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Date: Tue, 13 Jun 2006 17:22:31 +0000 (GMT+00:00)
From: "Laura Stewart (JIRA)" <derby-dev@db.apache.org>
To: derby-dev@db.apache.org
Subject: [jira] Commented: (DERBY-1397) Tuning Guide: Puzzling optimizer
 documentation
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    [ http://issues.apache.org/jira/browse/DERBY-1397?page=comments#action_12416037 ] 

Laura Stewart commented on DERBY-1397:
--------------------------------------

*** (LS) Questions
- Are we sure that the name of the property is derby.language.maxMemoryPerTable ? 
- What is the maximum value allowed? 
- Is there a value that we want to recommend for users to specify?
- The text says that it can be set to smaller values, what is the minimum value that we recommend?

> Tuning Guide: Puzzling optimizer documentation
> ----------------------------------------------
>
>          Key: DERBY-1397
>          URL: http://issues.apache.org/jira/browse/DERBY-1397
>      Project: Derby
>         Type: Bug

>   Components: Documentation
>     Versions: 10.2.0.0
>     Reporter: Rick Hillegas
>     Assignee: Laura Stewart
>      Fix For: 10.2.0.0

>
>  Selectivity and cardinality statistics
>    Working with cardinality statistics
>      When cardinality statistics are automatically updated
>        "For other operations, Derby automatically updates statistics for the table and all indexes on the table if they are already exist. Those operations are:
>    * (all indexes) When you execute SYSCS_UTIL.SYSCS_COMPRESS_TABLE.
>    * (index only) When you drop a column that is part of a table's index; the statistics for the affected index are dropped, and statistics for the other indexes on the table are updated.
> "
> What does the second bullet mean? Derby doesn't let you drop a column from a table right now. 
> ----------------------------------------------------------
> Here's another puzzling piece of optimizer documentation:
> I'm puzzled by the following paragraph in Tuning Guide->DML statements and performance->Performance and optimization->Joins and performance->Join strategies:
> "If memory use is not a problem for your environment, set this property to a high number; allowing the optimizer the maximum flexibility in considering a join strategy queries involving large queries leads to better performance. It can also be set to smaller values for more limited environments."
> I can't find the name of this property on that page of the Tuning Guide. I'm also confused about what we consider to be a "high number" versus what we consider to be "smaller values". Would appreciate advice here. 
> Satheesh adds this:
> The property it may be referring to is
> *derby.language.maxMemoryPerTable*. The default value is 1024 KB.
> Current default value is too small, so it would be a good tip for
> developers to know and tune this property. It would be great if Derby
> can configure this property value based on factors like max heap size,
> size of data cache and/or other parameters.

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