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Real World applications for read-only tablespaces

Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Read-only tablespaces in the real world

But how do we “pack” Oracle data for use as a read-only tablespace? Essentially, there are two areas where space is wasted within an Oracle table. On each data block, Oracle will reserve an amount for row expansion and this value is controlled by the PCTFREE table parameter. The PCTFREE parameter is useful when we are loading incomplete rows that have columns that are defined with VARCHAR data definitions. After initial loading, Oracle reserves space at the end of each data block for row expansion. For example, if we have defined the fact_96 table with 8K blocks and PCTFREE = 25, we have reserved 2000 bytes at the end of each and every data block for row expansion. Later when is issue an SQL UPDATE command to add column values for a null value, the row size will expand into this free space. The second area of free space in a tablespace is the space left over that the end of the table. (figure 8.15) Here we might see that the fact_96 table consumes only 75% of its tablespace, allowing for the table to extend as new rows are added. In short, table grow wider as column values are added (in each data block), and table grow longer as rows are added (at the end of the tablespace).

Figure 8.15 Free space distribution within a tablespace

If we know that the table chunk is now static, we can export the table into a flat file, and re-define the table and tablespace characteristics to maximize the amount of occupied space. At the table level, we set PCTFREE equal to 0, not reserving any space within the data blocks for growth. At the tablespace level, we redefine the tablespace to consume only as much space as the table requires. (The total amount of space for a table can be estimated fairly accurately by estimating the average row length, and multiplying by the number of rows)

This is an excerpt from "High Performance Data Warehousing", copyright 1997.

If you like Oracle tuning, you may enjoy the book Oracle Tuning: The Definitive Reference , with over 900 pages of BC's favorite tuning tips & scripts. 

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