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Optimizer statistics dynamic sampling when the statistics doesn't exist:

  • on the table
  • of for instance on a calculation
sal + comm > 2000

Dynamic sampling first became available in Oracle9i Database Release 2. It is the ability of the cost-based optimizer (CBO) to sample the tables a query references during a hard parse, to determine better default statistics for unanalyzed segments, and to verify its “guesses.”

This sampling takes place only at hard parse time and is used to dynamically generate better statistics for the optimizer to use, hence the name dynamic sampling More on Dynamic sampling - Oracle Magazine))

Parameter

Configuration Parameter 

show parameter dynami

NAME                           TYPE      VALUE
----------------------------   -------   -----
optimizer_dynamic_sampling     integer   2

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