SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20477, 20478, 20479, 20480, 20481, 20482, 
    20483, 20484, 20485, 20486, 20487, 
    20488, 20489, 20490, 20491, 20492
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (20477,20478,20479,20480,20481,20482,20483,20484,20485,20486,20487,20488,20489,20490,20491,20492) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}