_G = GLOBAL

[Maris]

I always use

_G = GLOBAL

Because GLOBAL._G == GLOBAL
i.e. _G == _G._G
So _G always defined and works fine irrespective of, whether the GLOBAL variable is defined.

My question is about "local" statement.
Is there difference for optimization between

_G = GLOBAL

and

local _G = GLOBAL

I noticed that I can get variables of local "global space" of other mods by using

local env = _G.ModManager:GetMod("modname").env

But there are only "globals", not "locals". E.g. I can use env.x if and only if there is global definition (x=5), and I can't use env.x if there is only local definition (local x=5).
So there is some difference. But is there difference for optimization?

I read some lua optimization tips. They said that local variables are very fast. But I'm not sure about variable in local global space (environment of the mod). It seems that they are in the same scope. Only difference that locals are invisible for accessing from outside. That's it. So is there any real difference for optimization? Does anybody know?

[ZupaleX]

It is not really clear what you are asking. I do not see the link between asking if you should write 

_G = GLOBAL

or

local _G = GLOBAL

and the impossibility to retrieve local variables from other mods (well, they are... local).