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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
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