Query Optimization Techniques and Performance Issues in XML ...
Query Optimization Techniques and Performance Issues in XML and Parallel databases CSE 8330 Instructor: Dr.Margaret H. Dunham Presenter: Akshaya Aradhya Topics to be covered Introduction Query optimization in XML databases Query optimization in Parallel databases Comparison Conclusion and Future work Bibliography Introduction
XML is an emerging standard for exchanging, storing and representing the data The data encoded in XML conforms to a DTD (Document Type Definition) XML structure is intuitive and it is easier to interpret it using its tree like structure. Introduction
XML data model is very complex when compared to other relational models, which renders a larger search space for optimizing XML queries In order to optimize XML queries, we need to study the equivalence issue related to the data and the query in order to find out the query equivalence before transforming the query Introduction
The techniques used to classify the XML query optimization techniques can be divided into groups based on the content and structure Content based query optimization Based on statistics or classification Query execution can be improved by classifying the elements, which transform the query based on constraints which are obtained from the data Introduction
The application of parallel database systems can be observed in decision support systems and a wide range of modern database applications. The machine architecture in parallel database systems are based on parallel dataflow architecture system, which make use of conventional, shared nothing hardware design. For each relation in the database, the tuples are declustered (partitioned) across disk storage units, which are attached to individual processors.
Introduction There are two properties demonstrated by parallelism, which makes it very desirable. The first one is called as linear scale-up, where the system can perform a task k times the size in a particular span of time, after the number of processors are increased by k. The second one is called as linear speedup where the response time is reduced by k times if we
increase the number of processors by k times Introduction During the query processing stage in parallel databases, parallelism can be exploited in three different ways. In the independent parallelism technique, different processors can execute different queries in parallel if the query operators do not depend on each other. By pipelining or by making use of inter-operator parallelism, the
output of the producer to the consumer can be passed on in parallel by two or more operators in a producer consumer relationship. Finally, in intra-operator or partitioned parallelism technique, copies of the same query operator can be run on multiple processors simultaneously, where each of them can be operated on a partition of the data. Optimization mechanism using ToXin tree ToXin indexing scheme was developed to overcome the limitation of applying optimization for path query processing.
This scheme was developed with the primary goal of exploiting the path structure of the XML databases in all the stages of query processing. There are two types of index structures in Toxin called Value index and Path index Optimization mechanism using ToXin tree Algorithm: ConstructIndexTree Output: Tree T ConstructIndexTree()
1. Perform a depth first traversal of the tree. 2. For each visited edge 2.1 Check whether the corresponding index edge has been added 2.1.1 For the current index edge of the XML element 22.214.171.124 Update the instance function in two redundant hash tables representing forward and backward navigation tables 126.96.36.199 Add the parent node and child node 2.2 If it has been added already, skip to the next index edge 3. Stop Optimization mechanism in Lore An input query is divided into a set of sub
queries where each operation is evaluated separately, as a part of the query. An effective execution order for these operations is obtained by creating evaluations for all the set of operations, which in turn helps in executing the queries faster. The final result can be obtained by joining all the aggregation of the results together. Optimization mechanism in Lore Algorithm: PlanSelectionAlgorithm
Input: Input list (for the query) Output: Plan P PlanSelectionAlgorithm (input list) 1. Create a structure in order to track the binding variables 2. while input list is not empty 2.1 For each element in the input list 2.1.1 Based on the current bound variables, find the cheapest access method for the remaining steps 2.1.2 If the step has the least cost, mark the variables as bound and add it to the plan P 2.1.3 Remove the chosen step 3. Return the final plan P obtained from the previous steps Optimizing queries in XML structured document databases
Using a set oriented algebraic technique named PAT algebra, a series of set related operations and rules are defined. PAT expressions are obtained by transforming input queries, after checking for the correctness of their syntax. Based on the relationship of elements in the DTD, the PAT expressions can be normalized with the help of the PAT algebra in order to get a new query.
Query optimization based on Schema Query optimization by pruning and rewriting queries Query optimization by classification of elements Join Strategy Selection Optimal Serial Plan (in identical processors) Comparison between Relational Database Management System vs. XML Database System
Comparison of algorithms Conclusion and Future work The tree generation algorithm and some of the optimal plan selection and generation algorithms run in polynomial time and hence, they need to be optimized to run in linear time. PAT algebra is being extended to make it more suitable for query optimization. Frequency search operations heavily make use of the indexing techniques in PAT.
The future research will also be focused more towards generation and use of partially correlated sub-plans, which depend on bindings passed between portions of query plan. When a significant number of paths pass through a small number of objects, a transformation which introduces a group-by clause can be useful. Further examination is being conducted in order to implement the Toxin Graph and to check if the Toxin Tree can be extended to be used as an alternative to DOM for querying, updating and storing XML documents Conclusion and Future work
Value based grouping and join techniques are being investigated along with multi-way structural joins, new access methods for merged operators and several structural pattern techniques. In addition to this, new optimization algorithms have to be implemented to improve caching in Web Service Management Systems, XQuery language constructs are to be optimized. Cost based decisions are to be integrated in earlier stages
of the query evaluation process and the cost model has to be refined in order model the CPU cost in a precise manner. Bibliography  Dunren Che, Karl Aberer, and Tamer. 2006. Query optimization in XML structured-document databases. The VLDB Journal 15, 3 (September 2006), 263-289.  Jason McHugh and Jennifer Widom. 1999. Query Optimization for XML. In Proceedings of the 25th International Conference on Very Large Data Bases (VLDB '99), Malcolm P. Atkinson, Maria E. Orlowska, Patrick Valduriez, Stanley B. Zdonik, and Michael L. Brodie (Eds.). Morgan Kaufmann Publishers Inc., San Francisco, CA, USA, 315326.
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