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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3926) Incorrect ORDER BY caused by index
Date Thu, 16 Apr 2009 15:20:14 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3926?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699728#action_12699728
] 

Mamta A. Satoor commented on DERBY-3926:
----------------------------------------

Bryan, I haven't read your comment completely but I noticed " I wish I had a clearer grasp
on the concept of a 'partial join order.  I think that this is an intermediate stage during
optimization, where the optimizer has so far chosen an ordering for some, but not yet all,
of the tables. It looks like the code is trying to handle the problem of how to make a sort
avoidance check at this intermediate point in optimization." Your understanding of partial
join order is correct. For our specific query case, the optimizer is eventually going to have
a join order consisting of 3 tables once it determines the best join order. But to arrive
to that join order, it goes through many iterations of possible join orders. It starts out
by considering only one optimizable in the first position of the join order, eg, 0,-1,-1 and
this will be considered partial join order.

> Incorrect ORDER BY caused by index
> ----------------------------------
>
>                 Key: DERBY-3926
>                 URL: https://issues.apache.org/jira/browse/DERBY-3926
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.1.3.3, 10.2.3.0, 10.3.3.1, 10.4.2.0
>            Reporter: Tars Joris
>         Attachments: derby-reproduce.zip
>
>
> I think I found a bug in Derby that is triggered by an index on a large column: VARCHAR(1024).
I know it  is generally not a good idea to have an index on such a large column.
> I have a table (table2) with a column "value", my query orders on this column but the
result is not sorted. It is sorted if I remove the index on that column.
> The output of the attached script is as follows (results should be ordered on the middle
column):
> ID                  |VALUE        |VALUE
> ----------------------------------------------
> 2147483653          |000002       |21857
> 2147483654          |000003       |21857
> 4294967297          |000001       |21857
> While I would expect:
> ID                  |VALUE        |VALUE
> ----------------------------------------------
> 4294967297          |000001       |21857
> 2147483653          |000002       |21857
> 2147483654          |000003       |21857
> This is the definition:
> CREATE TABLE table1 (id BIGINT NOT NULL, PRIMARY KEY(id));
> CREATE INDEX key1 ON table1(id);
> CREATE TABLE table2 (id BIGINT NOT NULL, name VARCHAR(40) NOT NULL, value VARCHAR(1024),
PRIMARY KEY(id, name));
> CREATE UNIQUE INDEX key2 ON table2(id, name);
> CREATE INDEX key3 ON table2(value);
> This is the query:
> SELECT table1.id, m0.value, m1.value
> FROM table1, table2 m0, table2 m1
> WHERE table1.id=m0.id
> AND m0.name='PageSequenceId'
> AND table1.id=m1.id
> AND m1.name='PostComponentId'
> AND m1.value='21857'
> ORDER BY m0.value;
> The bug can be reproduced by just executing the attached script with the ij-tool.
> Note that the result of the query becomes correct when enough data is changed. This prevented
me from creating a smaller example.
> See the attached file "derby-reproduce.zip" for sysinfo, derby.log and script.sql.
> Michael Segel pointed out:
> "It looks like its hitting the index ordering on id,name from table 2 and is ignoring
the order by clause."

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