just an excerpt from one of the reference manuals - refer manuals for detailed undestanding of statistics
- The purpose of collecting statistics is to compute statistical data that the Optimizer uses to optimize table access and join plans. The computed statistical data is retained in a synopsis data structure that is stored in the data dictionary for use during the optimizing phase of SQL statement parsing. Accurate demographics enable the query optimizer to determine the least costly access and join plans for a query. It is particularly important for the Optimizer to have accurate table demographic information for those situations in which data to be accessed using a column or index might be skewed.