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From "C.S. Nirmal J. Fernando (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4406) Wrong order when using ORDER BY on non-deterministic function
Date Tue, 30 Mar 2010 17:16:27 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12851471#action_12851471
] 

C.S. Nirmal J. Fernando commented on DERBY-4406:
------------------------------------------------

Hi,

After went through the source code and after debugging, I found some interesting points.

==================================================
ij> select random() from t order by random();
               
ascending; true
addedColumnOffset: -1
columnPosition: -1

expression instanceof ColumnReference = false     //random() is not a column
expression.isConstantExpression() = false    //random() is not a constant expression eg: x*2

ascending; true
addedColumnOffset: 0
columnPosition: 2

obc.getResultColumn().getExpression() instanceof ColumnReference = false     //Ordering on
an expression

1
----------------------
0.7460335991462618
0.22942514814484716
0.04022635294210675
0.39009678758326427
0.9438511615474983
0.803614720479578
0.7563735003656935

7 rows selected
===========================================

In OrderByColumn class at bindOrderByColumn(ResultSetNode target, OrderByList oblist) method,
Derby doesn't check for an expression (eg: random() ) in an order by clause. Therefore it
pulled up a column.

I have few questions:

1) How derby resolves random() function?
2) Does derby consider random() as a expression or else other (it can't be a column reference)?

> Wrong order when using ORDER BY on non-deterministic function
> -------------------------------------------------------------
>
>                 Key: DERBY-4406
>                 URL: https://issues.apache.org/jira/browse/DERBY-4406
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.5.3.0, 10.6.0.0
>            Reporter: Knut Anders Hatlen
>            Assignee: C.S. Nirmal J. Fernando
>
> If I read the SQL standard correctly, a statement such as "select random() as r from
t order by random()" should be treated as "select random() as r from t order by r". Derby
does however generate a second, hidden random() column by which the rows are ordered.
> ij> create table t(x int);
> 0 rows inserted/updated/deleted
> ij> insert into t values 1,2,3,4,5;
> 5 rows inserted/updated/deleted
> ij> -- wrong result, not ordered by r
> ij> select random() as r from t order by random();
> R                     
> ----------------------
> 0.1285512465366495    
> 0.5116860880915798    
> 0.21060042130229073   
> 0.2506706923680875    
> 0.6378857329935494    
> 5 rows selected
> ij> -- correct result, ordered by r
> ij> select random() as r from t order by r;
> R                     
> ----------------------
> 0.0749025910679918    
> 0.07694931688380491   
> 0.1724114605785414    
> 0.2268758969382877    
> 0.31900450349277965   
> 5 rows selected

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