SELECT 
  category_id 
FROM 
  cscart_categories 
WHERE 
  id_path REGEXP '(^|/)300(/|$)'

Query time 0.00050

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "15.57"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "ALL",
      "rows_examined_per_scan": 141,
      "rows_produced_per_join": 141,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "1.47",
        "eval_cost": "14.10",
        "prefix_cost": "15.57",
        "data_read_per_join": "572K"
      },
      "used_columns": [
        "category_id",
        "id_path"
      ],
      "attached_condition": "regexp_like(`CS_Cart_Dev`.`cscart_categories`.`id_path`,'(^|/)300(/|$)')"
    }
  }
}