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by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Tablespace Fragmentation

As rows are added to tables, the table expands into unused space within the space. Conversely, when rows are deleted, a table may coalesce extents, releasing unused space back into the tablespace. As this happens, it is possible for there to be discontiguous chunks, or fragments of unused space within the tablespace. Whenever the value for a table as specified by STORAGE (INITIAL xx) is exceeded, Oracle will create a new extent for the table. If the PCTINCREASE is set to 0, a new extent of the size specified in STORAGE (NEXT xx) will be added to the table. If PCTINCREASE is non-zero, the extent size will be equal to the value of the most recent extent size multiplied by PCTINCREASE.

Note: PCTINCREASE for a tablespace should never be set to zero, since this will disable the automatic coalesce facility for Oracle tablespaces. In general, all tablespaces except the system tablespaces (SYSTEM, RBS) should have PCTINCREASE set to 1. The PCTINCREASE parameter for tablespaces is generally only used when a table is allocated without a STORAGE clause--although Oracle also uses it for coalescing.

This allocation of new extents will be physically contiguous to the table’s initial location, as long as the next physical data blocks are empty. Unfortunately, as many tables populate a tablespace, a table may not have contiguous data blocks for its next extent, which means that it must fragment the extents onto another spot in the datafile, as shown here:

DATAFILE '/Data/ORACLE/sales/sales.dbf'

This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:


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