# Study on Effective Temporal Data Retrieval Leveraging Complex Indexed Architecture
## 2 Temporal Database Architecture
### Log Management
They are operated by the instance background processes (Log Writer, Process Monitor, etc.). The process is physically modeled by two phases (second and third phase), whereas the log data are partially in the instance memory (for not approved transactions and in online log repository located in the physical the storage (for active and inactive transactions). Finally, an optional step is presented (the fourth phase), if the online logs do not cover the defined time point. In that case, archived log files are used, extracted, and applied to the objects. After processing, a historical data image can be obtained. The limitation is just the necessity to scan all log files sequentially, whereas individual states influence the consecutive state, it cannot be applied in parallel, directly. In [9; 10], transaction operation precedence is discussed, by which just the most historical object is created, without the change monitoring. As the result, the historical image can be built sooner, and parallel data processing can be used, which dynamically improves performance. If the archive log mode is not enabled, historical data can be created in only a limited time manner [10]. Similarly, if any log file is not present, historical data cannot be obtained, at all. From the log header, it is not possible to detect, whether the particular transaction operates with the relevant object or not. Thus, all log files must be scanned in a time-ordered manner, even if they do not cover any change on the required object [8; 10]. Even more, the particular object does not need to be part of the current data set, it could be removed sooner by the Delete operation [11]. In [12], another solution dealing with historical images was defined. The starting position does not need to be a current data image, any historical backup can be used, as well. The profit can be reached by reducing the amount of the logs to be applied promptly (logs are always sorted on time by the SCN value--System Change Number, which evolve sequentially). Figure 3 shows the processing data flow. In the first step, the nearest data image is obtained, provided either by the current image or the backup. If the backup is used, the whole data image has to be referenced (whole/complete/level 0 backup). In the third step, incremental backups can be applied, the preferred type is cumulative, however, incremental can be used, as well, but in that case, the individual object can be referenced multiple times by applying all changes over the time, which limits the performance strategy [13]. At last, individual logs are used--first archived and then online. Reflecting on the performance, there is an assumption, that the number of logs can be lowered by using backups. In [14], performance limitation is discussed, just with an emphasis on backup loading, which can be resource-demanding, as well. There are several enhancements of the log file management to cover historical images. Flashback technology can automate the process [10]. Extension to the Flashback data archive provides a robust solution by defining attribute sets, which have to be monitored over time. Thanks to that, the amount of the data to be parsed and evaluated is lowered. Principles are defined in [6]. Performance improvement of the Flashback data archive can produce object specification distribution, where the individual objects are divided into sets covering the range of attributes. From the performance point of view, it can bring noticeable performance improvements [15], however, a strong problem can arise there if the references across the distributive architecture have to be used, to ensure data consistency. In that case, several segments have to be notified to cover reliability and integrity. Note please, that the references can be created, reflecting the foreign keys. If any object created as the composition of other objects is changed, also the secondary objects need to be monitored, etc. [15; 16]. Limitation of the above principles can be done by the temporal spheres.