Dynamic maintenance model for high average-utility pattern mining with deletion operation

The high average-utility itemset mining (HAUIM) was established to provide a fair measure instead of genetic high-utility itemset mining (HUIM) for revealing the satisfied and interesting patterns. In practical applications, the database is dynamically changed when insertion/deletion operations are performed on databases. Several works were designed to handle the insertion process but fewer studies focused on processing the deletion process for knowledge maintenance. In this paper, we then develop a PRE-HAUI-DEL algorithm that utilizes the pre-large concept on HAUIM for handling transaction deletion in the dynamic databases. The pre-large concept is served as the buffer on HAUIM that reduces the number of database scans while the database is updated particularly in transaction deletion. Two upper-bound values are also established here to reduce the unpromising candidates early which can speed up the computational cost. From the experimental results, the designed PRE-HAUI-DEL algorithm is well performed compared to the Apriori-like model in terms of runtime, memory, and scalability in dynamic databases.


Introduction
Knowledge Discovery in Database (KDD) [2,5,8,19,32] is a way to obtain useful and interesting patterns/information from the databases. At present, KDD has been widely used in many domains and applications. Association-rule mining (ARM) [1] is regarded as the generic and basic research in KDD for identifying frequent itemsets (FIs) by minimum support threshold and then discovering association rules (ARs) by minimum confidence threshold. Apriori algorithm [1] was firstly proposed to discover association rules (ARs), which utilizes the generate-and-test model for identifying the possible candidates level by level. In order to improve the mining efficiency, Han et al. [9] built a frequent pattern (FP)-tree structure to compress the database by keeping This article belongs to the Topical Collection: Emerging topics in Applied Intelligence selected from IEA/AIE2020 Guest Editors: Hamido Fujita, Philippe Fournier-Viger and Moonis Ali Jerry Chun-Wei Lin jerrylin@ieee.org Extended author information available on the last page of the article. necessary information and process an efficient FP-growth mining algorithm to reveal the set of frequent itemsets (FIs). Then, a new structure called N-list was introduced by Deng et al. [5] and PrePost algorithm was presented to find frequent itemsets (FIs) efficiently. In order to effectively discover different types of patterns, many approaches have presented in different fields and applications [8,19,32].
However, in traditional ARM, it is assumed that each item is purchased at most once in each transaction and only the amount of appearance of items in databases is considered without other meaningful factors, i.e., significance, weight, or unit profit. A frequent item/itemset can bring frequency information to retailers, but cannot guarantee high profits for retailers. High-utility itemset mining (HUIM) [7,22,23,28] was provided by looking at both quantity and unit of profit of items to reveal high-utility itemsets (HUIs). An itemset is considered as a high-utility itemset (HUI) if its measured utility is no less than a pre-defined minimum utility threshold. Regarding the downward closure (DC) property, the initial HUIM does not hold DC thus the "combinational explosion" happens in the mining process. Transaction-weighted utilization (TWU) model [23] was defined by Liu et al. that designs an over-estimated upper-bound utility on an itemset. Thus, the size of the unpromising candidates can be greatly reduced. Nevertheless, TWU model follows the generate-and-test process, in which a large number of profitable HUIs can thus be generated. To address this limitation, high-utility pattern (HUP)-tree [16] was introduced to keep the promising 1itemsets into the tree structure. Tseng et al. [25] further introduced the UP-growth+ approach based on the UP-tree structure to reveal the set of HUIs accurately. Moreover, utility-list (UL) structure [22] was proposed to obtain the k-itemsets based on the simple join operation. Many approaches have been respectively presented and discussed and most of them were used in the static applications [21,26,29,33]. Moreover, although the HUIM can discover high-utility itemsets in practical applications, the total profit value of an itemset increases along with its length. For instance, diamonds combine with any commodities would generate high utility and this combination might be regarded as a HUI. To evaluate the high utility patterns fairly, high average-utility itemset mining (HAUIM) [11] concerns the number of items in an itemset to reveal the set of high average-utility itemsets (HAUIs). If an itemset is called as a HAUI, the average-utility of this itemset exceeds the user-designed minimum threshold of average-utility. Hong et al. [11] proposed a level-wise manner called TPAU to reveal HAUIs, and this method introduced "average-utility upper-bound (auub)" to retain the DC property. This auub model is used to find high average-utility upper bound itemsets (HAUUBIs), that the average-utility of a HAUUBI is larger than the defined threshold value and the set of HAUUBIs also keeps the completeness and correctness of the satisfied HAUIs. To increase mining efficiency, a novel structure called high average-utility pattern (HAUP)-tree was designed by Lin et al. [15] to maintain 1-HAUUBIs. Lin et al. [18] further designed average-utility list (AUL)structure to retrieve the k-itemsets quickly based on the simple join operation. Other extensions of HAUIM [12,17] were also discussed and studied to improve the efficiency problem of knowledge discovery.
The above existing approaches related to HAUIM focused on mining HAUIs in the static database, but in practical applications, the content of a database is dynamically changed according to different operations, i.e., insertion or deletion. In this case, traditional methods to handle this situation is to re-perform the mining progress again to re-scan the updated database that causes an additional calculation. Cheung et al. developed Fast UPdated (FUP) [3] concept and FUP2 [4] concept to retain the discovered itemsets when some transactions are inserted or deleted, respectively. However, HAUIM, which applies the FUP concept, must also perform rescan process for mining the required information from the updated transactions, which is very costly. Other extensions of dynamic database [13,27,31] were also discussed and studied to improve the efficiency of mining process. In this paper, we first take advantage of pre-large concept [10] for updating the discovered HAUIs when some transactions are removed in the dynamic databases. Contributions of this paper are shown as follows.

