Key Retrieval Research Articles

Main-memory index structures with fixed-size partial keys

The performance of main-memory index structures is increasingly determined by the number of CPU cache misses incurred when traversing the index. When keys are stored indirectly, as is standard in main-memory databases, the cost of key retrieval in terms of cache misses can dominate the cost of an index traversal. Yet it is inefficient in both time and space to store even moderate sized keys directly in index nodes. In this paper, we investigate the performance of tree structures suitable for OLTP workloads in the face of expensive cache misses and non-trivial key sizes. We propose two index structures, pkT-trees and pkB-trees, which significantly reduce cache misses by storing partial-key information in the index. We show that a small, fixed amount of key information allows most cache misses to be avoided, allowing for a simple node structure and efficient implementation. Finally, we study the performance and cache behavior of partial-key trees by comparing them with other main-memory tree structures for a wide variety of key sizes and key value distributions.

An Efficient Retrieval Algorithm of Compound Words Using Extendible Hashing

An extendible hashing scheme resolves bucket overflows by reorganizing the hash function and file structure locally, so it is very suitable for fast key retrieval of dynamic key sets. However, it cannot search keys that contain a given string as substrings efficiently. In this paper, in order to design this substring search in extendible hashing, signature vectors are introduced as hash values, and a trie structure as an extendible hash table, where each vector is composed by a bit stream. Pseudo signature vectors are defined to identify the buckets, and a constrained depth-first search is presented to traverse the arcs of the trie structure. To construct a compact trie despite an increase in the number of keys, uniform signature vectors are introduced, and the method for an incremental expansion of the hash table is proposed. This approach can restrict the size of the bit stream for each key, making constrained depth-first search efficient. From simulation results, by applying the presented schemes to Japanese key sets, it showed a decrease in the number of accessed buckets of 85% in comparison with traditional extendible hashing for which only descriptors were used. In addition, the search time cost of the presented approach is 6 times faster.

An order searching algorithm of extensible hashing

Extensible hashing schemes provide fast key retrieval for dynamic key sets, but they can not keep order preserved searching. In order to solve this problem, Jonge et al. proposed the binary digital search tree (BDS-tree) that enables order preserved searching by extensible hashing schemes, and the method to change the BDS-tree into a compact bit stream (called the pre-order bit stream) However, searching and updating a key takes a lot of time in large key sets. This paper proposes a new method to preserve the key-order access property and to improve the time cost. The algorithms for retrieval, insertion and deletion of keys using this new method are introduced through examples. This method separates the BDS-tree into smaller trees of a certain depth, and creates the pre-order bit stream for each of the separated trees. The theoretical and experimental results, using 50,000 Japanese nouns, show that the presented method provides faster access than the traditional BDS-tree. Retrieval is 18 times, the insertion is 13 times and the deletion is 4 times faster. There is a small increase in the storage requirement for the separated BDS-tree. However, by nature, the pre-order bit stream is very compact in size, thus making the increase insignificant.

Performance of linear hashing schemes for primary key retrieval

Linear hashing is one of the most attractive dynamic hashing schemes. Linear hashing with partial expansions and linear hashing with priority splitting are two variations with improved space and time performance. Here, we propose a new structure, which is termed linear hashing with partial expansions and priority splitting. The above four structures are compared by simulation and it is shown that the new scheme outperforms its predecessors in both time and space costs.

A method of compressing trie structures

AbstractA trie structure can immediately determine whether a desired key is in a given key set or not, and can find its longest match easily. Thanks to these attractive properties, a trie structure is frequently used for various fields, such as natural language dictionaries, database systems and compilers. However, the total number of states of a trie becomes large, so space requirements are not good for a huge key set. To resolve this disadvantage a new structure which reduces the total number of states in a traditional trie, called a double‐trie, is introduced in this paper. Insertion and deletion operations, as well as key retrieval for this double‐trie, are presented. The efficiency of this method is shown by the results of simulations for various key sets.

Probability distributions for seek time evaluation

In this work, two known probability distributions based on the replacement model, namely, the Maxwell-Boltzmann and Bose-Einstein ones, are examined. Each of these distributions is assigned to a specific case of query processing, which is known in the literature as batching on primary key values or secondary key retrieval, respectively. These distributions are applied in evaluating the expected distance traveled by the read/write heads in disk searching. Two new probability distributions are derived, based on the previous ones, which are involved in the evaluation of the expected number of the cylinder hits. Therefore, seek time evaluation is faced globally. Numerical results are given.

Performance of a two-headed disk system when serving database queries under the scan policy

Disk drives with movable two-headed arms are now commercially available. The two heads are separated by a fixed number of cylinders. A major problem for optimizing disk head movement, when answering database requests, is the specification of the optimum number of cylinders separating the two heads. An earlier analytical study assumed a FCFS model and concluded that the optimum separation distance should be equal to 0.44657 of the number of cylinders N of the disk. This paper considers that the SCAN scheduling policy is used in file access, and it applies combinatorial analysis to derive exact formulas for the expected head movement. Furthermore, it is proven that the optimum separation distance is N/2 - 1 (⌈ N /2 - 1⌉ and ⌊ N /2 - 1⌋) if N is even (odd). In addition, a comparison with a single-headed disk system operating under the same scheduling policy shows that if the two heads are optimally spaced, then the mean seek distance is less than one-half of the value obtained with one head. In fact that the SCAN policy is used for many database applications (for example,batching and secondary key retrieval) demonstrates the potential of two-headed disk systems for improving the performance of database systems.

