DataBase Administrator (DBA) Interview Questions and Answers
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185. What does ROLLBACK do?
ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.
186. 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.
187. What are the values that can be specified for OPTIMIZER MODE Parameter?
COST and RULE.
188. What is COST-based approach to optimization?
Considering available access paths and determining the most efficient execution plan based on statistics in the data dictionary for the tables accessed by the statement and their associated clusters and indexes.
189. 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.
190. What is RULE-based approach to optimization?
Choosing an executing plan based on the access paths available and the ranks of these access paths.
191. What are the values that can be specified for OPTIMIZER_GOAL parameter of the ALTER SESSION Command?
CHOOSE,ALL_ROWS,FIRST_ROWS and RULE.
192. Define Transaction?
A Transaction is a logical unit of work that comprises one or more SQL statements executed by a single user.
193. What is Read-Only Transaction?
A Read-Only transaction ensures that the results of each query executed in the transaction are consistent with respect to the same point in time.
193. What is a deadlock?
Explain . Two processes waiting to update the rows of a table which are locked by the other process then deadlock arises. In a database environment this will often happen because of not issuing proper row lock commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically. These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally.
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