Review of related work
High average-utility itemset mining (HAUIM) and the concept of pre-large are separately introduced in this section.

High average-utility itemset mining
The fundamental algorithms of ARM [1] are designed to find patterns by only considering the frequency of itemsets. However, these algorithms can simply manage the binary database but the other interesting factors such as the importance, weight, or unit profit and quantity are ignored.
To discover more sufficient and significant information, HUIM (high-utility itemset mining) [6,23,28] concerns both quantity and unit profit of items to obtain the profitable patterns from databases. In order to maintain the downward closure (DC) property for the mining progress, transactionweighted utilization (TWU) model [23] was applied to keep the DC property of the high transaction-weighted utilization itemsets (HTWUIs). Lin et al. [16] further constructed high-utility pattern (HUP)-tree to store the 1-HTWUIs in a tree structure, which can improve the mining efficiency in HUIM. To apply HUP-tree structure, HUP-tree algorithm was designed to reveal the set of HUIs efficiently. Compared with the traditional Apriori-like method, HUP-tree approach is more efficient. To better enhance the mining performance, Tseng et al. [25] developed the UP-tree structure to reserve essential information for mining process, and designed UP-Growth+ algorithm and several pruning strategies for finding HUIs. Liu et al. [22] then proposed a utility-list (UL) structure to quickly obtain the k-candidate HUIs by a very simple join operation. Based on the UL structure, the HUI-Miner algorithm was designed to reveal HUIs efficiently.
Many studies of HUIM [20-22, 26, 29, 33] have been extensively discussed and developed to improve the mining process in KDD. Although HUIM has the ability to discover profitable information for decision-making, the utility value increases along with the size of the itemset. Due to this consequence, it might produce the misunderstanding information for decision-making. For example, any combinations with caviar/diamond may be considered as a HUI as well. To evaluate the high utility patterns fairly, high average-utility itemset mining (HAUIM) [11,15] was presented to take into account the length of itemsets for revealing high averageutility itemsets (HAUIs). The first algorithm related to HAUIM is called TPAU [11], which uses average-utility upper-bound (auub) model to keep downward closure (DC) property in a Apriori-like process. Lin et al. [15] then developed HAUP-tree, which is a compressed tree structure to maintain the 1-itemsets, thus the search space can be greatly reduced. HAUI-Miner algorithm applied a link-list structure to store crucial information based on a novel list structure. Several approaches for HAUIM [14,17,24,30] have been studied and the development of HAUIM is still in progress.

