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Archives of the TeradataForum
Message Posted: Wed, 14 Feb 2007 @ 10:10:13 GMT
| Subj: | | Re: How Do You Measure/Validate Compression Savings? |
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| From: | | Walter, Todd A |
| | I think MVC might actually lead to the *increase* in, say, CPU usage if MVC is liberally applied to "small" tables on a large scale. If
there is compressing then there must be uncompressing going on somewhere down the line, no? It is hard to measure all the effects without access
to a good test lab, though. | |
[taw]
The uncompression is as near to free as it can be. Because it is value based rather than block based or run length or... and because we keep the
list of values in memory any time we are accessing the table, we either do a pointer reference to the value in the current row or a pointer
reference to the value in the value list based upon the setting of the compress bits for the column. There is a small cost for compressing it when
the row is originally stored - have to compare the value to the list. And of course there is a one time cost to convert an uncompressed table to
add compression. But for queries, including against small tables, it is a net win as long as the chosen compression reduces the size of the
table.
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