14. Using the Query Optimizer : Database Statistics : Information Collected by the Optimizer
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Information Collected by the Optimizer
When you optimize a database, the following information is collected:
The number of unique values in those columns selected for optimization
A count showing what percentage of those column values are NULL
The number of duplicate values in those columns in the whole table, on average, or whether all values are unique. This is termed the repetition factor.
A histogram showing data distribution for each of those columns. Sample histograms and further information on their contents can be found in Histogram (Optimization Output).
The query optimizer can use this information to calculate the cost of a particular QEP.