Statistics, Data Distribution, and Cardinality
By now, you should have a good understanding of the importance of indexes. But, the index alone is not what the optimizer uses to determine how it’s going to access data. The optimizer must have information about the data that defines an index or a column. That information is referred to as a statistic. Statistics define both the distribution of data and the uniqueness or selectivity of the data. Statistics are maintained both on indexes and on columns within the system. You can even define statistics manually yourself.