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The Oracle rule-based optimizer
Oracle Tuning: The Definitive ReferenceOracle Tuning: The Definitive Reference

Oracle ranking scheme for SQL

This list describes the amount of time that is required by Oracle to perform a query. The fastest row access by ROWID happens very quickly since Oracle knows the exact data block where the row resides. The slowest, a full-table scan, could take hours to execute, depending upon the number of rows in the table.
The following list reflects the fastest to slowest time required to perform a query:
  1. Single row access by ROWID
  2. Single row by cluster join
  3. Single row by hash cluster with unique or primary key
  4. Single row by unique or primary key
  5. Cluster join
  6. Hash cluster key
  7. Indexed cluster key
  8. Using a multi-column (concatenated) index
  9. Using a single column index
  10. A bounded index range scan
  11. A full-table scan