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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, 07 May 2009 18:14:45 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3926?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12707021#action_12707021

Mamta A. Satoor commented on DERBY-3926:

It appears that we need to catch the case where an optimizable is not the outermost node and
optimizer is considering using an index on that optimizable (the index is on the order by
column) and there is no constant comparison predicate on that column. This pretty much makes
the index of no use but say the optimizer has been forced to use that index through optimizer
overrides (ie what we have done in our test queries in this jira). If the outer tables in
the join order are all one-row resultset, then it is not an issues because we will be doing
only one scan on the optimizable in question and all the rows returned for that optimizable
will be sorted on the index being considered on optimizable. The problem case is when there
are outer optimizable involved and the outer optimizables will qualify more than one row which
will be returned for them and for each one of those rows, we will be doing a scan on the optimizable
in question and hence the rows satisfied through multiple scans of the optimizable in question
will not be in any sorted order. To fix this, I am planning on adding additional code in OptimizerImpl.costBasedCostOptimizable
after the following existing if statement at line 2239
if (joinPosition == 0 ||  optimizableList.getOptimizable(proposedJoinOrder[joinPosition -
Following is the psudeo code of what I am planning on adding
if (joinPosition != 0) //if we are the outermost optimizable, we are good to go.
    if (optimizable.currentPlanUsingIndex() && optimizable.indexOnOrderByColumn()
&& optimizable.noConstantPredicateOnIndexColumn())
        Sorting can't be avoided on this optimizable
        Continue with the existing code which is 
        if (requiredRowOrdering.sortRequired(currentRowOrdering,assignedTableMap) == RequiredRowOrdering.NOTHING_REQUIRED)

I will try to implement this psedo code. Let me know if anyone has any comments if this does
not look like a good possible solution.

> 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
>            Assignee: Mamta A. Satoor
>         Attachments: d3926_repro.sql, derby-reproduce.zip, script3.sql, script3WithUserFriendlyIndexNames.sql,
> 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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