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Administration of Indexes

Indexes can make or break an application. A nonindexed query that takes tens of minutes can be made to return values within seconds if indexes are done properly. A critical subset of the Remote DBA’s tasks involves the placement, creation, sizing, and maintenance of the normal, bitmapped, partitioned, function-based, and subpartitioned indexes available in Oracle8, Oracle8i and Oracle9i.

Oracle8 offered new functionality in the form of partitioned, bitmapped, and reversed key indexes. Oracle8i offered all of these plus the capability to do INDEXTYPE, DOMAIN INDEX, and function-based indexes, as well as more advanced partitioning options for indexes. Partitioned indexes allow the spread of index data automatically by data value range across multiple partitions that can be placed on several disk arrays or platters. In Oracle8i, these partitions can be further subdivided into subpartitions. Bitmapped indexes allow for indexing of low-cardinality data, a feature that came about in and continued with Oracle8 and its later releases. Bitmapped indexes map data values as binary integers, allowing low-cardinality data to be quickly accessed with sometimes almost quantum decreases in access speed. For some specialized types of query, a reverse key index can improve data access speeds.      

In Oracle9i, two new types of index are offered: the bitmap join index and the skip scan index. A bitmap join index acts as an intersection between two tables, in a sense, prejoining them via a bitmap index. A skip scan index is a specially constructed index that allows Oracle to scan independent B-tree levels instead of the normal top-down scan. Indexes allow queries to rapidly retrieve data, with proper implementation. Single columns, or groups of columns, can be indexed. A Remote DBA can specify whether or not an index is unique. Remember, for proper table design, each table must have a unique identifier. A unique index is automatically created when a unique or primary key constraint clause is used in a CREATE or ALTER TABLE command.      

Indexes speed the search for queries when approximately 2 to 7 percent of the table or less is being retrieved. For larger retrievals, inserts, and updates to index columns, and deletes, indexes slow response. An exception to this is if you use a bitmapped index for low-cardinality data.     

How columns are indexed affects their efficiency. Order columns should be specified to reflect the way a select will retrieve them. The column accessed most often should be put first. Remember, the leading portion of the index is used to speed queries. A composite index can be created on up to 16 columns. Columns of type LONG and LONG RAW cannot be indexed.

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