DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

stores the selected columns from each qualifying base row in the result DTT and scrolling is
performed on this temporary table. DB2 deletes the result table when the cursor is closed.
Dynamic scrollable cursors are new with DB2 V8. Dynamic scrollable cursors do not
materialize the result table. Instead, they scroll directly on the base table and are therefore
sensitive to all committed inserts, updates, and deletes. Dynamic scrollable cursors are
supported for the index scan and table space scan access paths. DPSIs can also support
dynamic scrollable cursors. Dynamic scrollable cursors can also be used with row-level
operations or rowset-level operations.
3.12.1 Static scrollable cursor description
Static scrollable cursors introduced in V7 allow scrolling backwards as well as forwards. They
also provide the capability to place the cursor to an absolute position or a position relative to
the current cursor. In terms of performance, when a static scrollable cursor is opened, the
qualifying rows are copied to a DB2-declared temporary table, also commonly known as the
result table. This result table has a fixed (static) number of qualifying rows. Performance
impact is based upon the number of qualifying rows that must be copied from the base table
to the temporary table. Once the qualifying rows are copied to the temporary table, scrolling is
then performed on this table in either the forward or backward direction, sequentially or to a
specific row. When the cursor is closed, DB2 deletes the result table.
Dynamic scrollable cursors generally do not employ the usage of a temporary table.
Therefore, the single most important performance difference between static scrollable cursors
and dynamic scrollable cursors is the overhead in building the result table, maintaining a
consistent relationship between the base and result tables with an updatable cursor, and
deleting the result table at close cursor for static scrollable cursors.
Sensitivity options for static scrollable cursors
A static scrollable cursor can be sensitive or insensitive to update, and can be sensitive or
insensitive to changes made by other applications or outside of the cursor. These can be
specified in the DECLARE CURSOR statement and FETCH statement. Table 3-3 outlines the
available combinations, whether the cursor is read only or updatable, and whether the cursor
is sensitive to changes to the base table.
Table 3-3 Static cursor declare and fetch specifications
Specification
on DECLARE
Specification
on FETCH
92 DB2 UDB for z/OS Version 8 Performance Topics
Type of
Cursor
Comments
Insensitive Insensitive Read-only Not aware of updates, deletes and inserts to
base table
DB2 uses the predefined TEMP database and
segmented table space to create your DTT and
move in the results at execution time.
Sensitive Insensitive Updatable Aware of own updates and deletes within the
cursor.
Changes by another agent are not visible to the
cursor.
Any inserts are not recognized.
DB2 uses the predefined TEMP database and
segmented table space to create your DTT and
move in the results at execution time.