Function-Based Indexes (199)

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.

函式索引
1. 可以在某列後者多列上建立函式或者表示式式樣的索引
2. 用於建立索引的函式不能包含任何聚合函式，(DETERMINISTIC??)
3. 使用者不能在資料型別為 LOB，REF，或巢狀表的列上 ,或包含這些資料型別的物件型別列上建立函式索引。

[@more@]