About Model Combine Optimization?

I know the Unity3d ask to combine the models which use same material for resource optimization.
But I have some confusion in some case. For example, I made a prefab which is pillar model, I arrange it in the many places in the level. Should I combine all pillars into one model in place of the original prefab? If done, prefab won’t be existed.

This is kind of complex. If you’re using Unity Pro, then it’ll manage this stuff automatically if you turn all the batching options on. However, if you don’t have access to that, there are other things you can do, like collecting all the objects at the start of the game and automatically batching them into large meshes, but you have to be careful with them. For example, while it’s certainly faster to render 1 mesh of 100 pillars than to render 100 meshes of one pillar each, if you are only looking at one of those pillars at a time, chances are that there’s no point wasting your time rendering the whole 100 just to see the one that happens to be in your viewport.

Don’t worry about the prefab disappearing, btw- any solution to this I suggest would be implemented at runtime, just before the level loads- so your prefabs can be moved around independently in the scene editor, but as soon as you start the game they all get glued together until you press the stop button again.