SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    5921, 5995, 5927, 6052, 5895, 6033, 6005, 
    5901, 6060, 6076, 5903, 6024, 6092, 
    5915, 6100, 6128, 6121, 6137, 6151, 
    6169
  )

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "12.63"
    },
    "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": 29,
      "rows_produced_per_join": 29,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "6.83",
        "eval_cost": "5.80",
        "prefix_cost": "12.63",
        "data_read_per_join": "171K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (5921,5995,5927,6052,5895,6033,6005,5901,6060,6076,5903,6024,6092,5915,6100,6128,6121,6137,6151,6169))"
    }
  }
}