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From "Bryan Pendleton (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3926) Incorrect ORDER BY caused by index
Date Thu, 16 Apr 2009 14:44:15 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3926?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699721#action_12699721

Bryan Pendleton commented on DERBY-3926:

Thanks Army for taking the time to look at this, and for pointing us to a good theory!

I like Army's suggestion. 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
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.

I think that all *chosen* query plans eventually reach a *complete* join order, but a query
plan which
is *discarded* due to being too expensive may never get beyond a *partial* join order. Is
that true?

It seems to me that there may be two topics here:

1) it would be valid to make such a check for the purposes of doing some preliminary costing
the join order up to this point, but for such an algorithm, the code would have to then re-visit
sort avoidance decision later in the optimization, once the partial join order had become
a full join order.
That is, I wonder if the overall flow is something like:
 - optimizer investigates a partial join order, decides to cost it, determines that (so far)
a sort is not required.
 - optimizer later completes this join order, decides that it is acceptable, but does NOT
   whether a sort is now required for the complete join order
 - optimizer then chooses the correct full join order, but incorrectly avoids the sort due
to the decision
   it made when considering the partial join order.

2) The comment ("ORDER BY S.A, T.B, S.C") raises the interesting question of the situation
which each column, considered individually, is ordered properly, but because the ORDER BY
clause interleaves columns from different tables, a sort is still required. That is, if S
had an index
on (A, C), and T had an index on B, we might look at ORDER BY S.A, T.B, S.C and think that
sorting of the results was required, because the join from S -> T would emit the rows in
correct order, but that is wrong; the interleaving of the columns means that the sort must
still be performed.
And, presumably, there is the interesting situation where we need to cost out a query at a
where we have determined a partial join order that contains a position for S, but not T, or
vice versa.

It would be interesting to know more about the Wisconsin test cases, about the queries involved,
about the before- and after- query plan differences, with respect to sorting. Are *all* the
changes due
to situations where we formerly avoided a sort, but now we choose one? That's interesting,
I think; we
want to be careful to avoid introducing un-necessary sorts because that could be a substantial
performance regression (of course, if the old query was returning the rows in the wrong order,
the test didn't check, and the new query is now being performed correctly, that's important
to know, too!)

It would also be interesting to see if we can construct an example along the lines of the
ORDER BY S.A, T.B, S.C case, such that various other permutations (ORDER BY S.A, S.C, T.B
ORDER BY T.B, S.A, S.C) did not require sorting, but ORDER BY S.A, T.B, S.C did, and see how
the query plans emitted for these various cases behaved.

> 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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