Afterward, the space
requirements can be refined based on the actual usage statistics. In the following sections, you
will see the space estimation methods for tables, indexes, and clusters. You??™ll also see methods
for determining the proper settings for pctfree and pctused.
NOTE
You can enable Automatic Segment Space Management when you
create a tablespace; you cannot enable this feature for existing
tablespaces. If you are using Automatic Segment Space Management,
Oracle ignores the pctused, freelists, and freelist groups parameters.
Why Size Objects?
You should size your database objects for three reasons:
To preallocate space in the database, thereby minimizing the amount of future work
required to manage objects??™ space requirements
To reduce the amount of space wasted due to overallocation of space
To improve the likelihood of a dropped free extent being reused by another segment
You can accomplish all these goals by following the sizing methodology shown in the following
sections. This methodology is based on Oracle??™s internal methods for allocating space to database
objects. Rather than rely on detailed calculations, the methodology relies on approximations that
will dramatically simplify the sizing process while simplifying the long-term maintainability of the
database.
The Golden Rule for Space Calculations
Keep your space calculations simple, generic, and consistent across databases. There are far more
productive ways to spend your work time than performing extremely detailed space calculations
that Oracle may ignore anyway.
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