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Message Posted: Tue, 20 Mar 2001 @ 23:15:23 GMT


     
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Subj:   Re: Placement of VARCHAR and COMPRESSed Columns In A Row
 
From:   Fred Pluebell

Take a look at the "Teradata RDBMS Database Design" manual. In the version I have (B035-1094-060A) the information you want is in Chapter 12 - Database-Level Considerations.


David Carlson wrote:

  Has anyone run across recommendations of where to place VARCHAR and COMPRESSed columns in a table? Should these columns be placed at the end of the row or does it really matter? I know from work on other platforms that this could potentially be an performance issue but cannot find any documentation on this.  


Teradata automatically adjusts the physical order of different classes of columns within the row: all fixed length columns in the row, followed by uncompressed data from compressible columns, followed by variable length columns.



  Also noticed in the SQL Reference Manual that NCR recommends applying compression to 'five columns or less' to minimize the impact to access time. Does anyone know if this is still the recommendation? Seems really low considering the increases in performance over the years. Any thoughts.  


For the first few (e.g. "five or less") compressed columns, the performance impact is negligible even if there's not much space savings. But the more columns you compress, the more important it is to weigh the tradeoffs between space and processing time, and to consider other factors such as the relative order of compressed columns.



     
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