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Message Posted: Mon, 23 May 2011 @ 19:49:20 GMT

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Subj:   Re: Multi value compression and presence bits
From:   DWellman

Hi David,

I don't think so.

As far as I'm aware, there is only one byte reserved for presence bits by default. This seems to be confirmed by reading Chap 15 of the Database Design manual (TD 13.10). This says:

- there is 1 byte for "first presence bits array"

- there are 0-n bytes for "additional presence bits array"

I can't explicitly find it mentioned, but it always used to be that from this first "presence bits array" you only had 7 bits available. That may now have changed because there is a separate "flag byte" listed.

Also, you're correct that 255 values can be 'indexed' within 2 bytes, but remember that you can compress multiple columns within a table definition and each column that you compress 255 values for will require 8 presence bits. So if you compress 6 columns on 255 values (each) you will require 48 bits.

Does that help ?



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