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 (
    138, 139, 140, 141, 142, 143, 144, 145, 
    146, 147, 148, 149, 150, 151, 152, 153, 
    154, 155, 156, 157, 160, 161, 163, 164, 
    165, 167, 168, 169, 170, 172, 173, 175
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00249

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 (138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,160,161,163,164,165,167,168,169,170,172,173,175) and cscart_product_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}