SELECT 
  COUNT(DISTINCT cv.video_id) 
FROM 
  cscart_cp_videos cv 
  LEFT JOIN cscart_cp_video_descriptions cvd ON cvd.video_id = cv.video_id 
  AND cvd.lang_code = 'en' 
WHERE 
  1 
  AND cv.user_id = 21551 
  AND cv.status = 'A'

Query time 0.00104

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "209.35"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cv",
          "access_type": "ref",
          "possible_keys": [
            "status_idx",
            "user_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"
          ]
        }
      },
      {
        "table": {
          "table_name": "cvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "video_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "cscart_db.cv.video_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 457,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "114.25",
            "eval_cost": "45.70",
            "prefix_cost": "209.35",
            "data_read_per_join": "1M"
          },
          "used_columns": [
            "video_id",
            "lang_code"
          ]
        }
      }
    ]
  }
}