Pre-large concept
Traditional pattern mining only considered to find the significant information from a static dataset, such as association-rule mining (ARM) [1], frequent itemset mining (FIM) [2], and high-utility itemset mining (HUIM) [6]. When a database is dynamically changed, for instance, transaction deletion, insertion or modification, the batch approaches have to examine the updated database again to maintain the discovered knowledge. Thus, the previous calculations and results have become invalid and the additional cost is necessary and required.
To deal with the dynamic data mining efficiently, Cheung et al. then developed Fast UPdated (FUP) [3] concept and FUP2 [4] concept to keep the revealed itemsets when the size of the dataset is changed. For instance of transaction deletion in FUP2 concept, it separates the discovered patterns into four cases according to the original dataset and the deleted one, and each case has its own designed model to maintain and update the discovered information. However in some cases, the original database is still required to be rescanned for updating the discovered patterns, thus it takes some computational cost for knowledge maintenance.
To maintain the discovered information efficiently and decrease the number of database rescans, Hong et al. [10] furtherer proposed the pre-large concept for the maintenance progress. Based on the pre-large concept, two thresholds were designed that keeps not only the large itemsets but also the pre-large itemsets for knowledge maintenance. The pre-large itemsets can thus be considered as a buffer to avoid the extra movements of an itemset from large (high) to small and vice-versa. Based on the benefits of pre-large concept, the re-scanning process of databases can only be performed while a certain amount of transactions are inserted into the databases or deleted from the databases. This idea can greatly reduce the computational cost of multiple database scans. In pre-large concept for transaction deletion, nine cases are then arisen and shown in Fig. 1.
In Fig. 1, for the cases 2 to 4, 7, and 8, the final results are the same as the results before transaction deletion. However, the number of discovered knowledge may be reduced in case 1, and some new patterns can thus be produced in cases 5, 6, and 9. If the large itemsets and the pre-large itemsets are maintained and stored from the original database, it is simple to maintain cases 1, 5, and 6. For case 9, if the number of deleted transactions is no large than the safety bound (f ), the itemsets in case 9 are impossible to be a large itemset in the updated database. The safety bound (f ) is defined as: where S u and S l are respectively the upper-bound threshold and the lower bound threshold, which can be both defined by user or an expert. The possible results of the all cases are given in Table 1.
S / S P L , S

Partial upper-bound and high partial upper-bound itemset
In general, auub property has been widely used in the past methods of HAUIM. The average utility of an itemset in HAUIM is smaller than auub value since the auub is an over-estimated value in HAUIM that is used to keep the DC property in the mining progress. However, auub is too large thus there will be many unpromising candidates generated.
To solve this limitation, the proposed pub was designed and defined as below.
Definition 1 An item is an active item (ai) if this item at least exists in one of pubis. The set of ai is denoted as ais n and the formal definition is given below as: where pubis n−1 is a pubis length is set as (n − 1).

Definition 2 rmua(i, t)
is the remaining maximal utility of an item i in a transaction t and the formal definition is given below as: where I t is a set of the purchase items in this transaction t.
Definition 3 pub t i is the partial upper-bound of an item i in a transaction t and the formal definition is given below as: where m is the length of (ais ∩ I t ) \ i. Definition 4 pub i is the partial upper-bound of an item i and the formal definition is given below as: Definition 5 An itemset X is a high partial upper-bound itemset (hpubi) if its pub is not less than a predefined threshold r, which is indicates that pub i ≥ r.

Lead partial upper-bound and high lead partial upper-bound itemset
In this section, a smaller itemset is recommended to prevent the further generation of candidate sets. This itemset is called the high lead partial upper-bound itemset (hlpubi), and it is a subset of hpubi. The definitions of lpub and hlpubi are given below. The predefined item order is defined as Definition 6 lrmua(i, t) is the lead remaining maximal utility of an item i in a transaction t and the formal definition is given below as: Definition 7 lpub t i is the lead partial upper-bound of an item i in a transaction t and the formal definition is given below as: where m is the length of (ais ∩ I t ∩ s) \ i.
Definition 8 lpub i is the lead partial upper-bound of an item i and the formal definition is given below as:

Definition 9
An itemset X is a high lead partial upperbound itemset (hlpubi) if its lpub is not less than a predefined threshold r, which is indicates that lpub i ≥ r.

