The SCAN operator
reads rows into the query plan and makes them available for further
processing by the other operators in the query plan. The SCAN operator
is a leaf operator; that is, it never has any child operators. The SCAN operator
can read rows from multiple sources, so the showplan message identifying
it is always followed by a FROM message
to identify what kind of SCAN is
being performed. The FROM messages
are: FROM CACHE, FROM
OR, FROM LIST,
and FROM TABLE.
