Like indexes, IOTs are subject to inserts, deletes, block splits and coalsces, all of which affect the rowid. This means a standard rowid cannot always be used to retrieve an IOT record from a secondary index. For this reason secondary indexes on IOTs use a logical rowid which is made up of the original rowid and the primary key of the row. When a secondary index is referenced the rowid is used to find the block. If the block is not present at that disk address the primary key is used to find the block. This initial rowid access is know as a guess, since the block may not be at its origninal disk address anymore. Physical movements of the row do not affect the logical rowid, so long as the primary key is not updated.
With time the percentage of hits using the guess rowid will drop. When the hitrate gets sufficiently low the index should be dropped and recreated to refesh the guess rowids. The guess hitrate can be monitored using the
With time the percentage of hits using the guess rowid will drop. When the hitrate gets sufficiently low the index should be dropped and recreated to refesh the guess rowids. The guess hitrate can be monitored using the
PCT_DIRECT_ACCESS column of DBA_INDEXES, ALL_INDEXES and USER_INDEXES:[@more@]http://dbsnake.com/2009/06/logical-rowid-structure.html --原文连接
http://www.oracle-base.com/articles/9i/IndexOrganizedTableEnhancements9i.php#LogicalRowid
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