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 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    1278, 803, 788, 992, 796, 787, 798, 989, 
    987, 35691, 35710, 988, 793, 790, 35690, 
    794, 985, 804, 991, 786, 801, 35617, 
    35709, 35705, 35703, 795, 984, 802, 
    35708, 35687
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00742

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": "11",
          "used_key_parts": ["product_id", "lang_code"],
          "rows": 30,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (1278,803,788,992,796,787,798,989,987,35691,35710,988,793,790,35690,794,985,804,991,786,801,35617,35709,35705,35703,795,984,802,35708,35687) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}