Problem statement
In dynamic situation of transaction deletion based on the pre-large concept, there are nine cases that should be considered and arisen. Assume d is the deleted transactions, and |d| is the number of transactions in d. The problem statement of the HAUIM for transaction deletion is defined as follows: Problem Statement For the HAUIM with transaction deletion, an efficient model is required for development that can efficiently maintain the discovered knowledge without multiple database scans while the transactions are deleted from the databases. Thus, a satisfied itemset in the database for transaction deletion is considered as an HAUI if it satisfies the condition as: where au(X) U is the average-utility of itemset X in the updated database, T U D and T U d are respectively the total utility in D and d.

Proofs of nine cases
In this part, the proofs of nine cases of the proposed method based on the pre-large concept are given below.
, then it will be large, pre-large, or small in the updated database U . Proof For auub(X) D ≥ T U D × S u , auub(X) d < T U d × S l and S u > S l , then auub(

Lemma 4 For case 4, an itemset
, then it will be pre-large or small in the updated database U .
will be pre-large and small in the updated database U .

Lemma 5 For case 5, an itemset
, then it will be large, pre-large, or small in the updated database U .
then X is small in the updated database U .

Lemma 6
For case 6, an itemset X is pre-large in the original database D (T U D × S l ≤ auub(X) D < T U D × S u ) and small in the deleted database auub(X) d < T U d × S l ), then it will be large or pre-large in the updated database U .
will be large and pre-large in the updated database U .
Lemma 9 For case 9, an itemset X is small in both the original database D (auub(X) D < T U D × S l ) and the deleted database d (auub(X) d < T U d × S l ), then it will be pre-large or small in the updated database U .
Proof For auub(X) D < T U D × S l and auub(X) d <

Theorem 1 X is a small itemset (not a high average-utility itemset nor pre-large itemset) in the original database and the deleted database. It is impossible to be a high averageutility itemset in the updated database if the total utility of the deleted transactions is not larger than the following safety bound as:
Here, theorem 1 will be used in the proposed algorithm in which the pre-large itemsets serve as a buffer to store some extra information. It is thus sufficient to reduce the time by performing the multiple database scans and further diminish the updating cost of the deletion operation.

Proposed pre-large-based framework for transaction deletion
An innovative algorithm called PRE-HAUI-DEL is proposed to maintain and update the set of HAUIs in a dynamic database efficiently, and this algorithm is based on Apriorilike method. This section introduces the detailed Apriori framework for maintaining and updating the set of HAUI in dynamic databases. In the process of knowledge updating, the PRE-HAUI-DEL algorithm uses the novel upper-bound value to find new itemsets, thus pruning the optimistic candidates early and reducing the search space. First, a novel partial upper-bound (pub) is suggested to prune the unpromising candidates, and an itemset is named as a high partial upper-bound itemset (hpubi) if its utility exceeds the pub. Second, there will be a subset selected from hpubi, called hlpubi (high lead-pubi). In fact, these two sets are produced simultaneously, but it uses a tighter upper-bound than pub called lead-pub (lpub). The smalled upper-bound can further prevent the generation of candidate sets.

The designed framework
As shown in Fig. 2, the algorithm first mines the HAUI, pre-large itemsets and pubis, lead-pubis from the original database, If some data is deleted, then calculate the threshold r. According to the size of r, the next step is to re-scan the updated database again or update the new HAUI from the obtained itemsets. If you want to continue to delete the dataset, update the threshold r, and proceed to the next round.

