There are certain limitations and restrictions associated
with Design Advisor recommendations about indexes, materialized query
tables (MQTs), multidimensional clustering (MDC) tables, and database
partitioning.
Restrictions on index recommendations
- Indexes that are recommended for MQTs are designed to improve
workload performance, not MQT refresh performance.
- A clustering RID index is recommended only for MDC tables. The
Design Advisor will include clustering RID indexes as an option rather
than create an MDC structure for the table.
- The Version 9.7 Design Advisor does not recommend partitioned
indexes on a partitioned table. All indexes are recommended with an
explicit NOT PARTITIONED clause.
Restrictions on MQT recommendations
- The Design Advisor will not recommend incremental MQTs. If you
want to create incremental MQTs, you can convert REFRESH IMMEDIATE
MQTs into incremental MQTs with your choice of staging tables.
- Indexes that are recommended for MQTs are designed to improve
workload performance, not MQT refresh performance.
- If update, insert, or delete operations are not included in the
workload, the performance impact of updating a recommended REFRESH
IMMEDIATE MQT is not considered.
- It is recommended that REFRESH IMMEDIATE MQTs have unique indexes
created on the implied unique key, which is composed of the columns
in the GROUP BY clause of the MQT query definition.
Restrictions on MDC recommendations
- An existing table must be populated with sufficient data before
the Design Advisor considers MDC for the table. A minimum of twenty
to thirty megabytes of data is recommended. Tables that are smaller
than 12 extents are excluded from consideration.
- MDC recommendations for new MQTs will not be considered unless
the sampling option, -r, is used with the db2advis command.
- The Design Advisor does not make MDC recommendations for typed,
temporary, or federated tables.
- Sufficient storage space (approximately 1% of the table data for
large tables) must be available for the sampling data that is used
during the execution of the db2advis command.
- Tables that have not had statistics collected are excluded from
consideration.
- The Design Advisor does not make recommendations for multicolumn
dimensions.
Restrictions on database partitioning recommendations
The
Design Advisor can recommend database partitioning only for DB2® Enterprise
Server Edition.
Additional restrictions
Temporary simulation
catalog tables are created when the Design Advisor runs. An incomplete
run can result in some of these tables not being dropped. In this
situation, you can use the Design Advisor to drop these tables by
restarting the utility. To remove the simulation catalog tables, specify
both the -f option and the -n option (for -n, specifying the same
user name that was used for the incomplete execution). If you do not
specify the -f option, the Design Advisor will only generate the DROP
statements that are required to remove the tables; it will not actually
remove them.
Note: As of Version 9.5, the -f option is the default.
This means that if you run db2advis with the MQT
selection, the database manager automatically drops all local simulation
catalog tables using the same user ID as the schema name.
You
should create a separate table space on the catalog database partition
for storing these simulated catalog tables, and set the DROPPED TABLE
RECOVERY option on the CREATE or ALTER TABLESPACE statement to OFF.
This enables easier cleanup and faster Design Advisor execution.