SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.short_description, 
  IF(
    cscart_product_descriptions.short_description = '' 
    OR cscart_product_descriptions.short_description IS NULL, 
    cscart_product_descriptions.full_description, 
    ''
  ) AS full_description, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    121, 286, 103, 89, 110, 82, 100, 194, 202, 
    203, 111, 104, 123, 118, 119, 112, 114, 
    116
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00093

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_descriptions",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "product_id"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["product_id", "lang_code"],
          "rows": 18,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (121,286,103,89,110,82,100,194,202,203,111,104,123,118,119,112,114,116) and cscart_product_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}