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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3634) Cannot use row_number() in ORDER BY clause
Date Thu, 01 Oct 2009 20:18:23 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3634?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12761326#action_12761326
] 

Dag H. Wanvik commented on DERBY-3634:
--------------------------------------

The row_number() is part of SQL's OLAP machinery and has been in the standard since 1999,
I think.
Until SQL 008 added the new OFFSET/FETCH FIRST syntax, there was no way to achieve "LIMIT"
functionality via standard SQL without using the rather more indirect ROW_NUMBER window function.
If the underlying result set is not materialized (depends on the actual query), the OFFSET/FETCH
FIRST is better, because it will stop reading rows from the table once the the # of fetch
rows have been seen. For the time being, a WHERE predicate on ROW_NUMBER() is not optimized,
so as long as you keep draining the result set, all rows from the underlying (sub)query will
be read. There is currently no optimization for skipping (OFFSET); all rows are read internally
but discarded until we hit the offset.


> Cannot use row_number() in ORDER BY clause
> ------------------------------------------
>
>                 Key: DERBY-3634
>                 URL: https://issues.apache.org/jira/browse/DERBY-3634
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.4.1.3
>            Reporter: Rick Hillegas
>            Assignee: Dag H. Wanvik
>         Attachments: derby-3634-a.diff, derby-3634-a.stat, derby-3634-a.txt, derby-3634-b.diff,
derby-3634-b.stat, derby-3634-c.diff, derby-3634-c.stat, derby-3634-remove-2.diff, derby-3634-remove-2.stat,
derby-3634-remove.diff, derby-3634-remove.stat
>
>
> The following query works correctly:
> select abs(a), row_number() over ()
> from t
> where a > 100 and a < 111
> order by abs(a)
> I expected the following query to also work, but it raised an exception:
> select abs(a), row_number() over ()
> from t
> where a > 100 and a < 111
> order by row_number() over ()
> This is the error I saw: "ERROR 42X01: Syntax error: Encountered "over" at line 5, column
23".
> Here are the reasons why I think that this syntax is supposed to be supported:
> According to my reading of the 2003 SQL spec, the ORDER BY clause should be able to sort
on any expression in the SELECT list. That includes OLAP expressions. I believe this is so
because, according to part 2, section 10.10 (<sort specification>), a <sort key>
can be any <value expression> and if you follow the grammar for <value expression>,
it can resolve to be a <value expression primary> (see section 6.3), which can in turn
resolve to be a <window function>. This reasoning is supported by tracing the hotlinks
on the following page which lays out the SQL 2003 BNF: http://savage.net.au/SQL/sql-2003-2.bnf.html
This interpretation is further supported by the example of an ORDER BY clause referencing
an OLAP expression which is provided on page 23 of the introduction to OLAP written by Fred
Zemke, Krishna Kulkarni, Andy Witkowski, and Bob Lyle: www.cse.iitb.ac.in/dbms/Data/Papers-Other/SQL1999/OLAP-99-154r2.pdf

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