If we FFT a signal with number of points far less than the length of the signal, the result is unreliable?
As per the Matlab help, y = fft(x, 1024) where length(x) &gt; 1024 will result in just taking the FFT of x(1:1024).



So if talking the FFT of just the first 1024 samples of your 1,000,000 point signal is OK, then sure, what you say is correct.