You can create indexes on functions and expressions that involve one or more columns
in the table being indexed. A function-based index computes the value of the function
or expression and stores it in the index. You can create a function-based index as either
a B-tree or a bitmap index.
The function used for building the index can be an arithmetic expression or an
expression that contains a PL/SQL function, package function, C callout, or SQL
function. The expression cannot contain any aggregate functions, and it must be
DETERMINISTIC. For building an index on a column containing an object type, the
function can be a method of that object, such as a map method. However, you cannot
build a function-based index on a LOB column, REF, or nested table column, nor can
you build a function-based index if the object type contains a LOB, REF, or nested
table.

函数索引
1. 可以在某列后者多列上创建函数或者表达式式样的索引
2. 用于创建索引的函数不能包含任何聚合函数，(DETERMINISTIC??)
3. 用户不能在数据类型为 LOB，REF，或嵌套表的列上 ,或包含这些数据类型的对象类型列上建立函数索引。