ER/Studio - Embarcadero Technologies Product Documentation

USING ER/STUDIO > DEVELOPING THE PHYSICAL MODEL
• Where Not Null: (WHERE_NOT_NULL) If selected, specifies that multiple nulls can exist in a unique index. Useful
when an index contains at least one nillable column, but all non-null entries must be unique.
• Cluster: If selected, specifies to create a clustered index. A clustered index specifies that the rows of the table
should be inserted sequentially on data pages, which generally requires less disk I/O to retrieve the data.
Generally, the clustered index should be on a column that monotonically increases, such as an identity column,
or some other column where the value is increasing, and is unique. This is especially true if the table is subject to
many INSERTS, UPDATES, and DELETES. Clustered indexes can greatly increase access speed, but usually only
where the data is accessed sequentially in the same or reverse order of the clustered index, or when a range of
items are selected.
• Percent Free: (PCTFREE) Specifies the maximum percentage of space in each data block to reserve for future
updates. This reserved space helps to avoid row migration and chaining caused by an update operation that
extends a data row's length. Tables that won’t be updated should have this value set to 0.
• Min Percent Used: (MINPCTUSED) Specifies the minimum amount of used space on an index leaf page. If used,
online index defragmentation is enabled for this index.
• Allow Reverse Scans: Specifies that the index be created in a way so that scans can also be performed in the
direction opposite to that with which they were defined.
• Include: Specifies additional columns to be appended to the set of index key columns. The included columns
may improve the performance of some queries through index-only access.
IBM DB2 for OS/390 Index Options
• Unique: (UNIQUE) If selected, specifies that the index key contains no duplicate values and therefore every row
in the table is in some way unique. If not selected, a non-unique index is created that lets table rows have
duplicate values in the columns that define the index.
• Cluster: (CLUSTER) Select to create a clustered index. A clustered index specifies that the rows of the table
should be inserted sequentially on data pages, which generally requires less disk I/O to retrieve the data.
Generally, the clustered index should be on a column that monotonically increases--such as an identity column,
or some other column where the value is increasing--and is unique. This is especially true if the table is subject to
many INSERTS, UPDATES, and DELETES. Clustered indexes can greatly increase access speed, but usually only
where the data is accessed sequentially in the same or reverse order of the clustered index, or when a range of
items are selected.
NOTE: For IBM DB2 for OS/390 5.x, the Cluster option must be selected to enable the index partitioning
options on the Partitions tab of the Index Editor. For more information, see Partitioning a Table
Index.
• Where Not Null: (WHERE NOT NULL) Specifies that multiple nulls can exist in a unique index. Useful when an
index contains at least one nillable column, but all non-null entries must be unique.
• Concurrent Copy: (COPY) If selected, concurrent copies of the index are allowed, which can increase index
availability by supporting index recovery.
• Do Not Close Dataset: (CLOSE) If selected, the index will not be closed when not in use. If not selected, the
dataset can be closed if it is not in use and the maximum number of open datasets is reached or exceeded.
• Defer Index Creation: (DEFER) Specifies that index modifications are to be deferred during INSERT, UPDATE,
and DELETE operations. This allows the data updates to complete quicker, but queries may not return the most
up-to-date information until the index is synchronized. This option is not supported for an index on a declared
temporary or auxiliary table.
• Padded: (PADDED) If selected, the index entry is padded to the full length of its datatype which makes scanning
the index efficient if most of the index entries are close to or at the maximum length. If not selected, the index
supports true variable length key which allows for shorter indexes and more index entries on each page. It also
allows index-only access. If the index is not padded, data can be retrieved directly from the index without having
to look up the data length in the data page. Not padding the index can be more efficient if you’ll save a lot of disk
space by not padding them.
EMBARCADERO TECHNOLOGIES > ER/STUDIO® 8.0.3 USER GUIDE 170