Now in the real world, a lot of information is stored in the relational database. People store information and use it when they need. A lot of data are used everyday. “SQL is the standard programming language for creating, manipulating, and retrieving information stored in relational databases.”(Rachel Singer Gordon, 2006) People use SQL operate database for query.
But for one SQL statement, database management system (DBMS) can follow many plans to process it and produce the result. All plans would produce the same result, but their costs are different such as the amount of time of processing. Which plan is the best choice and how we find it? Query Optimization is very necessary and important in DBMS. (Yannis E. Ioannidis)
“There has been extensive work in query optimization since the early 70s.” (Surajit Chaudhuri) Query optimization was researched for such a long time, it still need to improve as there are a lot of factors need to think about such as Search Space, I/O Cost and CUP Cost and so on. These factors may affect the performance of query such as speed of query and resource used. (John G. Hughes, 1988, p 177)
“The query optimization phase in query processing plays a crucial role in choosing the most efficient strategy for executing a query.”(David Taniar, Hui Yee Khaw, Haorianto Cokrowijoyo Tjioe & Eric Pardede, 2007)
Friday 18 April 2008
about deadlock
Petri nets can be used to many areas. They are mathematical stuff. They can define, model, simulate many different system. They have been used to model and analyse some types of processes which including protocols, manufacturing systems, and business processes (Aalst 1999). For the distributed database systems, Petri nets are very useful as Petri nets can describe by modeling a finite concurrent system in many states.
“Petri nets are a widely used model for analyzing concurrent and distributed systems. Often such a system must exhibit reactive, non-terminating behavior, and one of the key analysis problems is that of deadlock-freedom: Do all reachable global states of the system (markings of the net) enable some action (net transition)?” (Keijo Heljanko, 1999) Petri net is very common and useful for distributed database systems. The hardest thing for using Petri net to prevent the deadlock is how to design a finite system. For a distributed database system, there must be many states. We can design a system model, but we can not make sure the system model is fully correct. There may be some condition we forget. For each state, whether deadlock happen will happen and how to prevent? That should be mostly considered. So it is not easy to design a finite system with analysis.
Using Petri net to prevent the deadlock must consider how the deadlock can not happen. So the four conditions of deadlock can not be happened at the same time. But as do not request while holding resource and lineate requests cause more waiting and the resources can not be used well, there are still a problem: how to improve the performance of the system? How to let the resources fully be used? Those are difficult.
Using Petri net to design a system to prevent the deadlock, we can check whether the deadlock happens at design time instead of to check at running time. It may let the system faster as there is no checking when system is running.
Using Petri net to design a finite system to prevent the deadlock, when the size of the system is very big, the number of the places, transitions and tokens may increase to a big number. We can not image how many there are, so it is difficult to design it and analyse it.
Therefore, some people make some other Petri net such as Colored Petri Net, Predicate/Transition Net, Adaptive Colored Petri Net, and Object Oriented Colored Petri Net and so on to fit many different systems and make design and analysis easy.
“Petri nets are a widely used model for analyzing concurrent and distributed systems. Often such a system must exhibit reactive, non-terminating behavior, and one of the key analysis problems is that of deadlock-freedom: Do all reachable global states of the system (markings of the net) enable some action (net transition)?” (Keijo Heljanko, 1999) Petri net is very common and useful for distributed database systems. The hardest thing for using Petri net to prevent the deadlock is how to design a finite system. For a distributed database system, there must be many states. We can design a system model, but we can not make sure the system model is fully correct. There may be some condition we forget. For each state, whether deadlock happen will happen and how to prevent? That should be mostly considered. So it is not easy to design a finite system with analysis.
Using Petri net to prevent the deadlock must consider how the deadlock can not happen. So the four conditions of deadlock can not be happened at the same time. But as do not request while holding resource and lineate requests cause more waiting and the resources can not be used well, there are still a problem: how to improve the performance of the system? How to let the resources fully be used? Those are difficult.
Using Petri net to design a system to prevent the deadlock, we can check whether the deadlock happens at design time instead of to check at running time. It may let the system faster as there is no checking when system is running.
Using Petri net to design a finite system to prevent the deadlock, when the size of the system is very big, the number of the places, transitions and tokens may increase to a big number. We can not image how many there are, so it is difficult to design it and analyse it.
Therefore, some people make some other Petri net such as Colored Petri Net, Predicate/Transition Net, Adaptive Colored Petri Net, and Object Oriented Colored Petri Net and so on to fit many different systems and make design and analysis easy.
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