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 (
    827, 840, 842, 889, 1085, 1100, 1106, 
    1150, 1201, 1212, 1216, 1226, 1242, 
    1253, 1309, 1434, 1436, 1462, 1514, 
    1570, 1618, 1851, 1969, 1979, 2036, 
    2046, 2047, 2063, 2165, 2185, 2315, 
    2345
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00107

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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": 32,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (827,840,842,889,1085,1100,1106,1150,1201,1212,1216,1226,1242,1253,1309,1434,1436,1462,1514,1570,1618,1851,1969,1979,2036,2046,2047,2063,2165,2185,2315,2345) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description
827
840
842
889
1085
1100
1106
1150
1201
1212
1216
1226
1242 <p>East test D</p>
1253 <p>Test REP</p>
1309
1434
1436
1462
1514
1570
1618
1851
1969
1979
2036
2046
2047
2063
2165
2185
2315
2345