SELECT 
  cscart_users.* 
FROM 
  cscart_users 
WHERE 
  user_id = 6

Query time 0.00005

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_users",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["user_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}