SELECT 
  COUNT(
    DISTINCT storefronts.storefront_id
  ) 
FROM 
  cscart_storefronts AS storefronts 
WHERE 
  1 = 1

Query time 0.00011

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0111808,
    "nested_loop": [
      {
        "table": {
          "table_name": "storefronts",
          "access_type": "ALL",
          "loops": 1,
          "rows": 2,
          "cost": 0.0111808,
          "filtered": 100
        }
      }
    ]
  }
}