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https://issues.apache.org/jira/browse/DERBY-2998?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12526227
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Bryan Pendleton commented on DERBY-2998:
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Hi Thomas, thanks for posting the patch! I'll try to have a look at the code
soon.
Perhaps we could treat the inability to reference the "AS R" renamed column in
the WHERE clause as a separate issue since, as you observe, it is a problem
with all column expressions, not just row_number() expressions. The most
commonly-requested use of row_number() is probably going to be for limiting the
number of rows retried, as in DERBY-581.
If you use the nested select form, to limit the results:
select * from (select row_number() as rownum, * from t) where runum <= 10
then does the execution logic actually short-circuit the evaluation properly
after the first 10 rows have been read? Or does the execution of this query run
through the entire table and then later end up discarding all the rows beyond
the first 10?
Since one of the primary goals of DERBY-581 is to efficiently limit the
processing to only a small subset of the rows, we want to make sure that we
achieve that result if possible.
> 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.diff, row_number_prototype.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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