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SELECT 
  storefronts.* 
FROM 
  cscart_storefronts AS storefronts 
  LEFT JOIN cscart_storefronts_countries AS countries ON storefronts.storefront_id = countries.storefront_id 
WHERE 
  1 = 1 
  AND (
    countries.country_code IN ('DE')
  ) 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}