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From "Bryan Pendleton (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-6938) Obtain cardinality estimates and true estimates for base tables as well as for intermediate results for queries involving multiple joins.
Date Sun, 11 Jun 2017 03:54:18 GMT

    [ https://issues.apache.org/jira/browse/DERBY-6938?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16045796#comment-16045796
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Bryan Pendleton commented on DERBY-6938:
----------------------------------------

I'm not certain I understand the question. I suspect we may have to study the code, and perform
some experiments, to determine the answer.

But we can start by looking at these resources, in addition to the documentation you mentioned:
*  https://wiki.apache.org/db-derby/LanguageOptimize
*  http://db.apache.org/derby/papers/optimizer.html

I looked at the explain.txt that you attached, but I'm not certain I am following your analysis.

I wonder if you might be able to annotate the explain.txt with your own comments, and point
directly at the section of the explain output that is of particular interest to you, so that
I can better understand the question you are asking.

Note that it is entirely possible that Derby's query optimizer is not as sophisticated as
you think it may be. That is one of the reasons that this testing is so important: we need
to understand where Derby's query optimizer is not considering all the information that is
available to it when it makes its decisions.


>  Obtain cardinality estimates and true estimates for base tables as well as for intermediate
results for queries involving multiple joins. 
> -------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: DERBY-6938
>                 URL: https://issues.apache.org/jira/browse/DERBY-6938
>             Project: Derby
>          Issue Type: Sub-task
>          Components: SQL
>            Reporter: Harshvardhan Gupta
>            Assignee: Harshvardhan Gupta
>         Attachments: explain.txt
>
>




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