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KNOWLEDGE-BASED SOFTWARE ENGINEERING phần 7

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Việc kết nối giữa các dịch vụ và VHE là dịch vụ có thể sử dụng xác thực, định tuyến chức năng vv được cung cấp bởi mạng thông qua giao diện khuôn khổ chung của hệ điều hành A. Một vấn đề ở đây hiện nay là một tán xạ của các công nghệ XML, đặc biệt là trong phần Schema, | M. Wojciechowski and M. Zakrzewicz Efficiency of Dataset Filtering Implementations 193 filtering constraint depends on the actual contents of the database. In general we observed that item constraints led to much better results reducing the processing time 2 to 8 times depending on constraint selectivity and filtering implementation method than constraints referring only to itemset size typically reducing the processing time by less than 10 . This is due to the fact that frequent itemsets to be discovered are usually smaller than transactions forming the source dataset and therefore even restrictive size constraints on frequent itemsets result in weak constraints on transactions. Fig. 1. Execution times for different values of selectivity of size constraints Fig. 2. Execution times for different values of selectivity of item constraints In case of item constraints all the implementations of dataset filtering and projection were always more efficient than the original Apriori with a post-processing constraint verifying step. Projection led to better results than filtering which can be explained by the fact that projection leads to the smaller number of Apriori iterations and slightly reduces the size of transactions in the dataset . Implementations involving materialization of the filtered projected dataset were more efficient than their on-line counterparts the filtered projected dataset was relatively small and the materialization cost was dominated by gains due to the smaller costs of dataset scans in candidate verification phases . However in case of size constraints rejecting a very small number of transactions materialization of the filtered dataset sometimes lead to longer execution times than in case of the original Apriori. The on-line dataset filtering implementation was in general more efficient than the original Apriori even for size constraints except for a situation unlikely in practice when the size constraint did not reject any transactions . Fig. 3.