Question

do you think history would have different if India were united when Mahmud of ghazi invaded India? give reason ?

Answer 1

Mauryan empire unites India and thwarts Greek invasion. Demise of Mauryas leads to invasion of central Asian tribes including Indo Greeks.Gupta empire unites India and thwarts Hunas. Demise of Guptas is followed by invasion of Arabs.India is united under the confederation of Gurjara Pratiharas and Rashtrakutas. This leads to defeat of Arab invaders. Demise of these empires is followed by invasion of Turks like Mahmud of Ghazni.India is once again united under Raja Suhaldev Pasi and Raja Bhoja. The united confederation demolishes the army of Ghazni. (This is a lesser known fact and basically the answer to your question. 

So, as you can see, the basic thing is that the Invaders like Mahmud of Ghazni were indeed able to enter the Indian heartlands whenever there was an absence of a central power in India and but once Indians recovered they were able to push back the invaders from where they came from.

So, if Mahmud of Ghazni had invaded an united India he would have met the same fate that his successor Salar Masud met.

Answer 2

if India was united Muhammad of Ghazni will return to his country but only his body will return.