SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19821, 19822, 19823, 19824, 19825, 19826, 
    19827, 19828, 19829, 19830, 19831, 
    19832, 19833, 19834, 19835, 19836
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00996

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 (19821,19822,19823,19824,19825,19826,19827,19828,19829,19830,19831,19832,19833,19834,19835,19836) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}