Compact 2002 DB versus Compression (zip)

[Kevin]

I have noticed that I cannot zip certain databases, but others will
zip without a problem. Two examples:

1) File1.mdb. Orig size 417 MB. After compacting, it was 391 MB.
After zipping it, it was 47 MB.

2) File2.mdb. Orig size 347 MB. After zipping (before compacting),
it was 331 MB. After compacting (the orig file), it was 329 MB. If I
try to zip the compacted file, it doesn't change the file size at all.

Can anyone provide any reason that some files would compress quite a
bit, whilst others won't deflate at all??? I haven't been able to
find anything online about it. Any help would be appreciated.

 

[Brendan Reynolds]

Compacting and zipping don't really bear any relation to each other, Kevin.
Compacting eliminates unused space within the MDB file. Zipping replaces
sequences of repeating patterns within the binary structure of the file.
(That's probably an over-simplified explanation of what zipping does, but
it's the best I can do, and probably good enough for our needs in this
discussion.) How much the size of the file is reduced by compacting depends
on how much unused space is in the file. How much the size is reduced by
zipping depends on how much of the file consists of repeating patterns.
(Probably another gross over-simplification, but you get the idea, I'm
sure).

One reason why zipping may make little if any difference to the size of an
MDB is encryption. Encryption eliminates the repeating patterns on which
zipping depends. If I remember correctly, I believe zipping is also
ineffective when drive compression is in use.

--
Brendan Reynolds (MVP)
http://brenreyn.blogspot.com

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