SELECT 
  state_id 
FROM 
  cscart_states 
WHERE 
  code = 'STA' 
  AND country_code = 'RU'

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_states",
      "access_type": "const",
      "possible_keys": ["cs", "code", "country_code"],
      "key": "cs",
      "key_length": "106",
      "used_key_parts": ["country_code", "code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}