ADDM recommends segments to shrink, and you simply choose to
accept the recommendations.
For Oracle9i databases, a common means of reducing fragmentation was through an
online reorganization accomplished through a CREATE TABLE...AS SELECT
online operation??”that is, the copying of the contents of one table to another while
the original table is updated. Changes to the original table were tracked and applied
to the new table. Physical and logical attributes of the table could be changed during
this online operation, thus allowing an online reorganization.
Prior to Oracle9i, reducing fragmentation was more difficult. The general recommendation
was to avoid fragmentation through careful planning. But the usual way to
solve fragmentation was to reorganize a table by exporting the table, dropping it, and
importing it. The data was unavailable while the table was in the process of being
reorganized. Many DBAs claimed that they saw improved performance after reorganizing
segments into a single extent. Over time, a decrease in performance
reoccurred as the number of extents the table occupied increased.
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