SELECT 
  category_id 
FROM 
  cscart_categories 
WHERE 
  category_type = 'D'

Query time 0.00709

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "368.89"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "ALL",
      "rows_examined_per_scan": 3444,
      "rows_produced_per_join": 344,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "334.45",
        "eval_cost": "34.44",
        "prefix_cost": "368.89",
        "data_read_per_join": "1M"
      },
      "used_columns": [
        "category_id",
        "category_type"
      ],
      "attached_condition": "(`s2cart`.`cscart_categories`.`category_type` = 'D')"
    }
  }
}