I have created a table A and added index. but now I want to modify column for index, so I select the table, click "Indexes" tab , select a index you want to change, click "alert index", the column you want to add in "Table Columns" area can not navigate to "Index columns" area, the navigation arrow is disable. why?
Oracle does not allow you to add/remove columns to an index.
If you want to do that, you have to drop and recreate the index.
thank you for your response, but the oracle tool "navicat" can do it, I don't understand why Toad cannot 
Navicat must be dropping and recreating the index, despite what the GUI makes you believe.
Toad won't fool you like that.
If you want to do this in Toad, it's still easy (not worth the crying emoji!). Find the index in schema browser. Go to the "Script" tab on the right. then you can send the script to the editor and modify it to drop and then recreate the index as desired, then run the script. Easy.
Yes, I can get the result by your way "Scrip", but i think there is a little trouble than GUI，but i make sure that Navicat can do it by GUI
I am not doubting that Navicat can do this with a GUI. What I am saying is there is no Oracle command to modify the columns of an index. So after you make changes in Navicat's GUI, Navicat must drop and then recreate your index.
Toad's "Alter Index" window lets you alter an index, but only in the ways that Oracle allows. Toad will not drop and recreate the index from the "Alter Index" window, because that would be misleading.
I see, thank you for your explanation 