SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6996, 7008, 7028, 7051, 7073, 7100, 7114, 
    7124, 7136, 7149, 7280, 7267, 7298, 
    7316, 7333
  )

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.42"
    },
    "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": 16,
      "rows_produced_per_join": 16,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "4.22",
        "eval_cost": "3.20",
        "prefix_cost": "7.42",
        "data_read_per_join": "94K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6996,7008,7028,7051,7073,7100,7114,7124,7136,7149,7280,7267,7298,7316,7333))"
    }
  }
}