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

► The data is accessible but until the table space is reorganized a performance penalty is
paid.
► Plans, packages and cached dynamic statements referring to the changed column are
invalidated. If auto-rebind is enabled, the plans and packages referencing the changed
table space are automatically rebound during the next access if not manually rebound
previously.
► Frequency statistics for columns are invalidated.
► The new column type must be large enough to hold the maximum value possible for
original column.
► If indexed, changes for character data type columns result in immediate index availability.
This includes columns defined as CHAR, VARCHAR, GRAPHIC, or VARGRAPHIC.
► If indexed, changes for numeric data type columns are immediate with delayed index
availability. This includes columns defined as SMALLINT, INTEGER, DECIMAL or
NUMERIC, FLOAT, REAL, or DOUBLE. Availability to the index is delayed until the index is
rebuilt.
► Scope of unavailability for dynamic SQL
– Deletes are allowed for table rows, even if there are indexes in RBDP.
– Updates and inserts are allowed for table rows, even if their corresponding non-unique
indexes are in RBDP state.
– Inserting or updating data rows which result in inserting keys into an index that is in
RBDP state is disallowed for unique or unique where not null indexes.
– For queries, DB2 does not choose an index in RBDP for an access path.
Recommendations
When using this feature, make sure that you do not forget to assess which programs need to
be changed. Host variables, for example, may need to be extended to cater to the extra
length. To minimize any performance degradation, schedule a REORG as soon as possible
after ALTER. This reestablishes fast column processing and eliminates the conversion from
old row format to the new one when retrieved by the application.
Rebuild any affected indexes and rebind plans and packages. Otherwise DB2 might pick an
inefficient access path during automatic rebind because the best suited index is currently not
available.
Schedule RUNSTATS to repopulate the catalog with accurate column and index statistics.
Alter index
Several changes can now be implemented via the ALTER INDEX statement:
► Add a column to the end of an index
Adding the column to the table and index in the same UOW will place the index in AREO*
state. The index is immediately available.
Adding an existing column to the index places the index in rebuild pending (RBDP) status.
► Scope of unavailability for dynamic SQL
Deletes are allowed for table rows, even if there are indexes in RBDP.
Updates and inserts are allowed for table rows, even if their corresponding non-unique
indexes are in RBDP state.
Inserting or updating data rows which results in inserting keys into an index that is in
RBDP state is disallowed for unique or unique where not null indexes.
Chapter 5. Availability and capacity enhancements 231