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From "Kim Haase (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-1397) Tuning Guide: Puzzling optimizer documentation
Date Tue, 15 Oct 2013 19:28:42 GMT

    [ https://issues.apache.org/jira/browse/DERBY-1397?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13795534#comment-13795534
] 

Kim Haase commented on DERBY-1397:
----------------------------------

This very elderly issue is still half valid. The first part is resolved, because you can now
drop a column. But the Tuning Guide reference to an unnamed and as yet undocumented property
remains. If it still seems worth doing, I can add a Reference Manual topic on the derby.language.maxMemoryPerTable
property.

> Tuning Guide: Puzzling optimizer documentation
> ----------------------------------------------
>
>                 Key: DERBY-1397
>                 URL: https://issues.apache.org/jira/browse/DERBY-1397
>             Project: Derby
>          Issue Type: Bug
>          Components: Documentation
>    Affects Versions: 10.2.1.6
>            Reporter: Rick Hillegas
>              Labels: derby_triage10_5_2
>
>  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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