# Estimations of rows scanned from disk for an index scan

When an index is available, the optimizer has to estimate the number of rows that will be scanned from disk. The accuracy of this estimate depends on the type of query being optimized.

- Queries with a known search condition
- Queries with an unknown search condition

Parent topic: Selectivity and cardinality statistics
Related concepts
Determinations of rows scanned from disk for a table scan
Statistics-based versus hard-wired selectivity
What are cardinality statistics?
Working with cardinality statistics