SELECT 
  COUNT(*) 
FROM 
  tmall_ec_livecommerce_live_users_count 
WHERE 
  video_id = 1

Query time 0.00749

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "tmall_ec_livecommerce_live_users_count",
          "access_type": "system",
          "possible_keys": ["idx_video_timestamp"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}