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https://issues.apache.org/jira/browse/DERBY-2998?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12527589
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Thomas Nielsen commented on DERBY-2998:
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When I step through the code executing the aliased nested selects above there
is (currently) no between the inner SELECT and the outer WHERE. This makes the
inner ROW_NUMBER() to be computed for *all* rows in the table, and then the
first N rows are selected in the WHERE clause. As previously stated there is no
short-cut in the execution. Not a very effective idea if you have a
million-and-one rows in the table and only want the first few.
It would be strange if all subqueries are executed like this in derby - so on
with the investigation...
> Add support for ROW_NUMBER() window function
> --------------------------------------------
>
> Key: DERBY-2998
> URL: https://issues.apache.org/jira/browse/DERBY-2998
> Project: Derby
> Issue Type: Sub-task
> Components: SQL
> Reporter: Thomas Nielsen
> Assignee: Thomas Nielsen
> Priority: Minor
> Attachments: row_number_prototype-2.diff, row_number_prototype-2.stat
>
>
> As part of implementing the overall OLAP Operations features of SQL
> (DERBY-581), implement the ROW_NUMBER() window function.
> More information about this feature is available at
> http://wiki.apache.org/db-derby/OLAPRowNumber
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