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Message Posted: Thu, 07 Jun 2007 @ 13:33:57 GMT


     
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Subj:   Re: Designing wide (fat) tables
 
From:   Victor Sokovin

  Normally these large tables are also designed with compression on some of the columns in mind. The bummer being that once you have too many columns, your table header increases in size, and then the number of values you can acutally compress reduces (as they need to be there in the table header, which is of fixed size). I am not sure on the size of table header and whether it has improved in later releases, but yeah it used to be a common occurrence in the past to see a table header overflow with too many cols+compression+indexes ...... So that would be one silent beast I would recommend you to watch for. (sometimes the pure motivation on going for a flat table is to avoid joins and make use of compression in storing those extra columns, a noble desire)  


Fair warning, Joseph. Table header size increased in recent releases but the number of factors contributing to the overflow also increased. Take, for example, the relatively new and tempting partitioning options. But also the good old VARCHAR columns should not be forgotten as they also contribute to the header.

I recall and can find the thread where some examples and calculations related to this topic can be found:

www.teradataforum.com/teradata/20050505_140933.htm

I can't add much more. I understand that wide tables might be unavoidable at times and they do the job they are designed to do but they are not my preferred way of storing data.


Regards,

Victor



     
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