Expert Indexing in Oracle Database 11g: Maximum Performance for Your Database

Optimizer Access Paths

An access path is the path chosen by the optimizer to retrieve data from the database. There are two basic types of access paths: index access paths and a full table scan. A full table scan is simply a scan of all the rows in a table, and the optimizer uses it mostly when a query requests a large portion of a table's blocks. Sometimes the percentage of rows retrieved by a query is relatively small, but due the way the table's data is distributed among the blocks in the table segment, rows that satisfy the query are present in the majority of blocks. Oracle reads blocks, and block selectivity is critical to access path selection. Index scans, on the other hand, are typically used to retrieve a small set of a table's rows, ...