SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6852, 6739, 6651, 6939, 6959, 6670, 6757, 
    6680, 6772, 6697, 6705, 6794, 6829, 
    6895, 6921, 6976, 6994
  )

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.02"
    },
    "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": 20,
      "rows_produced_per_join": 20,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "5.02",
        "eval_cost": "4.00",
        "prefix_cost": "9.02",
        "data_read_per_join": "118K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6852,6739,6651,6939,6959,6670,6757,6680,6772,6697,6705,6794,6829,6895,6921,6976,6994))"
    }
  }
}