SELECT 
  product_id 
FROM 
  cscart_product_popularity 
WHERE 
  product_id IN (
    21055, 21054, 21053, 21052, 21051, 21050, 
    21049, 21048, 21047
  )

Query time 0.00082

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_popularity",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "total"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 9,
          "filtered": 100,
          "attached_condition": "cscart_product_popularity.product_id in (21055,21054,21053,21052,21051,21050,21049,21048,21047)",
          "using_index": true
        }
      }
    ]
  }
}