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https://issues.apache.org/jira/browse/PHOENIX-1779?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14622598#comment-14622598
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James Taylor commented on PHOENIX-1779:
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For paged queries to work, you'd need to specify an ORDER BY clause as that
example does (http://phoenix.apache.org/paged.html).
Paged queries is just one use case for RVCs. There are many others. Basically
with SQL, the optimization is independent of the query results. The basic rule
is that a user needs to specify the intent of the query and realize that
there's no guarantee for ordering if there's no ORDER BY. That's pretty
standard with any RDBMS.
> Parallelize fetching of next batch of records for scans corresponding to
> queries with no order by
> --------------------------------------------------------------------------------------------------
>
> Key: PHOENIX-1779
> URL: https://issues.apache.org/jira/browse/PHOENIX-1779
> Project: Phoenix
> Issue Type: Improvement
> Reporter: Samarth Jain
> Assignee: Samarth Jain
> Fix For: 5.0.0, 4.4.0
>
> Attachments: PHOENIX-1779.patch, PHOENIX-1779_v2.patch,
> PHOENIX-1779_v3.patch, wip.patch, wip3.patch, wipwithsplits.patch
>
>
> Today in Phoenix we parallelize the first execution of scans i.e. we load
> only the first batch of records up to the scan's cache size in parallel.
> Loading of subsequent batches of records in scanners is essentially serial.
> This could be improved especially for queries, including the ones with no
> order by clauses, that do not need any kind of merge sort on the client.
> This could also potentially improve the performance of UPSERT SELECT
> statements that load data from one table and insert into another. One such
> use case being creating immutable indexes for tables that already have data.
> It could also potentially improve the performance of our MapReduce solution
> for bulk loading data by improving the speed of the loading/mapping phase.
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