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 (
    20877, 20878, 20879, 20880, 20881, 20882, 
    20883, 20884, 20885, 20886, 20887, 
    20888, 20889, 20890, 20891, 20892
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00090

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (20877,20878,20879,20880,20881,20882,20883,20884,20885,20886,20887,20888,20889,20890,20891,20892) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
20877 PCB block assy<p>Part number 91959T1100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20878 Wiring Assembly-Control<p>Part number 91410T1191</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20879 Bracket-wiring mounting<p>Part number 91931T1280</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20880 Bracket-wiring mounting<p>Part number 91931T1040</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20881 Bracket-wiring mounting<p>Part number 91931T1120</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20882 Bracket-wiring mounting<p>Part number 91931T1130</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20883 Low profile 50A fuse<p>Part number 1879001125</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20884 Fuse - Micro 10A<p>Part number 1879004948</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20885 Fuse - Micro 15A<p>Part number 1879004949</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20886 Fuse - Micro 20A<p>Part number 1879004950</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20887 Fuse - Micro 25A<p>Part number 1879004951</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20888 Fuse - Micro 30A<p>Part number 1879004952</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20889 Bracket-Connect Junction Box Side<p>Part number 919G5T1300</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20890 Wiring Assembly - Follower<p>Part number 91541T1890</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20891 Grommet-front door<p>Part number 919812H050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20892 Wiring assembly - rear door left<p>Part number 91620T1020</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>