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

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.50"
    },
    "table": {
      "table_name": "storefronts",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "storefront_id"
      ],
      "key_length": "4",
      "rows_examined_per_scan": 5,
      "rows_produced_per_join": 5,
      "filtered": "100.00",
      "using_index_for_group_by": "scanning",
      "cost_info": {
        "read_cost": "0.00",
        "eval_cost": "0.50",
        "prefix_cost": "0.50",
        "data_read_per_join": "11K"
      },
      "used_columns": [
        "storefront_id"
      ]
    }
  }
}