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SELECT 
  storefronts.* 
FROM 
  netting_storefronts AS storefronts 
  LEFT JOIN netting_storefronts_promotions AS promotions ON storefronts.storefront_id = promotions.storefront_id 
WHERE 
  1 = 1 
  AND (
    promotions.promotion_id IN ('15')
  ) 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}