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 (
    1371, 1493, 340, 293, 379, 1159, 311, 
    406, 304, 1058, 1188, 1293, 1116, 1368, 
    1020, 1136, 2500, 2513, 1135, 1252, 
    286, 118, 932, 1138, 1144, 1160, 1200, 
    354, 989, 2503, 1247, 1145
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00120

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 (1371,1493,340,293,379,1159,311,406,304,1058,1188,1293,1116,1368,1020,1136,2500,2513,1135,1252,286,118,932,1138,1144,1160,1200,354,989,2503,1247,1145) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description
118
286
293 <p>Aberrus Heroic Description</p>
304 <p>test</p>
311 <p>test quest</p>
340
354
379
406
932
989
1020
1058
1116
1135
1136
1138
1144
1145
1159
1160
1188
1200
1247 <p>Test</p>
1252 <p>Test PVP</p>
1293 <p>Test Builds</p>
1368
1371
1493
2500 <p>test other donation petstest other donation petstest other donation pets</p>
2503 <p>test-test</p>
2513