What is query optimization ? List distributed query optimization algorithm and explain any one from tha?
Answers
Answer:
Query optimization is a function of many relational database management systems. The query optimizer attempts to determine the most efficient way to execute a given query by considering the possible query plans.
Explanation:
Query Trading
In query trading algorithm for distributed database systems, the controlling/client site for a distributed query is called the buyer and the sites where the local queries execute are called sellers. The buyer formulates a number of alternatives for choosing sellers and for reconstructing the global results. The target of the buyer is to achieve the optimal cost.
The algorithm starts with the buyer assigning sub-queries to the seller sites. The optimal plan is created from local optimized query plans proposed by the sellers combined with the communication cost for reconstructing the final result. Once the global optimal plan is formulated, the query is executed.
Reduction of Solution Space of the Query
Optimal solution generally involves reduction of solution space so that the cost of query and data transfer is reduced. This can be achieved through a set of heuristic rules, just as heuristics in centralized systems.
Following are some of the rules −
Perform selection and projection operations as early as possible. This reduces the data flow over communication network.
Simplify operations on horizontal fragments by eliminating selection conditions which are not relevant to a particular site.
In case of join and union operations comprising of fragments located in multiple sites, transfer fragmented data to the site where most of the data is present and perform operation there.
Use semi-join operation to qualify tuples that are to be joined. This reduces the amount of data transfer which in turn reduces communication cost.
Merge the common leaves and sub-trees in a distributed query tree.