An index is an ordered subset of the table on which it is created 索引是創(chuàng)建索引所基于的表的有序子集。
To alter the arrangement of a set of items from two or more ordered subsets to one or more other subsets each containing a number of items , commonly one , from each of the original subsets in a specified order that is not necessarily the order of any of the original subsets 將兩個或多個有序子集中項目集合的排列按所指明的順序改變成一個或多個子集,每個子集包含若干個(通常為一個)來自每個原先子集中的項目,但不一定和任何原先子集中的排序一樣。