SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    2918, 2923, 2927, 2932, 2937, 2939, 2943, 
    2946, 2950, 2955, 2959, 2962, 2965, 
    2974, 2979, 2986, 2991, 2997, 3000, 
    3003, 3009
  )

Query time 0.00055

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.42"
    },
    "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": 21,
      "rows_produced_per_join": 21,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "5.22",
        "eval_cost": "4.20",
        "prefix_cost": "9.42",
        "data_read_per_join": "124K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (2918,2923,2927,2932,2937,2939,2943,2946,2950,2955,2959,2962,2965,2974,2979,2986,2991,2997,3000,3003,3009))"
    }
  }
}