SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20765, 20766, 20767, 20768, 20769, 20770, 
    20771, 20772, 20773, 20774, 20775, 
    20776, 20777, 20778, 20779, 20780
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00923

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 (20765,20766,20767,20768,20769,20770,20771,20772,20773,20774,20775,20776,20777,20778,20779,20780) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}