Introduction to NonStop SQL/MP

Physical Database Structure
NonStop SQL Architecture
113425 Tandem Computers Incorporated 3–5
Index-Only Access
For some queries, NonStop SQL/MP achieves even greater access speed if the index
columns contain all the requested data. To satisfy these queries, the system retrieves
data from the index table only and does not read the base table at all. This feature is
called index-only access.
Suppose an index on the EMPLOYEE table includes the LAST_NAME, FIRST_NAME,
and EMPNUM columns. Now suppose a query selects only those columns and
requests the result to be ordered by employee name. In this case, NonStop SQL/MP
uses index-only access. If the query also selects the DEPTNUM column,
NonStop SQL/MP scans the index, uses it to locate the appropriate rows in the base
table, and then reads the base table to retrieve the data.
Figure 3-2 illustrates the index XEMPNAME, which speeds access to the EMPLOYEE
table for selections sequenced by employee name. (When you create the index, you do
not have to specify the EMPNUM column because it is the primary key of the
EMPLOYEE table. NonStop SQL/MP automatically includes the primary key in an
index.)