通过减少执行查询时需要检索的元组（tuple）的数量，以及减少查询所需的计算，函数索引能够显著改善性能。
They improve performance by reducing the number of tuples fetched during query execution.
如果JIT编译会延迟启动又不能显著改善应用程序的长期性能，则执行这种编译就非常浪费。
Performing JIT compilations that delay start-up yet do not significantly improve the application's long-term performance are particularly wasteful.
DB 2 Optimization Expert识别执行情况较差的SQL并提供一组全面的专家建议程序，帮助改善性能并降低总体所有权成本。
DB2 Optimization expert identifies poorly performing SQL and offers a comprehensive set of expert advisors that can help improve system performance and lower total cost of ownership.
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