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From "Jim Newsham" <jnews...@referentia.com>
Subject RE: INPLACE Table Compression
Date Tue, 27 Mar 2007 18:51:15 GMT


> -----Original Message-----
> From: Bryan Pendleton [mailto:bpendleton@amberpoint.com]
> Sent: Tuesday, March 27, 2007 5:42 AM
> To: Derby Discussion
> Subject: Re: INPLACE Table Compression
> 
> > "Inplace" Compression utility, no disk space is recovered and the size
> > of the "*.dat" files is not reduced.
> 
> That is correct. In-place compression re-arranges the records on
> the existing pages of the existing file, gathering the existing
> records together and shifting all the free space to be together.
> This makes access to the existing records more efficient, and
> allows new records to efficiently make use of the available free
> space, but does nothing to reduce the overall size of the file.
> 
> Think of it as somewhat similar to the "defragment" process that
> your operating system offers. Your overall disk volume is still
> the same size, but the use of the disk is improved.
> 
> To release disk space back to the operating system, you must use
> the version of compression which copies the data to a new file,
> then deletes the old file.

Are you serious?  Then surely the documentation is wrong.  It says there are
three operations:  purge, defrag, and truncate; when truncate is used, it
releases disk space to the operating system (by the way, it's not clearly
stated, but I assume that defrag will do nothing unless a purge has been
previously performed, and that truncate will do nothing unless a defrag has
been previously performed).  I'm looking at the 10.2.2 documentation
(refderby.pdf, page 108).

I haven't been able to test this for myself because in-place compress
currently fails for me with ArrayIndexOutOfBoundsException.  I assume this
is due to DERBY-606 (though I don't have a table anywhere remotely near 2
GB), so I'm waiting to see if it works for us in 10.3.

Jim




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