Concurrency Control in DBMS: Resolving Conflicts

is timestamp ordering. In timestamp ordering, transactions are assigned a unique timestamp when it begins.

Each object in the system has a read and a write timestamp associated with it. The read timestamp is the timestamp of the last committed transaction that read the object. The write timestamp is the timestamp of the last committed transaction that modified the object. Not maintain rights to what processes can access the data which could ultimately produce a deadlock would be negligent. Use of concurrency control ensures that the DBMS is able to run smooth and error free. What is the difference between a rule-based optimizer and a cost-based optimizer? Cost-Based Optimization(CBO) and Rule-Based Optimization (RBO), hese are the optimization strategies used by Database engines for executing a query or a stored procedure. RBO is a set of rules that determine how exactly a query is executed. This is often the simplest implementation, but it is far from the best choice in most cases. Much of the more recent performance enhancements, such as hash joins, improved star query processing, and histograms, are not available through rule-based optimization. This make the rule-based approach inferior to cost-based in most instances. As indicated by its name the CBO was developed with costs in mind. It was developed to reach the cheap cost of execution.

The cheapest plan is the one that will use the least amount of resources (CPU, Memory, I/O, etc. ) to get the desired output . The cost-based approach generally chooses an execution plan that is better than the plan chosen by the rule-based approach. This becomes especially evident for large queries with multiple joins or multiple indexes. The cost-based

approach also eliminates having to tune your SQL statements; this greatly improves productivity.

• Coronel, C. , Morris, S. , & Rob, P. (2011). Database Systems (Design, Implementation and Management) 9th Edition. Boston: Cengage Learning.