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 (
    231, 232, 233, 234, 235, 236, 237, 238, 
    239, 78, 79, 80, 81, 82, 83, 84, 85, 86, 
    87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 
    97, 100, 101, 102
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00109

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": 32,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (231,232,233,234,235,236,237,238,239,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,100,101,102) and cscart_product_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}