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https://issues.apache.org/jira/browse/PHOENIX-2700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15155425#comment-15155425
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James Taylor commented on PHOENIX-2700:
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Yes, this particular query only needs to sum the duplicates. Would a sliding
window be able to prevent all the keys from being returned from the server? The
trickiest case is the boundary case, though - where a duplicate may straddle
across two regions. Nope, no windowing in Phoenix yet.
> Push down count(group by key) queries
> -------------------------------------
>
> Key: PHOENIX-2700
> URL: https://issues.apache.org/jira/browse/PHOENIX-2700
> Project: Phoenix
> Issue Type: Bug
> Reporter: James Taylor
>
> Queries that attempt to detect duplicates potentially return a lot of data to
> the client if the column being deduped is near unique. For example:
> {code}
> SELECT SUM(DUP_COUNT)
> FROM (
> SELECT DEDUP_KEY, COUNT(1) As DUP_COUNT
> FROM TABLE_TO_DEDUP
> GROUP BY DEDUP_KEY
> )
> WHERE DUP_COUNT > 1
> {code}
> If all of the following are true, then we can detect duplicates on the region
> server in our coprocessors instead of returning every unique DEDUP_KEY to the
> client for a final merge:
> - each scan won't be split on the same DEDUP_KEY
> - the DEDUP_KEY is the leading primary key column
> - we can push the DUP_COUNT > 1 evaluation through our coprocessor
> The first requirement is the hardest, but potentially there could be a custom
> split policy added.
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