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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3926) Incorrect ORDER BY caused by index
Date Sat, 11 Apr 2009 01:23:14 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3926?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12698038#action_12698038

Mamta A. Satoor commented on DERBY-3926:

Yes, even if the cost calculation for a plan is wrong, Derby should not return wrong resutls
and hence for this jira entry, at this point, it might not be worth it to pursue the lead
if the cost calculation is wrong.

I did find one consistently reproducible SQL which will cause the problem behavior whether
we are in the same session where the tables/indexes were created or whether we start a fresh
database session. So, once the database has been setup, one can open a new ij session and
consistently repro the problem case with following optimizer overrides (this way, one does
not have to setup the whole database in the same session as the origina problem SQL to repro
the problem)
SELECT table1.id, m0.value, m1.value FROM  --DERBY-PROPERTIES joinOrder=FIXED 
table2  m1 -- DERBY-PROPERTIES index=key3
, table2 m0 -- DERBY-PROPERTIES index=key3
, table1 
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;

So, the important thing is have both m1 and m0 use the index KEY3 which is on the column value
on which ordering is happening for table m0.

Now that I have a simple repro case (ie I don't have to go through countless iteration of
optimizer for all different join orders and different predicate pulling down in different
join orders), I can focus on the problem 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:,,,
>            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
> 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 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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