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From Harri Pesonen <fue...@nic.fi>
Subject Re: A few questions about index usage
Date Fri, 27 Jan 2006 19:17:48 GMT
Thanks for you comments and for creating a JIRA item. I think that the 
first case is a separate thing, because the following is slow as well:

SELECT MIN(Id), MIN(Id) + 1 FROM Customer

This case could be optimized to do only one MIN-traversal. I think that 
this is a more fundamental thing, perhaps requiring more thought, and 
some kind of global optimizations.

Sorry for the format of this message, I am using the digest.

--
Harri

Jeffrey Lichtman wrote:
>
>> 1) "SELECT MIN(Id) FROM Customer" and "SELECT MAX(Id) FROM Customer" 
>> are both fast, but "SELECT MIN(Id), MAX(Id) FROM Customer" is slow, 
>> taking 5 seconds. Why?
>
>
> Derby knows how to use an index to quickly find a minimum or a maximum 
> (by traversing down one side of the B-tree or the other). It doesn't 
> know how to do both in the same query, which would take two traversals.

Is the work to fix this the same as making IN list use multiple probes,
and/or makeing OR lists do multiple probes?  There are existing JIRA
items for those, or is it different enough to have a separate JIRA?
>
>> 2) "SELECT * FROM Customer ORDER BY Id" is fast but "SELECT * FROM 
>> Customer ORDER BY Id DESC" is slow. Why? Can't it scroll the index 
>> backwards?
The structure could support this, the work just has not been done.  With
row level locking, concurrent tree splitting, and current assumptions
throughout the access method that scans go top/down, left to right the
work to do a backward scan is harder than doing a forward scan.  Also
once the store portion is done, there would be necessary changes to:
optimizer, execution engine, and scan interface to allow the backward
scan.  This would be a hard first project.

The current workaround is that you can create both an ascending and
a descending index if you need ordering in both directions to go fast.

You should check out the access/btree white papers Dibyendo (sp?) 
produced, on the web site.

I will go ahead and log an enhancement in JIRA (DERBY-884)


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