Covering Index in Oracle

A friend in csdn asked about the index coverage concept. This concept is very small. I have explained "" In My Forum. But as an oracle moderator, I cannot add an external Address link to the reply, so I will post a copy in csdn.

 

For example, a composite index has three fields: F1 + F2 + F3. Could you tell me:

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 '.

Some people say that 1 is consistent with the index coverage. Some people say that only 2 is consistent. Which one is correct?

 

 

 

Covering Index in Oracle (which should be translated as overwrite index) refers

Http://www.inthirties.com/thread-458-2-1.html

# A covering index is an index, which provided des 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.

 

 

In this definition, 2 and 1 are non-conforming.