Proposed PRE-HAUI-DEL algorithm
To efficiently update the discovered information for transactions deletion, a novel algorithm called PRE-HAUI-DEL is developed. The proposed algorithm is shown in Algorithms 1 and 2. For Algorithm 1, it is an Apriori-based process to discover the sets of high average-utility itemsets and pre-large itemsets from the original database. Moreover, it utilizes two upper-bounds (pub and lpub) to generate the candidate itemsets.
First in Algorithm 1, the proposed method calculates the safety-bound value for database rescan in line 2 and scans the whole original database for each item to reveal the set of HAUIs with one item and initialize pubis and leadpubis in lines 3 to 15. Note that the proposed process uses auub to initialize pubis and lead-pubis in line 12. Then a while loop in lines 16 to 38 is performed to evaluate the candidate itemsets and reveal HAUIs. This while loop first generates the candidate itemsets by performing a sub-loop for lead-pubis in lines 18 to 20. The first n-1 itemsets of each candidate itemset with n items should be included in lead-pubis. For instance, if the itemsets "{a, b, c}" are the candidate itemset, the itemset "{a, b}" definitely exists in lead-pubis and the itemsets "{a, c}" and "{b, c}" absolutely exists in pubis. In fact, according to the above definitions, "{a, b}" are also included in pubis since lead-pubis is a smaller set than pubis, it can reduce the searching space for the generation progress of the candidate itemsets. The second loop here is to evaluate the whole candidate itemsets into the sets of pubi, lead-pubi, pre-large itemset and high average-utility itemset by checking their pub, lpub and au, respectively. The sets of pubi and lead-pubi are used to generate the candidate itemsets for the next round and the sets of pre-large itemset and high average-utility itemset will be used in Algorithm 2 to update the final result with the safety-bound value r.
In Algorithm 2, it is an updating process for the itemsets in a dynamic database with several deletion operations. By obtaining the benefit of pre-large concept, the prelarge itemsets come across as a buffer to retain the edge itemsets which are almost high average-utility itemsets. The re-scan threshold makes sure the other itemsets out of the pre-large itemset is impossible to become a high averageutility itemset if the number of deleted transactions does not achieve the safety-bound value. In line 2, the updating process first checks the size of the deleted transactions and updates the re-scan threshold. Suppose the size of the deleted transactions already achieves the threshold, in that case, the proposed method will generate the updated dataset and perform Algorithm 1 again to update the prelarge itemsets, high average-utility itemsets, and re-scan  Fig. 2 The diagram of the designed framework threshold r in lines 4 to 7. Otherwise, the proposed updating progress will utilize the pre-large itemsets and previous high average-utility itemsets to update discovered information, as well as updating the threshold value in lines 9 to 11. Finally, the proposed method will output the final P (pre-large itemsets), H (high average-utility itemsets) and re-scan threshold r as the final results in line 13.

An illustrated example
In this section, an example for maintaining and updating HAUIs from a dynamic database is given. Table 2 is used as the original database, and suppose the last 4 transactions (7 to 10) are then deleted from the database. In addition, Table 3 is the profit table, and we assume that the S u = 0.3 and S l = 0.15. The process of the designed algorithm is described below. First, the designed algorithm calculate the safety-bound value as: r = (S u −S l )×T U D S u = 280 (Algorithm 1, line 2). After that, the designed algorithms generates 1-HAUIs, 1-HAUUBIs, 1-PAUUBIs, pubis and lead-pubis from original database (Algorithm 1, lines 3 to 15). The results of the auub values for 1-itemsets in the original database are then shown in Table 4.
Here, we can obtain that S u × T U D = 168 and  lines 1 to 2). We then can also obtain the sets of 1-HAUIs, 1-HAUUBIs, 1-PAUUBIs, pubis and lead-pubis from the deleted database (Algorithm 2, line 4). Also, the auub values for the deleted transactions of 1-itemsets are shown in Table 5.
The designed algorithm then calculate S u × T U d = 63 and S l × T U d = 31.5. In addition, HAUIs = {C, D},     Tables 6 and 7.
Since T U U = 350, we then can obtain that T U U × S u = 105. Thus, the final HAUIs is generated as: {C, E}. After that, the candidates from pubis and lead-pubis are then used to generate k-HAUIs (Algorithm 1, lines 16 to 38). After the recursive process of the designed algorithm based on the genertae-and -test approach, we then can get the maintenance results as the output of the developed algorithm.

