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SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.short_description, 
  IF(
    cscart_product_descriptions.short_description = '', 
    cscart_product_descriptions.full_description, 
    ''
  ) AS full_description, 
  cscart_product_descriptions.meta_keywords, 
  cscart_product_descriptions.meta_description, 
  cscart_product_descriptions.search_words, 
  cscart_product_descriptions.promo_text 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (7, 4, 3, 6) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "ALL",
      "possible_keys": [
        "PRIMARY",
        "product_id"
      ],
      "rows": 14,
      "filtered": 28.571,
      "attached_condition": "((`_testcscart`.`cscart_product_descriptions`.`product_id` in (7,4,3,6)) and (`_testcscart`.`cscart_product_descriptions`.`lang_code` = 'ja'))"
    }
  }
}

Result

product_id	short_description	full_description	meta_keywords	meta_description	search_words	promo_text
3
4
6
7