SELECT 
  storefront_id 
FROM 
  cscart_hybrid_auth_storefronts_providers 
WHERE 
  provider_id = 8

Query time 0.00021

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.40"
    },
    "table": {
      "table_name": "cscart_hybrid_auth_storefronts_providers",
      "access_type": "index",
      "key": "PRIMARY",
      "used_key_parts": [
        "storefront_id",
        "provider_id"
      ],
      "key_length": "8",
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 1,
      "filtered": "50.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "1.20",
        "eval_cost": "0.20",
        "prefix_cost": "1.40",
        "data_read_per_join": "16"
      },
      "used_columns": [
        "storefront_id",
        "provider_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_hybrid_auth_storefronts_providers`.`provider_id` = 8)"
    }
  }
}