SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6051, 6057, 6066, 6073, 6081, 6095, 6097, 
    6106, 6112, 6120, 6131, 6136, 6146, 
    6157, 6168, 6175, 6184, 6188, 6197, 
    6206, 6213, 6220, 6239, 6244, 6251
  )

Query time 0.00057

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "11.03"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 25,
      "rows_produced_per_join": 25,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "6.03",
        "eval_cost": "5.00",
        "prefix_cost": "11.03",
        "data_read_per_join": "148K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6051,6057,6066,6073,6081,6095,6097,6106,6112,6120,6131,6136,6146,6157,6168,6175,6184,6188,6197,6206,6213,6220,6239,6244,6251))"
    }
  }
}