I have three questions on rebuilding oracle table indexes.

1.What affects the table index tree, is it updates/inserts/deletes ?

2.I have a index defined over three columns in a table, and the table gets updated very frequently but those columns indexed are not updated.
Would those updates also affect the index tree..Do i need to rebuild the indexes in sucn cases too...

3.When does rebuilding index tree take more time, is it when the table has more number of records or the table had involved in more number of transactions ?

The core thing i would want to know is when to rebuild indexes and what indexes of a table need to be rebuilt.