SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    7641, 7648, 7657, 7716, 7723, 7731, 7737, 
    7744, 7753, 7770, 7775, 7788, 7793, 
    7843, 7851
  )

Query time 0.00051

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.82"
    },
    "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": 17,
      "rows_produced_per_join": 17,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "4.42",
        "eval_cost": "3.40",
        "prefix_cost": "7.82",
        "data_read_per_join": "100K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (7641,7648,7657,7716,7723,7731,7737,7744,7753,7770,7775,7788,7793,7843,7851))"
    }
  }
}