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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-3926) Incorrect ORDER BY caused by index
Date Fri, 15 May 2009 16:37:45 GMT

     [ https://issues.apache.org/jira/browse/DERBY-3926?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Mamta A. Satoor updated DERBY-3926:

    Attachment: DERBY3926_patch3_051509_stat.txt

I have attached a patch (DERBY3926_patch3_051509_diff.txt)  that fixes the reproducible order
by query case. The problem was that when considering inner optimizable nodes that required
ordering(but the user query has no constant comparison predicate(s) on those columns), we
did not check if the previous optimizables all returned single rows before deciding to avoid
sorting. If the previous optimizables return more than one row, then that would require multiple
scans into the inner optimizable and the rows satisfied by the multiple scans may or may not
be ordered as per the user query requirement. 

The new logic has gone into impl\sql\compile\OptimizerImpl.java and the supporting code to
determine if the current inner optimizable requires ordering on columns with no constant comparison
predicates on them is in impl\sql\compile\OrderByList.java

The logic has also been explained in detail in OptimizerImpl.java I will appreciate if someone
can take a look at it. I have run junit tests and derbyall. There are two failures in derbyall.
One of them is for T_RawStoreFactory which I think is existing known jira issue DERBY-3993.
The other failure is in wisconsin test. Only one sql is failing in wisconsin. That sql with
some optimizer overrides looks as follows.
select * from   --DERBY-PROPERTIES joinOrder=FIXED 
where TENKTUP1.unique1 = TENKTUP2.unique1
order by TENKTUP1.unique1, TENKTUP2.unique1;

As the name of the columns suggest, there are unique indexes on the columns we are dealing
with in the query above. With my changes in the patch, now we are requiring a sort on the
top of the query plan. Prior to my changes, we were avoiding sort on this query. The reason
we are requiring sort as per the new logic is - The outermost optimizable is TENKTUP1 and
it is going to return more than one row. Next, we consider TENKTUP2 as the inner optimizable.
We see that user has requested ordering on
TENKTUP2.unique1 and there is no constant predicate on TENKTUP2.unique1 AND the previous optimizable
is not one row resultset and because of these conditions, we require that sorting is necessary.
I think ideally, we should be able to avoid sort because even though the previous optimizable
is returning more than one row, the current optimizable has equality check with the previous
optimizable (on the ordered columns) and hence even though there will be multiple scans into
current optimizable, the rows will
all be ordered because of the equality check. I haven't given this additional logic much thought.
I will look more into it.

> 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, DERBY3926_notforcheckin_patch1_051109_diff.txt,
DERBY3926_notforcheckin_patch1_051109_stat.txt, DERBY3926_notforcheckin_patch2_051109_diff.txt,
DERBY3926_patch3_051509_diff.txt, DERBY3926_patch3_051509_stat.txt, 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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