覆盖索引的概念

比如有复合索引为3个字段：f1 + f2 + f3,请问:

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'.

有人说1符合索引覆盖, 有人说只有2才符合, 请问哪个对?

Oracle中的covering index(应该翻译成覆盖索引)是指

[url]http://www.inthirties.com/thread-458-2-1.html[/url]

[quote][color=red] A covering index is an index, which includes all of the columns referenced in the query[/color]. 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.[/quote]

从这个定义上讲，2符合，1不符合。