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From Knut Anders Hatlen <knut.hat...@oracle.com>
Subject Re: Reclaiming space from Derby
Date Fri, 10 Sep 2010 08:48:57 GMT
Paul Taylor <paul_t100@fastmail.fm> writes:

> Knut Anders Hatlen wrote:
>> Paul Taylor <paul_t100@fastmail.fm> writes:
>>> Knut Anders Hatlen wrote:
>>>> Paul Taylor <paul_t100@fastmail.fm> writes:
>>>>> HI, I use derby in embedded mode, I let the user specify a maxmium
>>>>> size for the derby database files, my program monitors this and if it
>>>>> goes over the size I use SQL to delete records that are no longer
>>>>> necessary in the hope of shrinking the database files size, but it
>>>>> doesn't, how can I force Derby to shrink back down ?
>>>> Hi Paul,
>>>> The database files won't shrink unless you compress the tables.
>>>> http://db.apache.org/derby/docs/10.6/ref/rrefaltertablecompress.html
>>>> http://db.apache.org/derby/docs/10.6/ref/rrefproceduresinplacecompress.html
>>>> Hope this helps,
>>> Thanks guys that does help, the only problem is that it takes an
>>> exclusive lock on the table and I wanted to run it in the background
>>> as and when required when other tasks might be going ahead. But can
>>> probaly work round this.
>> In the issue tracker there's a request for allowing more concurrent
>> activity while compressing a table, but there hasn't been any work on it
>> yet. https://issues.apache.org/jira/browse/DERBY-3974
> I dont quote get the difference between the two procedures, one makes
> adjustments in place, the other doesnt, but they both exclusively lock
> the table so what is the advantage of one over the other ?

Yes, they both lock the table exclusively. The main advantage of the
in-place variant is that it requires less disk space during the
operation, since it doesn't create a new copy of the table.

SYSCS_COMPRESS_TABLE, on the other hand, will also update the index
cardinality statistics while it's compressing the table, whereas the
in-place variant doesn't update the statistics. And I think there are
some edge cases where it will be able to free more space than in-place.

Knut Anders

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