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Message Posted: Thu, 08 Nov 2007 @ 11:24:06 GMT

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Subj:   Re: Calculating number of compressible values as a
From:   Dieter Noeth

Etienne E Stieger wrote:

  The above information from the manual is a bit confusing because:  

  a) Is 8192 characters the only absolute cut-off point I should worry about? In other words, is this the only real constraint I should use to calculate how many values I could compress without running into systems limits?  

I once tried to find out that limit, too. It's probably the 8k limit, e.g. i could compress up to 31 values for a CHAR(255).

  b) Under which conditions would you get a thin header vs a fat header?  

If you're running V2R6 and the header size hits 64k.

  c) Does thin vs. fat header have any impact here? I.e., could this become 2x8192 for a fat header?  


  d) Are MVC values stored as varying length values in the table header, or is the datatype&size of the compressed column used here?(i.e. padded with spaces).  

They are stored with trailing blanks, that's why it looks so ugly in SHOW TABLE

  e) Does UNICODE/2-byte char columns have a different impact on the amount of header space used than LATIN/1-byte character columns? In other words, are these 8192 character positions, or 8192 Bytes?  

The dbc.tvfields.compressvaluelist is a VARCHAR(8192), so these are characters.


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