Even if you follow the most rigorous sizing calculations, you
cannot be sure how Oracle will load the data into the table or index.
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144 Oracle Database 11g DBA Handbook
In the following section, you??™ll see how to simplify the space-estimation process, freeing you
to perform much more useful DBA functions. These processes should be followed whether you
are generating the default storage values for a dictionary managed tablespace or the extent sizes
for locally managed tablespaces.
NOTE
In an Oracle 10g database, you should be using locally managed
tablespaces. If you have upgraded from a prior release that used
dictionary-managed tablespaces, you should replace them with
locally managed tablespaces.
The Ground Rules for Space Calculations
Oracle follows a set of internal rules when allocating space:
Oracle only allocates whole blocks, not parts of blocks.
Oracle allocates sets of blocks rather than individual blocks.
Oracle may allocate larger or smaller sets of blocks, depending on the available free
space in the tablespace.
Your goal should be to work with Oracle??™s space-allocation methods instead of against them.
If you use consistent extent sizes, you can largely delegate the space allocation to Oracle even in
a dictionary-managed tablespace.
The Impact of Extent Size on Performance
There is no direct performance benefit gained by reducing the number of extents in a table. In
some situations (such as in Parallel Query environments), having multiple extents in a table can
significantly reduce I/O contention and enhance your performance.
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