Getting Started with IBM Data Studio for DB2

116 Getting Started with IBM Data Studio for DB2
• In-place reorganization (called Incrementally reorganize the table in place in the
Options tab shown in Figure 3.15) allows reorganization to occur while the table or
index is fully accessible. If you select this option, you can set the table access
control to allow read, or read and write access.
• Offline reorganization (called Rebuild a shadow copy of the table in the Options
tab) means that reorganization occurs in offline mode. You can specify whether to
allow read access or not during offline reorganization.
While offline reorganization is fast and allows perfect clustering of the data, online
reorganization lets the table remain online to applications. If applications must write to the
table while reorganization is occurring, use the in-place reorganization method. When you
use in-place reorganization, you have more control over the process, and you can pause
and restart the process. However, in-place reorganization requires more time to complete.
You can reorganize the table by using an existing index. When you reorganize by using an
existing index, the data is accessed more quickly while data is being reorganized.
When you choose offline reorganization, you can use the temporary table space to store
the copy of the reorganized table. You can use the temporary table space by selecting the
Use system tempspace to temporarily store a copy of the reorganized table option. You
can also reorganize long fields and large object data, as well as the option to reset the
dictionary if data compression is enabled.
4. After you choose the options that are required for your organization, you can run
the command by clicking Run, as shown in Figure 3.15 above.
5. Close the editor before you start the next task.
3.2.4 Gathering statistics
When an application runs a query, the DB2 optimizer compiles the query and creates an
access plan, which describes the sequence of steps to run the query and fetch the data
that is returned. Access plans give estimations of the cost and time that is required to run a
query. The sequence of steps that is created as part of the access plan depends on a
number of factors, such as:
Size of the database table, indexes and views
Distribution of data in the specific columns of the tables
Average length and the cardinality of the column values
Amount of null values and the highest and lowest values of the columns
As update, insert, and delete transactions happen on a database, the data grows or shrinks
and often changes its distribution. This means the statistics that the optimizer currently
knows about are outdated and no longer reflect reality. If the information stored in the
catalogs is not up to date, the steps created as part of the access plan may not be accurate
and can generate a less than optimal access plan, which may negatively affect
performance.