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Oracle Features for the Data Warehouse

Oracle 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 was introduced with Oracle 7.2 to change the behavior of full-table scan reads. When a table is created with the CACHE option, full-table scan fetches against this table will be treated as if they were not full-table scans, and they will be placed at the most-recently-used end of the data buffer. Once the data blocks have been read to the buffer cache, they will “age-out” and move toward the least-recently-used side of the buffer just like any other data block in Oracle.

In this sense the term “cache” is somewhat misleading. For example, when Oracle packages are cached in the library cache, they are “pinned” (i.e. ineligible from being aged-out ) and the packages remain in the library cache for the life of the Oracle instance. Data blocks, on the other hand, are not “pinned” into the data buffer with the CACHE option and the data blocks will age-out of the buffer at the same rate as other data blocks. Of course, the data blocks will remain in the buffer for a much longer period than data block fetched with full-table scans, but they will not stay in the buffer indefinitely. At this time, the only way to “pin” a data block in an Oracle buffer is to utilize Oracle parallel server to dedicate an Oracle instance for a particular table, and make the buffer large enough to hold the entire contents of the table.

The CACHE_SIZE_THRESHOLD init.ora parameter must be set to use the table CACHE option. This parameter controls the space in the buffer cache that is used exclusively for full-table scans. The Oracle documentation falsely insinuates that the CACHE_SIZE_THRESHOLD parameter is used exclusively with Oracle parallel server. In reality this parameter applies to all Oracle databases.
Also, note that the CACHE_SIZE_THRESHOLD parameter must be smaller than the table size. The default for this value is (.10 * db_block_buffers), such that any table that is larger than 10% of the buffer pool will not be effected by the CACHE option.

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

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