Before you can change which maps tables use, you need to decide how these changes will occur.
The Time Limit Decision
- Without a time limit, planned map reassignments listed in ActionsTbl will occur until either all the planned reassignments complete or until the DBA manually stops the reassignments.
- With a time limit, the Mover process might skip a group of tables if the time limit is almost reached and instead reassign another, smaller group of tables that can be processed in the available time. One example of using a time limit is if you want to limit actions to the weekend and have map reassignment work expire during normal business hours.
The Serial or Parallel Processing Decision
- Serial processing: It is a good idea to use serial processing when changing maps for large tables. Changing the map for one large table at a time is a strategy less likely to run out of space than changing the maps for several large tables at once. The ALTER TABLE request that changes the map for a table first makes a copy of the table and then deletes the old copy. ALTER TABLE requires 2 times the space of the original table to complete map reassignment. Column-partitioned and NoPI tables require 3 times the space of the original table. If you use only 1 worker session, Mover reassigns only one large table at a time. If you try to do more than that, you risk running out of space.
- Parallel processing: It is a good idea to use parallel processing when you are changing the map for many smaller or medium-sized tables. Parallel processing is faster than serial processing and is not risky when table sizes are moderate.
The Workload Decision (Optional)
SET QUERY_BAND = 'WDClassification=MoveTableToMap;' FOR SESSION;
- Results of specifying the workload: If you set the query band as shown in the preceding example, the Mover session will execute in the WD-MapsMover workload. This workload is dedicated to reassigning tables from one map to another. The default priority of this workload is Timeshare High.
- Results of not specifying the workload: If you do not set the query band, TASM workload classification rules will determine the workload and the corresponding priority of the Mover process. For more information, see Teradata Workload Management User Guide, B035-1197.
You can move the WD-MapsMover workload to a different Timeshare access level using Viewpoint Workload Designer. For example, if you wanted to run map reassignments in the background, you could move WD-MapsMover to Timeshare Low. If you wanted to run this work at a higher priority, you could move WD-MapsMover to Timeshare Top.