The code changes about built-in functions in TiKV can be minimized.

After the investigation about several built-in functions, I found some common patterns:

1. Create a new encoding.
2. Convert the arguments into new charset from utf8.
3. Apply these changes to the vectorized-version evaluation.

We can extract these patterns into another Sig named builtinToBinarySig(to_binary()) and wrap the cast function as follows:

The rewrite process before this proposal:

select some_builtin(arg1, arg2...)

is rewritten to

select some_builtin(cast_as_string(arg1), cast_as_string(arg2)...)

The rewrite process after this proposal:

select some_builtin(arg1, arg2...)

is rewritten to

select some_builtin(to_binary(cast_as_string(arg1)), to_binary(cast_as_string(arg2))...)

Then we can maintain a built-in function list to determine which function needs to be wrapped, instead of putting the boilerplates everywhere.

var toBinaryMap = map[string]struct{}{
	ast.Hex: {}, ast.Length: {}, ast.OctetLength: {}, ast.ASCII: {},
	ast.ToBase64: {},
}