Experimental results
In this paper, two suggested upper-bound (pub and lpub) are further utilized to reduce the search space of candidate itemsets. This section describes two experimental processes of the proposed PRE-HAUI-DEL regarding two upperbound models; one is with the lpub and another is without the lpub, and the experiments showed that the proposed lpub can reduce more candidate sets from being generated. The experimental results show the effectiveness of PRE-HAUI-DEL with different parameter settings on six different real datasets. The datasets applied in the experiments and their characteristics are given in Table 8. The experiments were executed on a Mac with Mojava operating system, which is equipped with Intel i5-5257U CPU and 8 GB main memory. The Java language is then used for the program implementation of the compared algorithms.
In the experiments, four algorithms are then compared and discussed, which are Apriori, Apriori (lpub), PRE-HAUI-DEL and PRE-HAUI-DEL (lpub). Apriori is the algorithm that preforms the re-scan process by Algorithm 1 but does not use lpub; PRE-HAUI-DEL is the algorithm that  performs the updated process (Algorithm 2) but does not use lpub, Apriori (lpub) is the algorithm that re-scans the updated dataset when some transactions are deleted in the original dataset by applying Algorithm 1, and PRE-HAUI-DEL (lpub) is the algorithm that performs the updating process by Algorithm 2.

Runtime with different upper-bound utility threshold
The runtimes of the developed PRE-HAUI-DEL under different upper-bound utility thresholds in six real datasets are discussed in this section. Experimental results are given in Fig. 3. From Fig. 3, it can be seen that whether the lpub is applied in the designed algorithm, the proposed PRE-HAUI-DEL is always better than the previous models. Without lpub strategy, the designed PRE-HAUI-DEL still well performs than the generic Apriori-like approaches. This reason is that the designed model does not require multiple database scans each time while the transactions are deleted from the database. However, Apriori (without pre-large technique) needs to scan the database even if there is a tiny amount of transactions deleted from the database. This computational cost is huge, especially when the database is frequently updated. Mathematically, the utilized lpub definitely is a tighter upper-bound than that of both pub or auub. However, the algorithm with lpub still suffers the computational cost in some cases, which can be seen in Fig. 3e while the threshold is set as 1.74%. The reason is even the number of reduced candidates is reduced, the designed algorithm requires to take the huge computational cost by holding the lpub property. In general, the lpub usually enhances the performance in most cases, and it can be seen from Fig. 3d that the Apriori with lpub sometimes can obtain better performance than the proposed PRE-HAUI-DEL without lpub.

Memory usage and number of candidate with different upper-bound threshold
The number of candidates of the compared algorithms is then evaluated, and the results are then shown in Fig. 4.
Since the designed PRE-HAUI-DEL and the Apriori use the different framework to maintain and update the discovered information, it is meaningless to compare them in terms of number of the candidates. Thus, we only compare the Aprior algorithm with and without lpub strategy in this section. Note that an itemset is considered as a candidate if it is required to check the utility of it.
In the Section 7.3, it can be seen that lead partial upper bound (lpub) is more suitable for dense database, and from Fig. 4, it can be seen that there are fewer itemsets needed to be examined by using lpub in the dense databases, such as accidents, mushroom, and chess. This is because the lpub is a rigorous upper-bound and can reduce the size of candidates effectively. In a sparse database, it is not sensitive to be affected by shift values of upper-bounds. There might be a big utility cap between each possible itemset. Therefore, the influence between pub and lpub is not very obvious. To solve this issue, a dynamic selfadjusting technique should be developed in future works that can be used to select a suitable upper-bound for the current database during the mining process.
In addition, it can be seen from Fig. 5 that the Apriori uses more memory than Apriori (lpub). It shows that our new lead partial upper bound (lpub) strategy is effective for pruning the candidates in the search space. Moreover, the Lpub can cut more candidate sets and use less memory to store them. Secondly, we find that memory usage increases with the increase of candidates, because when there is more candidates generated, the more itemsets are stored in the memory.

