SELECT 
  * 
FROM 
  cscart_reward_points 
WHERE 
  usergroup_id = 12

Query time 0.00025

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_reward_points",
      "access_type": "ALL",
      "rows": 6,
      "filtered": 100,
      "attached_condition": "cscart_reward_points.usergroup_id = 12"
    }
  }
}