SELECT 
  category_id 
FROM 
  cscart_categories 
WHERE 
  category_type = 'D'

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0304148,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_categories",
          "access_type": "ALL",
          "loops": 1,
          "rows": 120,
          "cost": 0.0304148,
          "filtered": 100,
          "attached_condition": "cscart_categories.category_type = 'D'"
        }
      }
    ]
  }
}