Function-based indexes provide an efficient mechanism for evaluating statements that
contain functions in their WHERE clauses. The value of the expression is computed and
stored in the index. When it processes INSERT and UPDATE statements, however,
Oracle must still evaluate the function to process the statement.

Function-based indexes defined on UPPER(column_name) or LOWER(column_name) can facilitate
case-insensitive searches.

A function-based index can also be used for a globalization support sort index that
provides efficient linguistic collation in SQL statements.

使用函数索引
1. 如果一个 SQL 语句的 WHERE 子句中使用了函数，那么建立相应的函数索引是提高数据访问性能
2. 使用 UPPER(column_name) 或 LOWER(column_name) 函数建立函数索引需要注意大小敏感性