SELECT 
  category_id 
FROM 
  cscart_csc_virtual_categories_conditions 
WHERE 
  is_virtual = 'Y'

Query time 0.00022

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "table": {
      "table_name": "cscart_csc_virtual_categories_conditions",
      "access_type": "ALL",
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "0.25",
        "eval_cost": "0.10",
        "prefix_cost": "0.35",
        "data_read_per_join": "24"
      },
      "used_columns": [
        "category_id",
        "is_virtual"
      ],
      "attached_condition": "(`nuie_scalesta_net`.`cscart_csc_virtual_categories_conditions`.`is_virtual` = 'Y')"
    }
  }
}