Selectivity and cardinality statistics

Selectivity and cardinality statistics The optimizer determines the number of rows that will be scanned from disk when deciding on an access path for a particular table (whether to use an index or to scan the table). Statistics Cardinality statistics

For table scans, the optimizer knows exactly the number of rows that will be scanned from disk (see ).
For index scans, the optimizer must estimate the number of rows that will be scanned from disk (see ).
might be able to use cardinality statistics to make a better estimate of the number of rows that will be scanned from disk, as described in this section.