132 | Chapter 5: Managing Oracle
Fragmentation and Reorganization
Fragmentation is a problem that can negatively impact performance??”and one that
many DBAs have struggled to manage in the past. Fragmentation can be an
unwanted phenomenon if it results in small parts of noncontiguous ???free space??? that
cannot be reused.
In Oracle, a collection of contiguous blocks is referred to as an extent. A collection of
extents is referred to as a segment. Segments can contain anything that takes up
space??”for example, a table, an index, or a rollback segment. Segments typically consist
of multiple extents. As one extent fills up, a segment begins to use another
extent. As fragmentation occurs, by database activity that leaves ???holes??? in the
contiguous space represented by extents, segments acquire additional extents. As
fragmentation grows, increased I/O activity results in reduced performance.
Resolving Fragmentation
As of Oracle Database 10g, resolving fragmentation issues became fairly trivial. You
can perform an online segment shrink using the Segment Advisor interface accessible
through EM.
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