dFdx dFdy textureGrad and texture

texture and textureGrad have any diffrence?
because I found use texture and set GL_LINEAR have same effect.
I thinked texture no filter and textureGrad filter…

but since texture filter also,what mean is:
vec2 dPdx = dFdx( index );
vec2 dPdy = dFdy( index );
textureGrad(texID, index, dPdx, dPdy);
?

http://www.opengl.org/wiki/GLSL_Sampler#Gradient_texture_access

thus mean :function texture don’t select mipmap,and function textureGrad do?

textureGrad allow to change the mipmap that would be used by the default algorithm, by tweaking the gradient values which are then used by the card to select the appropriate mipmap level(s).

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