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Message Posted: Fri, 03 Feb 2012 @ 20:57:26 GMT


     
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Subj:   Re: MVC and Presence bits
 
From:   Dave Wellman

Hi,

Not quite. You're correct in that there is always 1 presence byte in the row header of which only 7 bits are available for use.

However, remember that the presence bits are used to indicate both NULL values and compressed values. So, if your table definition includes only NOT NULL columns and there are no compressed columns then those 7 bits are available.

In that situation, if you added up to 128 (2 ^ 7 = 128) MVC values for a ** single ** column then you shouldn't need any further presence bytes. So you can actually compress more than 7 values in the single column and not use more bytes.

I think that as soon as you start compressing more than one column then you'll find that you cannot fit so many MVC values into the same number of bytes.


Cheers,

Dave

Ward Analytics Ltd - Information in motion (www.ward-analytics.com)



     
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