SELECT 
  category_id 
FROM 
  cscart_categories 
WHERE 
  category_type = 'D'

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "ALL",
      "rows": 54,
      "filtered": 100,
      "attached_condition": "cscart_categories.category_type = 'D'"
    }
  }
}