I noticed that "local" is the default manner for setting session variable.

While global variable starts with @@, we don't support any of them now:

Statement::SetVariables(set_var) => {
let var_name = set_var.variable.to_string().to_uppercase();
match var_name.as_str() {
// known vars
    _ => {
        return NotSupportedSnafu {
            feat: format!("Unsupported set variable {}", var_name),
        }
        .fail()
    }
}}

https://dev.mysql.com/doc/refman/8.0/en/set-variable.html

We don't have a plan to set global variables. This PR is trying to fix that set local {var} and set session {var} are not working, they are the same as set {var}.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.03%. Comparing base (4aa756c) to head (43e173d).

@@            Coverage Diff             @@
##             main    #4064      +/-   ##
==========================================
- Coverage   85.33%   85.03%   -0.31%     
==========================================
  Files         987      987              
  Lines      171204   171224      +20     
==========================================
- Hits       146094   145597     -497     
- Misses      25110    25627     +517