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Understanding Dynamic Sampling

From: <Rajesh.Rao_at_jpmchase.com>
Date: Thu, 21 Jun 2007 16:45:23 -0400
Message-ID: <OFF28FC8D1.D9735114-ON85257301.007024D3-85257301.0072051A@jpmchase.com>

From the 10gR2 Docs, the definition for the values 3 and 4, for the 
parameter optimizer_dynamic_sampling:

       Level 3: Apply dynamic sampling to all tables that meet Level 2 criteria, plus all tables for which standard selectivity estimation used a guess for some 
predicate that is a potential dynamic sampling predicate. The number of blocks sampled is the default number of dynamic sampling 
blocks. For unanalyzed tables, the number of blocks sampled is two times 
the default number of dynamic sampling blocks.
       Level 4: Apply dynamic sampling to all tables that meet Level 3 criteria, plus all tables that have single-table predicates that reference 2 or more 
columns. The number of blocks sampled is the default number of dynamic sampling 
blocks. For unanalyzed tables, the number of blocks sampled is two times 
the default number of dynamic sampling blocks.

I understand how dynamic sampling would assist in the scenario described 
in Level 4, but can someone help me understand "all tables for which standard selectivity estimation used a guess for some 
predicate that is a potential dynamic sampling predicate" ? 

Regards
Raj



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Received on Thu Jun 21 2007 - 15:45:23 CDT

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