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When a SQL query is submitted to an Oracle
database, Oracle must decide how to access the data. The process of making this
decision is called query optimization, because Oracle looks for the optimal way to
retrieve the data. This retrieval is known as the execution path. The trick behind
query optimization is to choose the most efficient way to get the data, since there
may be many different options available.
For instance, even with a query that involves only a single table, Oracle can take
either of these approaches:
??? Use an index to find the ROWIDs of the requested rows and then retrieve those
rows from the table.
??? Scan the table to find and retrieve the rows; this is referred to as a full table scan.
Although it??™s usually much faster to retrieve data using an index, the process of getting
the values from the index involves an additional I/O step in processing the
query. Query optimization may be as simple as determining whether the query
involves selection conditions that can be imposed on values in the index.


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