SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21951, 21952, 21953, 21954, 21955, 21956, 
    21957, 21958, 21959, 21960, 21961, 
    21962, 21963, 21964, 21965, 21966
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.01063

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 (21951,21952,21953,21954,21955,21956,21957,21958,21959,21960,21961,21962,21963,21964,21965,21966) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}