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Oracle 8 Tips  

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Oracle’s Table CACHE Option

The table cache option is a major benefit of the Oracle architecture for data warehouses, and deserves the warehouse designers complete attention. While the term cache is a misnomer in the sense that an Oracle table is not permanently pinned in memory, the table cache option dramatically improved the buffer hit ration for small, frequently accessed warehouse tables. When a request is made for Oracle to retrieve a row from a table, Oracle performs several steps. First, Oracle checks the data buffer to see if the database block that contains the row already resides in Oracle’s data buffer. (Figure 8.1)

Figure 8.1 The Oracle data buffer

This scan is made from the most-recently-used end to the least-recently-used end of the buffer, and since this buffer is in the SGA as RAM memory, this check happens very quickly. Only after Oracle has determined that the database block is not already in the buffer, does Oracle perform a physical I/O to fetch the data block.
Note: the UNIX operating system is configured to check the cache area or physical RAM before UNIX issues a physical I/O against a disk. Therefore, all I/O requests from Oracle do not always result in a disk I/O since it is possible that an Oracle data block is in UNIX RAM even when it is not in the Oracle buffer cache.

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