Partial match retrieval using recursive linear hashing

Recursive linear hashing is a hashing technique proposed for files which can grow and shrink dynamically. The scheme is an extension of linear hashing, a method originally proposed by Litwin, but unlike Litwin's scheme, it does not require conventional overflow pages. In this paper, we investigate the application of recursive linear hashing to partial match retrieval problems. Consistent with the results for primary key retrieval, recursive linear hashing performs better than the conventional scheme on these problems, especially at high load factors.

Clinical data evaluation and the PL health control information system

In a health control service environment, that is, a periodic, membership AMHTS type of comprehensive health check-up system, where clinical data evaluation, especially an evaluation in terms of subject-specific normal ranges, is most important, the medical information system is required to handle: (1) Various network types files; (2) real-time immediacy; (3) an asserted reliability to meet personal health control purposes. As in other computer applications already successfully used, an indexed direct access method (IDAM) developed is our solution. It allows us to provide multiple indices for the file network, instead of inverted files, a unique index-to-record relationship, preventing any unrecoverable chaining destruction and, thereby, provides any network type access a stable access time. Further-more, for research purposes, a data integrity for on-line access and batch access was attained as well as a retrieval language system with a multiple key retrieval function.

Minimum cost selection of secondary indexes for formatted files

Secondary indexes are often used in database management systems for secondary key retrieval. Although their use can improve retrieval time significantly, the cost of index maintenance and storage increases the overhead of the file processing application. The optimal set of indexed secondary keys for a particular application depends on a number of application dependent factors. In this paper a cost function is developed for the evaluation of candidate indexing choices and applied to the optimization of index selection. Factors accounted for include file size, the relative rates of retrieval and maintenance and the distribution of retrieval and maintenance over the candidate keys, index structure, and system charging rates. Among the results demonstrated are the increased effectiveness of secondary indexes for large files, the effect of the relative rates of retrieval and maintenance, the greater cost of allowing for arbitrarily formulated queries, and the impact on cost of the use of different index structures.

On the use of bit maps for multiple key retrieval

The traditional file structures used to support fast response to complex user queries have been based on the inverted list organization, using either the pointer or bit string representation. In this paper, the use of bit maps for executing multiple key searches is studied. Bit maps turn out to be less precise inverted lists Where the inversion is kept for a quantization of attribute domains and the objects referenced are blocks of data records. The goal is to reduce the total number of I/O accesses required to execute a retrieval based on a Boolean qualification. An evaluation of the method is given for both storage space and expected retrieval time under simplified assumptions. Key Words and Phrases: Multiple key retrieval, inverted lists, bit strings, bit maps, Boolean queries, data base management. CR Categories: 3.70, 3.71, 3.73, 3.74, 4.33, 4.34

On the use of bit maps for multiple key retrieval

The traditional file structures used to support fast response to complex user queries have been based on the inverted list organization, using either the pointer or bit string representation. In this paper, the use of bit maps for executing multiple key searches is studied. Bit maps turn out to be less precise inverted lists Where the inversion is kept for a quantization of attribute domains and the objects referenced are blocks of data records. The goal is to reduce the total number of I/O accesses required to execute a retrieval based on a Boolean qualification. An evaluation of the method is given for both storage space and expected retrieval time under simplified assumptions. Key Words and Phrases: Multiple key retrieval, inverted lists, bit strings, bit maps, Boolean queries, data base management. CR Categories: 3.70, 3.71, 3.73, 3.74, 4.33, 4.34

Organization and maintenance of large ordered indexes

Organization and maintenance of an index for a dynamic random access file is considered. It is assumed that the index must be kept on some pseudo random access backup store like a disc or a drum. The index organization described allows retrieval, insertion, and deletion of keys in time proportional to logk I where I is the size of the index and k is a device dependent natural number such that the performance of the scheme becomes near optimal. Storage utilization is at least 50% but generally much higher. The pages of the index are organized in a special datastructure, so-called B-trees. The scheme is analyzed, performance bounds are obtained, and a near optimal k is computed. Experiments have been performed with indexes up to 100000 keys. An index of size 15000 (100000) can be maintained with an average of 9 (at least 4) transactions per second on an IBM 360/44 with a 2311 disc.

Secondary key retrieval using an IBM 7090-1301 system

The secondary key retrieval method involves the preparation of secondary storage lists from primary data records. Search requests are satisfied by logical operations on appropriate lists, producing a complete set of addresses of primary records relevant to the request. Experimental results are presented and a comparative analysis is given.