# Queries with a known search condition

When the exact start and stop conditions are known at compilation time, the optimizer uses the index itself to make a very precise estimate of the number of rows that will be scanned from disk. An example of a query with a known search condition:

    SELECT *
    FROM Flights
    WHERE orig_airport = 'SFO'

The search value, 'SFO', is known. The optimizer will be able to make an accurate estimate of the cost of using the index orig_index.

In addition, if the index is unique, and the WHERE clause involves an = or IS NULL comparison to all the columns in the index, the optimizer knows that only a single row will be scanned from disk. For example:

    -- there's a unique key on city_id
    SELECT * FROM Cities WHERE city_id = 1

Parent topic: Estimations of rows scanned from disk for an index scan
Related concepts
Queries with an unknown search condition