SELECT 
  COUNT(DISTINCT video_id) as video_count 
FROM 
  cscart_cp_videos 
WHERE 
  user_id = 21551 
  and status = 'A'

Query time 0.00075

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "49.40"
    },
    "table": {
      "table_name": "cscart_cp_videos",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "status_idx",
        "user_id_idx",
        "video_id_idx",
        "idx_cp_videos_status_userid_videoid",
        "idx_status_timestamp"
      ],
      "key": "idx_cp_videos_status_userid_videoid",
      "used_key_parts": [
        "status",
        "user_id"
      ],
      "key_length": "7",
      "ref": [
        "const",
        "const"
      ],
      "rows_examined_per_scan": 457,
      "rows_produced_per_join": 457,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "3.70",
        "eval_cost": "45.70",
        "prefix_cost": "49.40",
        "data_read_per_join": "714K"
      },
      "used_columns": [
        "video_id",
        "status",
        "user_id"
      ]
    }
  }
}