Performance improvement

[ngaut]

There are some redundant computing in charset.go(https://github.com/pingcap/parser/blob/master/charset/charset.go), some of the functions may just return constants. There is no need to do compute every time if we save a map/slice and initialize them. Such as:

GetAllCharsets, GetCharsetInfoByID.

We may also change the type of argument from string to integer in GetCharsetDesc.
With better redesign, we can save more memory and CPU.

[hawkingrei]

I will try to improve GetAllCharsets. it is so interesting.

[winkyao]

@hawkingrei Thanks, some tips here:

1. GetAllCharsets can be inited at the init function, and GetAllCharsets can return the inited array directly.
2. GetCharsetInfoByID can use a map to fetch Collation effectively, like var collationsMap = make(map[int]*Collation), and init the map in the init function too.

[lkk2003rty]

I'll try to improve GetCharsetInfoByID

[winkyao]

Actually, you should modify the code in https://github.com/pingcap/parser/blob/master/charset/charset.go, and update the TiDB go.mod.

[winkyao]

@lkk2003rty Are you interested in this issue #9638?

[lkk2003rty]

@lkk2003rty Are you interested in this issue #9638?

yep, I can do it