SELECT 
  cscart_products.* 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    390989, 390988, 390999, 390998, 278, 
    390982, 390983, 400917, 390995, 400914, 
    391022, 312422
  )

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0216566,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_products",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "loops": 1,
          "rows": 12,
          "cost": 0.0216566,
          "filtered": 100,
          "attached_condition": "cscart_products.product_id in (390989,390988,390999,390998,278,390982,390983,400917,390995,400914,391022,312422)"
        }
      }
    ]
  }
}