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The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse
Oracle Tablespace Management

* MAXEXTENTS--The maximum allowable number of extents (Note: Oracle 7.3 supports MAXEXTENTS UNLIMITED)

* PCTINCREASE--The percentage by which each subsequent extent grows (normally set to 1)

* PCTFREE--The percentage of space to be kept on each data block for future expansion

The PCTFREE parameter is used to reserve space on each data block for the future expansion of row values (via the SQL UPDATE command). Table columns may be defined as allowing null values that do not consume any space within the row, or with varchar data types. A varchar data type specifies the maximum allowable length for the column instance, but the acceptable range of values may be anywhere from 4 bytes (the size of the length holder) to the size of the field plus 4 bytes. Hence, a varchar(2000) may range in size from 4bytes to 2004 bytes.

If an application initially stores rows with empty values and later fills in the values, the PCTFREE parameter can dramatically reduce I/O contention. If a block of storage is filled by the addition of a row, subsequent updates to that row to fill in column values will cause the row to fragment--usually onto the next available contiguous block.

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

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