Troubleshooting Oracle Performance, Second Edition

CHAPTER 7

System Statistics

The query optimizer used to base its cost estimations on the number of physical reads needed to execute SQL statements. That method is known as the I/O cost model. The main drawback of this method is that single-block reads and multiblock reads are equally costly.¹ Consequently, multiblock read operations, such as full table scans, are artificially favored. Before system statistics were introduced, especially in OLTP systems, the optimizer_index_caching and optimizer_index_cost_adj initialization parameters were used to work around this problem (see Chapter 9 for coverage of both parameters). In fact, the default values ...