Scalability of database size
In this section, the runtimes of the proposed PRE-HAUI-DEL with different database sizes for deletion are then compared and discussed. Six different databases are utilized to verify the effectiveness of the proposed method. The experimental results are provided in Fig. 6. In Fig. 6, it can be seen that the utilized lpub has better performance regarding the runtime cost. In most real datasets, the running time of Apriori (lpub) is less than that of the generic Apriori, especially in dense datasets, i.e., the mushroom and the chess databases. However, for instance, in both the foodmart and the retail datasets, the execution efficiency of Apriori (lpub) is not as good as that of Apriori. Thus, we can say that the lpub is more suitable for dense databases, but lpub does not play a better role in sparse databases. The main reason can be concluded that even the number of the candidates is reduced but the cost for performing the lpub strategy is high in some cases with the additional calculation. Thus, the lpub is not suitable for spares databases.
In addition, the designed PRE-HAUI-DEL could handle the decreasing process and reduce the runtime effectively. However, it can also be seen that the runtime of the PRE-HAUI-DEL is sometimes larger than Apriori. That is because some non-large and non-small itemsets are generated and maintained by using pre-large concept during the maintenance process. Moreover, PRE-HAUI-DEL algorithm needs to examine the updated utility value to achieve the final results for further maintenance or knowledge generation. Similarly, in these cases, the runtime of Apriori (lpub) is smaller than that of PRE-HAUI-DEL (lpub). In some cases, the performance of PRE-HAUI-DEL (lpub) is still better than that of PRE-HAUI-DEL, although its execution performance is worse than Apriori.
In Fig. 6a, we can see that the database size is reduced by fewer transactions each time, and in Fig. 6b, the database size is reduced by more transactions. As shown in Fig. 6a,   Fig. 4 The numbers of the candidates with different upper-bound utility thresholds   the re-scan operation will be performed after three times of transaction deletion, but the re-scan operation will be performed frequently in Fig. 6b. Based on (1), if the size of the decreased data is large, the safety bound will be small; according to (1), the re-scanning process will be frequently performed; it causes that the pre-large concept is not suitable to be applied in a situation with a huge size of the deletion database. Another example can be observed from Fig. 6 that two designed PRE-HAUI-DEL algorithms showed good performance in the case of accidents database with the 3,400 transactions for deletion. When the size of deleted transactions grows up to 17,000 for each operation, the two PRE-HAUI-DEL algorithms will perform the rescan process every time; this is because the lpub strategy of the over-estimated upper-bound value requires the extra computational cost, thus the developed PRE-HAUI-DEL requires more time than that of the Apriori approaches. Although this issue can be solved by increasing the size of the pre-large concept, but the developed model will suffer the huge computational cost in the updating progress. Thus, the proposed PE-HAUI-DEL with the pre-large concept is not suitable to be applied in the case by removing a very huge size of the transactions for deletion.

Conclusion and future work
In this paper, we first utilized pre-large concept into HAUIM for handling transaction deletion in dynamic databases. Besides, the lpub upper-bound model is also utilized in the developed algorithm that can be used to greatly reduce the size of the examined candidates in the search space.
Compared to the generic model for updating the discovered knowledge in a batch mode, the designed PRE-HAUIM-DEL can well maintain the discovered HAUIs without multiple database scans; the computational cost can be greatly reduced and the discovered knowledge regarding HAUIs can be maintained correctly and completely. In the future, we will then discuss the big data issue for HAUIM and also consider to apply different platforms (i.e., Hadoop or Spark) to handle the large-scale problems in HAUIM. At the same time, we will continuously study the concept of pre-large and apply it to the other mining tasks in dynamic databases, i.e., transaction modification. More effective model rather than the pre-large concept will be further discussed and studied in the future works.
Funding Open access funding provided by Western Norway University Of Applied Sciences.
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