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From "RPost" <rp0...@pacbell.net>
Subject Re: Optimizer hints?
Date Fri, 17 Dec 2004 23:10:16 GMT
Do you have any specific examples of queries that could benefit from an
optimizer hint?

Does anyone know of any documented instances where Derby has selected an
execution plan that is ineffective or suboptimal? Were these instances able
to be corrected by tuning the system using existing

Oracle allows hints to be provided as SQL comments.

Re providing an offline tool. Does this method imply that the statements
exist independent of any given running database instance. If so are you
proposing that a prepared statement be saved and made available for future
execution without preparing it again?

----- Original Message ----- 
From: "Satheesh Bandaram" <satheesh@Sourcery.Org>
To: "Derby Development" <derby-dev@db.apache.org>
Sent: Friday, December 17, 2004 2:41 PM
Subject: Optimizer hints?

> Hash: SHA1
> I have been thinking about adding optimizer hints to Derby. Though Derby
> optimizer does perform a reasonable job, it may be useful to have
> optimizer hints for cases 1) when updated index statistics is not
> available, or even incorrect 2) Rare cases, like this one?, when the
> optimizer doesn't do the job right 3) when applications issue queries
> not designed for Derby (like very large number of tables).
> Derby optimizer is primarily tasked to handle 1) access method (whether
> to use table scan or index) 2) join order and 3) join strategy (nested
> loop or HashJoin) A complete optimizer hints (should they actually be
> called optimizer overrides?) should be able to provide means of
> specifying all these. There are several ways to do this, including
>    1. Enhance the SQL to recognize additional keywords, like properties.
> These properties could specify optimizer hints. That is what Cloudscape
> had before, but this causes non-portable SQL to be written. Attempts to
> run these statements against any other SQL server could cause syntax
> errors. Not an ideal solution, according to me.
>    2. Provide optimizer hints as SQL comments. These comments are
> recognized only by Derby parser and other SQL engines would simply
> ignore these. There are some limitations in Derby parser currently to
> implement this option.
>    3. Provide an offline tool which registers hints for SQL statements.
> When those SQL statements are executed, Derby could look up previously
> registered hints for those statements and automatically apply them.
> Advantage of this scheme is that applications don't need to be modified
> to add hints and they can easily be removed when not needed. How exactly
> to register the hints could be worked out. I personally prefer this
> approach.
> Any comments?
> Satheesh
> Gerald Khin (JIRA) wrote:
> >HashJoinStrategy leads to java.lang.OutOfMemoryError
> >----------------------------------------------------
> >
> >         Key: DERBY-106
> >         URL: http://nagoya.apache.org/jira/browse/DERBY-106
> >     Project: Derby
> >        Type: Bug
> >    Reporter: Gerald Khin
> >
> >
> >My application is running out of memory: I encounterd a
> java.lang.OutOfMemoryError. I used -Xmx256M. Unfortunatley, I cannot
> spend an arbitrary amount of JVM memory.
> >
> >Then, I commented out the line in class OptimizerFactoryImpl which was
> adding the HashJoinStrategy to the set of Join strategies:
> >
> >        if (joinStrategySet == null)
> >        {
> >//            JoinStrategy[] jss = new JoinStrategy[2];
> >            JoinStrategy[] jss = new JoinStrategy[1];
> >            jss[0] = new NestedLoopJoinStrategy();
> >//            jss[1] = new HashJoinStrategy();
> >            joinStrategySet = jss;
> >        }
> >
> >And with these changes the OutOfMemoryError has gone away! And it works
> even with -Xmx128M!!!
> >
> >So I guess that there is a major memory issue with this HashJoin
> strategy implementation.
> >
> >If it turns out to be too complicated to make the memory consumption
> more predicatble or even bounded to some configurable limit, then I need
> at least as a workaround a way to turn off the HashJoin strategy
> completely: I did it by patching and building my own derby.jar, but if
> there would be an official solution with some kind of switch like a
> system property, it would be great!
> >
> >
> >
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