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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 (729) 
  AND cscart_product_descriptions.lang_code = 'fr'

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "const",
      "possible_keys": ["PRIMARY", "product_id"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}

Result

product_id	short_description	full_description	unit_name
729	<div style="border: 2px solid #d32f2f; border-radius: 12px; padding: 16px; font-size: 16px; line-height: 1.6; font-family: sans-serif;"> <strong style="color: #d32f2f;">En sélectionnant cette prothèse,</strong> vous optez pour la fabrication d'une gouttière de contention, de blanchiment ou de bruxisme et bien plus encore... </div>