. .
T_AUTOALLOC 15 65536 8
T_AUTOALLOC 16 1048576 128
. . .
T_AUTOALLOC 22 1048576 128
23 rows selected.
Unless a table is truncated or the table is dropped, any blocks allocated to an extent remain
allocated for the table, even if all rows have been deleted from the table. The maximum number
of blocks ever allocated for a table is known as the high-water mark (HWM).
Segments
Groups of extents are allocated for a single segment. A segment must be wholly contained within
one and only one tablespace. Every segment represents one and only one type of database object,
such as a table, a partition of a partitioned table, an index, or a temporary segment. For partitioned
tables, every partition resides in its own segment; however, a cluster (with two or more tables)
resides within a single segment. Similarly, a partitioned index consists of one segment for each
index partition.
Temporary segments are allocated in a number of scenarios. When a sort operation cannot fit
in memory, such as a select statement that needs to sort the data to perform a distinct, group by,
or union operation, a temporary segment is allocated to hold the intermediate results of the sort.
Chapter 6: Monitoring Space Usage 171
Index creation also typically requires the creation of a temporary segment. Because allocation and
deallocation of temporary segments occurs often, it is highly desirable to create a tablespace
specifically to hold temporary segments.
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