SELECT 
  COUNT(*) 
FROM 
  cscart_ec_seats 
WHERE 
  layout_id = 1 
  AND category_id = 1

Query time 0.00060

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "5.03"
    },
    "table": {
      "table_name": "cscart_ec_seats",
      "access_type": "ref",
      "possible_keys": [
        "idx_layout_category"
      ],
      "key": "idx_layout_category",
      "used_key_parts": [
        "layout_id",
        "category_id"
      ],
      "key_length": "8",
      "ref": [
        "const",
        "const"
      ],
      "rows_examined_per_scan": 20,
      "rows_produced_per_join": 20,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "1.03",
        "eval_cost": "4.00",
        "prefix_cost": "5.03",
        "data_read_per_join": "4K"
      },
      "used_columns": [
        "layout_id",
        "category_id"
      ]
    }
  }
}