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Segments in Oracle
A segment is a container for Objects (such as tables, indexes....). A segment consists of extents.
Segment types
There are 11 types of segments in Oracle 10g:
table
table partition
index
index partition
cluster
rollback
deferred rollback
temporary
cache
lobsegment
lobindex
These types can be grouped into four segment classes:
data segments: table and cluster
index
rollback
temporary data
In order to find out the size of the respective segments, use dba_segments (or user_segments, or all_segments). A segment can either be created so that it can have an unlimited or limited number of extents. Oracle keeps track of free blocks for each data segment.
Segment Headers
The segment header is stored in the first block of the first extent. It contains:
The extents table
The free lists descriptors
The high water mark

Physical attributes
See here.
Temporary segments
The following sql statements require a temporary segment if they cannot be completed in memory:
create index.
select order by
select distinct
select group by
select union
select intersect
select minus by
analyze table
unindexed joins
certain correlated subqueries
Actually, most of these statements require a sort. Temporary segments can be stored in any tablespace. However, it makes more sense to store temporary segments in a temporary tablespace. SMON frees temporary segments.


To Be Continue
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