These are the columns that are all columns referenced in the query, not just
columns in a WHERE clause. If so,
never has to go to the
data pages at all, but can retrieve all data through index access alone. For
example, in the following queries, OrigIndex covers the
query:
SELECT orig_airport
FROM Flights
SELECT DISTINCT lower(orig_airport) FROM Flights
FROM Flights
can get all required
data out of the index instead of from the table.
If the query produces an updatable result set,
will retrieve all data
from the data pages even if there is an index that covers the query.