SELECT 
  cscart_products.* 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (21780, 2136, 2132, 2133, 2135, 2138)

Query time 0.00059

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_products",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 6,
          "filtered": 100,
          "index_condition": "cscart_products.product_id in (21780,2136,2132,2133,2135,2138)"
        }
      }
    ]
  }
}