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Message Posted: Fri, 02 Feb 2007 @ 17:48:33 GMT

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Subj:   Re: How Do You Measure/Validate Compression Savings?
From:   Judge, James

A few replies on this topic, based on some observed measurements

  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.  

There was a Partner's user presentation on this topic a few years back (R5.x timeframe) which quantified different scenarios. But in my observations (unless the workload is PI, singleton, row inserts-retrievals) a set process that gets the benefit of 30% compression resulting in 30% fewer IOs for that object, always benefits from reduced CPU consumption.

  The idea of having a tool for MVC never occurred to me before I read the thread, probably because of the fear of compressing "small" tables and causing weird adverse effects elsewhere.  

Haven't come across that scenario

  Granted on 'large' systems (more than 100 amps), you generally try to avoid redistribution of tables around 1Gb if at all possible.  

If the 1GB table is being joined to table(s) of 00'sGB or TBs then I have seen this as not uncommon in the R6.x releases

  But how would you calculate queries that touch many objects. What percent of spool reduction is due to compression as opposed to minus natural data growth over time.  

Granted the benefit is diluted across an "n-way" join, but a simple capture of AMPcpu & AMPIO before and after the test shows the benefit. Why couldn't a tool do that (at least estimate based on explain plan IO requirements and making the assumption that 30% compression = 30% less IO for that object)?

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