Each step in a path expression can also contain a predicate that filters the elements that are returned by that step, retaining only elements that satisfy some condition.
路径表达式中的每一步还可以包含一个谓词，用于过滤该步返回的元素，只保留满足某种条件的元素。
Before access path selection, the DB2 optimizer transforms SQL statements into forms that are semantically identical, such as by using predicate pushdown or predicate transitive closure.
在选择访问路径之前，DB 2优化器将sql语句转换成语意上一致的形式，例如使用谓词下推或谓词传递闭包。
Missing statistics can cause the optimizer to overestimate or to underestimate the selectivity of a predicate, and eventually to select an inefficient access path.
丢失的统计数据会让优化器高估或低估谓词的选择性，最终会选择低效的访问路径。
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