to cause memory problems by requesting large
counts.
The user cannot page the count himself, because you need a start and stop
column to do that, and get_count only returns an integer.
So the best fix is for us to do the paging under the hood, in CassandraServer.
Add a limit to the slicepredicate they specify, and page through it.
We could add a global setting for count_slice_size, and document that counts of
more columns than that will have higher latency (because they make multiple
calls through StorageProxy for the pages).
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