HIGH and LOW Options

This option directs the optimizer to use a sophisticated cost-based algorithm that examines all reasonable query-plan choices and selects the best overall alternative.

For large joins, this algorithm can incur more overhead than you desire. In extreme cases, you can run out of memory.

This option directs the optimizer to use a less sophisticated but faster to design optimization algorithm, based on the lowest-cost path at each stage. This algorithm eliminates unlikely join strategies during the early stages of optimization and reduces the time and resources spent during optimization.

When you specify the LOW level of optimization, the database server might not select the optimal strategy because that strategy was eliminated from consideration during the early stages of the algorithm.