Databases Reference
In-Depth Information
The Index Skip Scan operation is a compromise between the Index Range Scan and Fast Full
Scan. In our example, the database looks up the first leaf in the index in which entries have
CUST_GENDER
=
"F"
, then scans all the possible values for
CUST_YEAR_OF_BIRTH
and, for
each value, scans the index for
CUST_FIRST_NAME
=
"Yvette"
.
See the Oracle Database Concepts 11
g
R2 documentation for a
complete description of how B-tree indexes work and what B-tree
branch and leaf nodes are, available online at:
http://download.oracle.com/docs/cd/E14072_01/
server.112/e10713/indexiot.htm#CNCPT811
This operation is slower than an Index Range Scan, but faster than a Fast Full Scan because
less data blocks have to be read.
In step 7, we query rows with a predicate not involving the first field of the
CUSTOMERS_IXMULTI
index, the results are as follows:
We can see that even in this case the Index Skip Scan is chosen by the optimizer, and the
behavior in executing this plan will be similar to the previous one.
