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https://issues.apache.org/jira/browse/DERBY-2526?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12487253
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A B commented on DERBY-2526:
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Thank you for your investigation of this issue, Bryan. I've been a bit busy so
I haven't had time to look at this more, but it sounds like you're doing a
great job of narrowing this down.
One suggestion (without having actually run done any investigation of my own):
does this problem reproduce if you set "derby.optimizer.noTimeout=true"? If it
does, then you could set that property and then step through the code to see if
there are any obvious differences in the optimization paths for the two
statements. I usually start by putting a break point in the
"getNextPermutation()" method of OptimizerImpl and then doing a quick step
through of that method for the different queries, to see if any there are any
obvious differences in code flows (esp. number of times the method is called).
Another technique I use (which is a tad less scientific) is to put System.outs
in the "rememberBestCost()" method of OptimizerImpl to print out: a) the join
order that is being saved as "best", and b) the cost of that join order. Then
add a similar System.out to the "modifyAccessPaths()" method of OptimizerImpl
and print out the final value of bestJoinOrder, along with the final cost
estimate. If you do that and then run the two different queries, you can
often see a difference in plans/costs that helps track down the issue at hand...
> Wrong query results due to column ordering in UNION view
> --------------------------------------------------------
>
> Key: DERBY-2526
> URL: https://issues.apache.org/jira/browse/DERBY-2526
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.0.2.1, 10.1.3.1, 10.2.2.0, 10.3.0.0
> Reporter: Bryan Pendleton
> Attachments: badQuery.log, derby-2526.sql, goodQuery.log
>
>
> I think both select statements in the attached repro script should return 1
> row, but in fact the first statement returns 1 row and the second returns
> zero rows.
> The only difference between the two statements is that the columns in the
> UNION view are listed in a different order (bvw vs. bvw2).
> This seems like a bug to me; the order of the columns in the view definition
> shouldn't matter, should it?
> As Army noted on the derby-dev list, the fact that this reproduces with 10.0
> means that it is not caused by some of the 10.2 optimizer changes. Something
> else is going wrong.
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