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https://issues.apache.org/jira/browse/PHOENIX-2606?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15151990#comment-15151990
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James Taylor commented on PHOENIX-2606:
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We currently do a sort per scan on the client-side for group by and then do the
final merge/aggregation. This sort uses memory mapped files, so in a way it's a
kind of spooling. We could potentially do the sort on the server-side instead
and then only pull over the data as the final merge is done. Here are some
relevant JIRAs for improvements: PHOENIX-1217, PHOENIX-1006.
> Cursor support in Phoenix
> -------------------------
>
> Key: PHOENIX-2606
> URL: https://issues.apache.org/jira/browse/PHOENIX-2606
> Project: Phoenix
> Issue Type: New Feature
> Reporter: Sudarshan Kadambi
>
> Phoenix should look to support a cursor model where the user could set the
> fetch size to limit the number of rows that are fetched in each batch. Each
> batch of result rows would be accompanied by a flag indicating if there are
> more rows to be fetched for a given query or not.
> The state management for the cursor could be done in the client side or
> server side (i.e. HBase, not the Query Server). The client side state
> management could involve capturing the last key in the batch and using that
> as the start key for the subsequent scan operation. The downside of this
> model is that if there were any intervening inserts or deletes in the result
> set of the query, backtracking on the cursor would reflect these additional
> rows (consider a page down, followed by a page up showing a different set of
> result rows). Similarly, if the cursor is defined over the results of a join
> or an aggregation, these operations would need to be performed again when the
> next batch of result rows are to be fetched.
> So an alternate approach could be to manage the state server side, wherein
> there is a query context area in the Regionservers (or, maybe just a
> temporary table) and the cursor results are fetched from there. This ensures
> that the cursor has snapshot isolation semantics. I think both models make
> sense but it might make sense to start with the state management completely
> on the client side.
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