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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 (50, 51, 49, 48) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.61"
    },
    "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": 4,
      "rows_produced_per_join": 4,
      "filtered": "100.00",
      "index_condition": "((`danishecarter_31jan`.`cscart_product_descriptions`.`product_id` in (50,51,49,48)) and (`danishecarter_31jan`.`cscart_product_descriptions`.`lang_code` = 'en'))",
      "cost_info": {
        "read_cost": "8.81",
        "eval_cost": "0.80",
        "prefix_cost": "9.61",
        "data_read_per_join": "18K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description",
        "unit_name"
      ]
    }
  }
}

Result

product_id	short_description	full_description	unit_name
48		<p>This is marine audio at its best. This 8" two-way speaker is built specifically for marine use, with heavy-duty construction that strenuously resists water, corrosion, heat—and boring music. Pioneer research showed that more bass is required for boating applications, and this 8" size delivers.</p>
49		<p>Whether you're out on the water or at the dock, this 6-1/2” two-way speaker adds a new dimension to your boat. It's built specifically for marine use, with heavy-duty construction inside and out that strenuously resists water, corrosion, heat, and sunlight.</p>
50		<p>Absolute fidelity to musical sources takes form from speakers that reproduce the ambience in which sounds originate. Stage size, musicians’ movement, aural reflection and other distinctive details all bring these sounds to life. Pioneer D-Series speakers are available in 2-way component packages or 2-way coaxial designs in multiple sizes that fit most vehicles.</p>
51		<p>Absolute fidelity to musical sources takes form from speakers that reproduce the ambience in which sounds originate. Stage size, musicians’ movement, aural reflection and other distinctive details all bring these sounds to life. Pioneer D-Series speakers are available in 2-way component packages or 2-way coaxial designs in multiple sizes that fit most vehicles.</p>