GBD tema 6 inglés

Given the following code SQL: what will return the sentence select of the row 8 (transaction T2). 10. 20. an exception will happen in T1. an exception will happen in T2.

Given two transactions T1 and T2 and the below execution plan, we can affirm. At a level of isolation READ UNCOMMITED a dirty reads would happen. At a level of isolation READ UNCOMMITED a phanton reads would happen. At a level of isolation READ COMMITED a dirty reads would happen. At a level of isolation READ COMMITED a phanton reads would happen.

In Oracle, a serializable transaction: It can generate an exception if it tries to modify a data that has been modified in another transaction T2, after beginning T1. It can read not updated data. It is the minimal level of necessary isolation to guarantee that there will not be dirty reads. It is the minimal level of necessary isolation to guarantee that there will not be phantom reads.

Given two transactions T1 and T2 and a schedule P, it is possible to guarantee that: If P is serializable, it is guaranteed that there are not conflict operations. If P is serial, it is guaranteed that that there are not conflict operations. If P2 is an alternative schedule to P, P2 is serial, it is known that both obtain the same database state at the end of the execution plan. If P is serial then it is strict.

Taking the following case as an example, write a schedule to be: NO SE CUAL ES. Serial and strict. Serializable and not serial, recoverable and that avoids delete in cascade. Serializable and not serial, strict. Not serializable and it does not avoid delete in cascade. Serializable and not recoverable.

A planner that implements the protocol of strict 2PL. It guarantees that any schedule will be serializable. It guarantees that any schedule will be recoverable. It guarantees that there will not be deadlocks. It guarantees that no transaction will stay in indefinite waiting.

A planner that guarantees concurrence, the serializability, prevents deletes in cascade, is not free from of deadlocks and does not know the data that every transaction is going to modify to the beginning of this one, might implement a strategy: Serial. Conservative 2PL. Strict 2PL. Rigorous 2PL.

With regard to Oracle SERIALIZABLE isolation level, it is posible to affirm: It prevents lost updates. It prevents cascade deletes. It prevents deadlocks. It is a 2PL strategy.

With regard to Oracle READ COMMITED isolation level, it is possible to affirm: It prevents lost updates. It prevents erasures in cascade. It prevents deadlocks. It prevents dirty readings.