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 (
    15842, 15840, 15785, 15841, 16154, 16846, 
    16060, 16066, 16061, 16062, 16063, 
    16064, 16065, 15769, 15496, 15497, 
    15768, 15770
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00080

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "12.61"
    },
    "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": 18,
      "rows_produced_per_join": 18,
      "filtered": "100.00",
      "index_condition": "((`dev_yourmart_co_bw`.`cscart_product_descriptions`.`product_id` in (15842,15840,15785,15841,16154,16846,16060,16066,16061,16062,16063,16064,16065,15769,15496,15497,15768,15770)) and (`dev_yourmart_co_bw`.`cscart_product_descriptions`.`lang_code` = 'en'))",
      "cost_info": {
        "read_cost": "10.81",
        "eval_cost": "1.80",
        "prefix_cost": "12.61",
        "data_read_per_join": "68K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ]
    }
  }
}