CSDN里的一個朋友問到了這個索引覆蓋的概念。 這個概念很小的知識點，在我的論壇里有解釋“”，不過作為Oracle版主，不能在回帖里加上網外的地址鏈接，所以這里在CSDN里帖上一份
比如有復合索引為3個字段：f1 + f2 + f3,請問:
1: select f1, f2, f3, f4 from table where f1 = 'XX' and f2 = 'XX'.
2: select f1, f2, f3 from table where f1 = 'XX' and f2 = 'XX'.
有人說1符合索引覆蓋, 有人說只有2才符合, 請問哪個對?
Oracle中的covering index(應該翻譯成覆蓋索引)是指
http://www.inthirties.com/thread-458-2-1.html
# A covering index is an index, which includes all of the columns referenced in the query. So the creating covering index can imPRove performance because all the data for the query is contained within the index itself and only the index pages, not the data pages, will be used to retrieve the data. Covering indexes can bring a lot of performance to a query, because it can save a huge amount of I/O Operations.
從這個定義上講，2符合，1不符合
