oracle statistics

http://www.oracle.com/ocom/groups/public/@otn/documents/webcontent/1354477.pdf

Oracle Database 11g introduced a new sampling algorithm that is hash based and provides deterministic statistics. This new approach has the accuracy close to a 100% sample but with the cost of, at most, a 10% sample. The new algorithm is used when ESTIMATE_PERCENT is set to AUTO_SAMPLE_SIZE (the default)

It is highly recommended that from Oracle Database 11g onward you let ESTIMATE_PRECENT default.

The METHOD_OPT parameter controls the creation of histograms during statistics collection. Histograms are a special type of column statistic created when the data in a table column has a non-uniform distribution.

A unique column will not have a histogram created on it if it is only seen in equality predicates.