SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20077, 20078, 20079, 20080, 20081, 20082, 
    20083, 20084, 20085, 20086, 20087, 
    20088, 20089, 20090, 20091, 20092
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00038

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 (20077,20078,20079,20080,20081,20082,20083,20084,20085,20086,20087,20088,20089,20090,20091,20092) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}