The optimizer determines the number of rows that will be scanned from disk
when deciding on an access path for a particular table (whether to use an
index or to scan the table).
- The optimizer knows "exactly" the number of rows that will be
scanned from disk for table scans (see ).
- For index scans, the optimizer must estimate the number of rows that will
be scanned from disk. (see ). might
be able to use cardinality statistics to make a better estimate of the number
of rows that will be scanned from disk as described in this chapter.