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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 (
    85974, 85975, 85977, 85978, 85979, 85980, 
    85981, 85982, 85983, 85984, 85985, 
    85986, 85987, 85988, 85989, 85990, 
    85991, 85992, 85993, 85994, 85995, 
    85996, 85997, 85998
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00065

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "10.82"
    },
    "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": 24,
      "rows_produced_per_join": 24,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "8.42",
        "eval_cost": "2.40",
        "prefix_cost": "10.82",
        "data_read_per_join": "91K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`nuie_scalesta_net`.`cscart_product_descriptions`.`product_id` in (85974,85975,85977,85978,85979,85980,85981,85982,85983,85984,85985,85986,85987,85988,85989,85990,85991,85992,85993,85994,85995,85996,85997,85998)) and (`nuie_scalesta_net`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id	short_description	full_description
85974	nuie Handles Bar Handle 96mm Centres - Chrome
85975	nuie Handles D Handle 96mm Centres - Chrome
85977	Hudson Reed Handles D Handle 128mm Centres - Chrome
85978	nuie Handles D Handle 320mm Centres - Chrome
85979	nuie Handles Handle 420mm Centres - Chrome
85980	Hudson Reed Profile Handle 352mm Centres - Chrome Black
85981	nuie Handles Seven Shaped Handle 480mm Centres - Chrome
85982	nuie Handles Knurled Bar Handle 96mm Centres - Chrome
85983	nuie Handles Knurled Bar Handle 160mm Centres - Chrome
85984	nuie Handles Knurled Bar Handle 192mm Centres - Chrome
85985	nuie Handles D Handle 192mm Centres - Chrome
85986	nuie Handles D Handle 192mm Centres - Brushed Brass
85987	nuie Handles D Handle 192mm Centres - Matt Black
85988	nuie Handles D Handle 192mm Centres - Copper
85989	nuie Handles Knurled Bar Handle 96mm Centres - Matt Black
85990	nuie Handles Knurled Bar Handle 160mm Centres - Matt Black
85991	nuie Handles Knurled Bar Handle 192mm Centres - Matt Black
85992	nuie Handles D Handle 160mm Centres - Chrome
85993	nuie Handles D Handle 160mm Centres - Brushed Brass
85994	nuie Handles D Handle 160mm Centres - Matt Black
85995	nuie Handles D Handle 160mm Centres - Copper
85996	nuie Handles Knurled Bar Handle 96mm Centres - Brushed Brass
85997	nuie Handles Knurled Bar Handle 160mm Centres - Brushed Brass
85998	nuie Handles Knurled Bar Handle 192mm Centres - Brushed Brass