SELECT 
  COUNT(*) 
FROM 
  cscart_storefronts AS storefronts 
  LEFT JOIN cscart_storefronts_currencies AS currencies ON storefronts.storefront_id = currencies.storefront_id 
WHERE 
  1 = 1 
  AND (
    currencies.currency_id IN ('3')
  )

Query time 0.00082

JSON explain

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

Result

COUNT(*)
0