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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.
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