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 (
    20781, 20782, 20783, 20784, 20785, 20786, 
    20787, 20788, 20789, 20790, 20791, 
    20792, 20793, 20794, 20795, 20796
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00186

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 (20781,20782,20783,20784,20785,20786,20787,20788,20789,20790,20791,20792,20793,20794,20795,20796) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}

Result

product_id short_description full_description unit_name
20781 End piece - air duct, right<p>Part number 86546T1100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20782 Wiring Harness - Front Bumper<p>Part number 91890T1081</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20783 Cover - Rear Bumper Lower<p>Part number 86612T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20784 Bracket - rear bumper side lower, right<p>Part number 86614T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20785 Bracket - rear bumper side lower, left<p>Part number 86613T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20786 Piece - rear bumper side, left<p>Part number 86697T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20787 Piece - rear bumper side, right<p>Part number 86698T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20788 Bracket-rear beam upper mounting<p>Part number 86633T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20789 Bracket-rear bump, upper<p>Part number 86632T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20790 Bracket-rear beam lower mounting, left<p>Part number 86637T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20791 Bracket - rear beam lower mounting, right<p>Part number 86638T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20792 Bracket-Rear Beam Lower Mounting<p>Part number 86636T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20793 Trim assembly - tail, left<p>Part number 866C1T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20794 Trim assembly - tail, right<p>Part number 866C2T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20795 SKID PLATE-RR BUMPER<p>Part number 86671T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20796 Wiring Harness-Rear Bumper<p>Part number 91880T1111</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>