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 (
    21245, 21246, 21247, 21248, 21249, 21250, 
    21251, 21252, 21253, 21254, 21255, 
    21256, 21257, 21258, 21259, 21260
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00141

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 (21245,21246,21247,21248,21249,21250,21251,21252,21253,21254,21255,21256,21257,21258,21259,21260) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}

Result

product_id short_description full_description unit_name
21245 Wiring Assembly-Control<p>Part number 91400T6041</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21246 Bracket-wiring mounting<p>Part number 91931T1310</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21247 Bracket-wiring mounting<p>Part number 91931T6040</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21248 Protector-wiring<p>Part number 91961T1330</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21249 Protector-wiring<p>Part number 91961T1340</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21250 Protector-wiring<p>Part number 91961T1460</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21251 Protector-wiring<p>Part number 91961T6160</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21252 Protector-wiring<p>Part number 91961T6170</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21253 Protector-wiring<p>Part number 91961T6330</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21254 BOLT<p>Part number 1128106163</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21255 Wiring Assembly - Follower<p>Part number 91508T6092</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21256 Bracket-relay box mounting<p>Part number 919G5T6510</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21257 Bracket-relay box mounting<p>Part number 919G5T6520</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21258 Junction Block-Sub<p>Part number 91957T6500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21259 Protector-wiring<p>Part number 91961T6720</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21260 Protector-wiring<p>Part number 91961T6100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>