create or replace table t1 (col1 char(10) character set ucs2, filler char(100), key(col1)) ;
insert into t1 values ('a', 'a');
insert into t1 values ('a', 'a');
set optimizer_trace=1;
explain format=json select * from t1 force index(col1) where col1 >='a';
EXPLAIN
{
  "query_block": {
    "select_id": 1,
    "cost": "COST_REPLACED",
    "nested_loop": [
      {
        "table": {
          "table_name": "t1",
          "access_type": "range",
          "possible_keys": ["col1"],
          "key": "col1",
          "key_length": "21",
          "used_key_parts": ["col1"],
          "loops": 1,
          "rows": 2,
          "cost": "COST_REPLACED",
          "filtered": 100,
          "index_condition": "t1.col1 >= 'a'"
        }
      }
    ]
  }
}
select JSON_DETAILED(JSON_EXTRACT(trace, '$**.analyzing_range_alternatives')) AS JS from INFORMATION_SCHEMA.OPTIMIZER_TRACE;
JS
[
    {
        "range_scan_alternatives": 
        [
            {
                "index": "col1",
                "ranges": 
                ["(a) <= (col1)"],
                "rowid_ordered": false,
                "using_mrr": false,
                "index_only": false,
                "rows": 2,
                "cost": 0.003808422,
                "chosen": true
            }
        ],
        "analyzing_roworder_intersect": 
        {
            "cause": "too few roworder scans"
        }
    }
]
drop table t1;