For the exceptional situations where the optimizer cannot choose an optimal query execution plan, DB2 has added features such as the SELECTIVITY clause and optimization guidelines.

对于优化器不能选择最佳查询执行计划的例外情况，DB 2已经增加了诸如selectivity子句和优化指南之类的特性。

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For comparison, Figure 3 illustrates the access plan where I forced the DB2 optimizer to ignore the index by adding the clause SELECTIVITY 1 to the predicate in the where clause.

为了便于比较，图3显示了通过将SELECTIVITY 1子句添加到where子句的谓词中，强迫db2优化器忽略索引的访问计划。

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