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https://issues.apache.org/jira/browse/DERBY-6305?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14177893#comment-14177893
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Myrna van Lunteren edited comment on DERBY-6305 at 10/21/14 2:32 PM:
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According to a comment in Optimizer.java, useStatistics can be used in a query
and is a 'Property name for disabling statistics use for all queries.' It
matches the derby.optimizer.useStatistics property.
I believe this is a left-over from the Cloudscape-style optimizer properties. I
think these are index, or cardinality, statistics, not runtime statistics.
There is an open enhancement suggesting to revive this - DERBY-4113.
The property is used once in the tests in SelectivityTest.java was converted
from an old cloudscape test after optimizer overrides/directives were supported
in Derby. In that test it gives the same results with useStatistics=false or
true or without. But the value of the estimated rowcount is different with
useStatistics=false, so this override is working.
was (Author: myrna):
According to a comment in Optimizer.java, useStatistics can be used in a query
and is a 'Property name for disabling statistics use for all queries.' It
matches the derby.optimizer.useStatistics property.
I believe this is a left-over from the Cloudscape-style optimizer properties. I
think these are index, or cardinality, statistics, not runtime statistics.
There is an open enhancement suggesting to revive this - DERBY-4113.
The property is used once in the tests in SelectivityTest.java was converted
from an old cloudscape test after optimizer overrides/directives were supported
in Derby. In that test it gives the same results with useStatistics=false or
true or without. The value of the estimated rowcount is different with
useStatistics, but it is the same whether the value is true or false, so either
it's not a useful test or it's not working.
> Tuning Derby information on optimizer overrides needs improvement
> -----------------------------------------------------------------
>
> Key: DERBY-6305
> URL: https://issues.apache.org/jira/browse/DERBY-6305
> Project: Derby
> Issue Type: Bug
> Components: Documentation
> Affects Versions: 10.10.1.1
> Reporter: Kim Haase
> Attachments: DERBY-6305.diff, DerbyDashProperties.html,
> ctundepthoptover.html
>
>
> The topic "Optimizer overrides" in the Tuning Derby manual talks about using
> the "-- DERBY-PROPERTIES" clause to specify overrides but doesn't include a
> link to the topic later in the manual, "Overriding the default optimizer
> behavior", that explains how to use the clause. This undoubtedly results in
> user confusion.
> The first topic is
> http://db.apache.org/derby/docs/10.10/tuning/ctundepthoptover.html; the
> second is
> http://db.apache.org/derby/docs/10.10/tuning/ctunoptimzoverride.html.
> I'll add the link, and I welcome additional suggestions on how to improve
> these topics.
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