Choosing a Base-Table Access Strategy

Three methods are available for accessing a base table:

• Table scan. This is generally the slowest method because every data page must be read. When a table scan is used, single-table restrictions are checked for each record in the table, but no base-table page is read from disk more than once.
  
  
• Index range. When an index range is used, a table is opened with a particular index on one of the single-table restrictions (or selection criteria). Records in the index are then checked against the remaining restrictions. A base-table page can be read from disk more than once.
  
  
• Rushmore restriction. A Rushmore restriction is used when there are restrictions on multiple-indexed fields. By using multiple indexes to resolve the query, the number of base-table pages that need to be read from disk is minimized. For more information, see the following section.