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(yīng)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.
