SELECT 
  category_id 
FROM 
  cscart_csc_virtual_categories_conditions 
WHERE 
  is_virtual = 'Y'

Query time 0.00015

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_csc_virtual_categories_conditions",
      "access_type": "ALL",
      "rows": 400,
      "filtered": 100,
      "attached_condition": "cscart_csc_virtual_categories_conditions.is_virtual = 'Y'"
    }
  }
}