The optimizer cost model relies on statistics collected about the objects involved in a query,and the database and host where the query runs.

Statistics are critical to the optimizer's ability to pick the best execution plan for a sql statement.

Optimizer statistics include the following:

Table statistics

Number of rows
Number of blocks
Average row length

Column statistics

Number of distinct values (NDV) in a column
Number of nulls in a column
Data distribution (histogram)
Extended statistics

Index statistics

Number of leaf blocks
Number of levels
Index clustering factor
System statistics

I/O performance and utilization
cpu performance and utilization
As shown in Figure 10-1,the database stores optimizer statistics for tables,columns,indexes,and the system in the data dictionary. You can access these statistics using data dictionary views.