Why doesn't derivative show the complex maxima minima? 
Let's find the global minimum of $y=x^2$
First we calculate its first derivative, and make it equal to zero. $y'=2x=0$; $x=0$
Then we check its second derivative, if it's positive then it is minimum. $y''=2$
$(0,0)$ turned out to be global minimum.

Why doesn't derivative show the complex minimum? $(i,-1)$ is below it and that derivation should show the minima are  $(∞i,-∞)$ and $(-∞i,-∞)$.
How can I calculate complex minima of other functions?
 A: I see several problems in your understanding.
First, for general complex values $z\in\Bbb C$ there is no meaning in asking for the minimum of $z^2$. For exmaple, is $$i<1\quad \text{or} \quad 1 <i\;?$$ Is $-i<-1$? The complex numbers are "2-dimensional" and you cannot order the plane. There is no minimum because there is no way to decide what is smaller.
The second thing is that you seem to have a wrong understanding of the term minimum itself. Just because $(0,0)$ is a minimum does not mean that there cannot be a value of the function below $0$. There is just no smaller value in some neighborhood of your minimum. If this shows you something (at all), then that $(0,0)$ is no global minimum here. You can ask why the derivative of $x^3$ will only give you $(0,0)$ (which is just a saddle point) but not $(-1,-1)$. That is because there are smaller values close to $-1$. It also will not give you $(-\infty,-\infty)$. So this is not a problem with complex numbers.
A: If a codomain is $\Bbb C$, then there is a nonsense to speak about the reasonable order. That is why we don't consider extrema of complex-valued functions.
