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DATA ACCESS 90. Define Transaction ? A Transaction is a logical unit of work that comprises one or more SQL statements executed by a single user. 91. When does a Transaction end ? When it is committed or Rollbacked. 92. What does COMMIT do ? COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions transactions that start only after transaction is committed. 93. What does ROLLBACK do ? ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction. 94. What is SAVE POINT ? For long transactions that contain many SQL statements, intermediate markers or savepoints can be declared which can be used to divide a transaction into smaller parts. This allows the option of later rolling back all work performed from the current point in the transaction to a declared savepoint within the transaction. 95. What is Read-Only Transaction ? A Read-Only transaction ensures that the results of each query executed in the transaction are consistant with respect to the same point in time. 96. What is the function of Optimizer ? The goal of the optimizer is to choose the most efficient way to execute a SQL statement. 97. What is Execution Plan ? The combinations of the steps the optimizer chooses to execute a statement is called an execution plan. 98. What are the different approaches used by Optimizer in choosing an execution plan ? Rule-based and Cost-based. 99. What are the factors that affect OPTIMIZER in choosing an Optimization approach ? The OPTIMIZER_MODE initialization parameter Statistics in the Data Dictionary the OPTIMIZER_GOAL parameter of the ALTER SESSION command hints in the statement. 100. What are the values that can be specified for OPTIMIZER MODE Parameter ? COST and RULE. |