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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 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    389637, 366904, 366903, 368409, 366900, 
    366968, 424699, 435209, 435210
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00113

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "12.62"
    },
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "product_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "product_id",
        "lang_code"
      ],
      "key_length": "9",
      "rows_examined_per_scan": 9,
      "rows_produced_per_join": 9,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "10.83",
        "eval_cost": "1.80",
        "prefix_cost": "12.63",
        "data_read_per_join": "61K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_descriptions`.`product_id` in (389637,366904,366903,368409,366900,366968,424699,435209,435210)) and (`webmarco`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id	short_description	full_description
366900		<ul><li>Taglia Unica</li><li>Scollatura a Cuore</li><li>Tessuto Leggermente Plissettato</li><li>Spalline Cadenti</li><li>Manica Lunga</li></ul>
366903
366904		<ul><li>Aderente</li><li>Scollatura a Cuore</li><li>Dettagli in Strass sulla Scollatura</li><li>Taglia unica</li></ul>
366968		<ul><li>50% Viscosa,50% Nylon</li><li>Intrecciato sul Davanti</li><li>Taglia Unica</li></ul><p><br /></p>
368409		<ul><li>Taglia unica </li><li>100% Acrilico</li><li>Manica a Camicia</li></ul>
389637		<ul><li>Aderente</li><li>Monospalla</li><li>Scollatura a Cuore</li><li>Manica Lunga con dettaglio in Piuma</li><li>Taglia Unica</li></ul>
424699		<ul><li>Aderente</li><li>Monospalla</li><li>Scollatura a Cuore</li><li>Manica Lunga con dettaglio in Piuma</li><li>Taglia Unica</li></ul>
435209
435210