How to separate key bindings of TAB and C-i?

[gniuxiao]

I want to insert a tab when type TAB and indent current line when type
C-i, so I wrote the following codes:

(defun my-insert-tab()
 (interactive)
 (insert " "))

(global-set-key (kbd "TAB") 'my-insert-tab)
(global-set-key (kbd "C-i") 'lisp-indent-line)

but both TAB and C-i invoke lisp-indent-line, then I changed the
setting sequence to:

(global-set-key (kbd "C-i") 'lisp-indent-line)
(global-set-key (kbd "TAB") 'my-insert-tab)

now both TAB and C-i invoke my-insert-tab.

I think TAB and C-i are internally stick to each other, and I wanna
know how to separate them, thanks ;-)