selectivity clause的意思|示意

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。

To that end, you can append the selectivity keyword right after the predicate in the WHERE clause, followed by an estimated or guessed selectivity value in the range of 0 and 1.

为此，可以在where子句中将关键字SELECTIVITY放在谓词的后面，后面带一个0到1之间的对选择性的估计值。

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。