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From Bryan Pendleton <bpendle...@amberpoint.com>
Subject How do I tell whether a table has unused space or not?
Date Tue, 28 Mar 2006 02:53:57 GMT
Recently, I had a table which was able to substantially
benefit from compression. I determined that I had a problem
by setting derby.logQueryPlan to TRUE, running a "select *"
query against the table, and then viewing the page and
row count statistics that were emitted in the query plan,
where I saw that the optimizer was expecting to process many
more pages than I thought needed to be in the table.

Compressing the table made my problem go away, and the
optimizer returned to choosing my desired query plans.

But I am left with the desire for an easier way to figure
out whether my table needs compression or not.

I tried reading the manual, for example I found
http://db.apache.org/derby/docs/10.1/adminguide/cadminspace21579.html
but that page does not offer any clear way to tell whether a
table needs to be compressed or not.

What is the preferred way to decide whether a table has unused space?

thanks,

bryan


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