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。
The affirmative-negative questions are formed by juxtaposing the affirmative and negative forms of the main element of the predicate.
正反疑问句是并列谓语的肯定形式与否定形式来表示疑问。
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