Thursday, April 30, 2020

DBMS Quiz Questions with Answers 07

Database management systems DBMS TRUE or FALSE questions with answers explained for competitive and entrance exams.

TRUE or FALSE questions in DBMS 

1. We can optimize query plans by pushing a selection down an expression tree, but not by moving a selection up the tree.
(a) TRUE                                          (b) FALSE

View Answer

Answer: FALSE
Usually it is good to push down the selection operation. That is, we usually execute a selection operation as early as possible in a query plan.
It depends on the type of selection operation. For example, the selection condition with join operation may be done later than the projection operation.
employee ssn=essnessn(dependent)
In this case, the project operation is performed before because we may need only essn attribute from dependent table to proceed further.

2. The LRU buffer replacement algorithm should not be used for certain query operations, such as nested-loop join.
(a) TRUE                                         (b) FALSE

View Answer

Answer: TRUE
The goal of a replacement strategy for blocks in the buffer is to minimize accesses to the disk. For general-purpose programs, it is not possible to predict accurately, which blocks will be referenced. Therefore, operating systems use the past pattern of block references as a predictor of future references. The assumption generally made is that blocks that have been referenced recently are likely to be referenced again. Therefore, if a block must be replaced, the least recently referenced block is replaced. This approach is called the least recently used (LRU) block-replacement scheme.
As a database system is able to predict the pattern of future references more accurately than an operating system. MRU is the best strategy for nested-loop join. In MRU, for instance, in the join operation, we move the most recently used block of inner table to the disk once they are processed.

3. To process join operation R S, we can choose any join methods: nested-loop, indexed nested-loop, sort merge, hash, or hybrid. The only difference is their costs.
                   (a) TRUE                                          (b) FALSE

View Answer

Answer: FALSE
No. For example, we cannot perform indexed nested-loop join if we don’t have indices on the join attribute.
Queries involving a natural join may be processed in several ways, depending on the availability of indices and the form of physical storage for the relations;
If tables are sorted, we can use merge join
If join result is similar to that of Cartesian product, block nested-loop may be desirable.
[include reference]

4. Given an SQL query, there are often multiple ways of writing it in relational algebra.
                   (a) TRUE                                         (b) FALSE

View Answer

Answer: TRUE
An SQL query can be written in multiple ways in relational algebra. It is very true for the SQL queies that uses joins, selection of specific attributes and multiple conditions etc.

5. Triggers can operate on insertion, deletion, and updates.
                   (a) TRUE                                         (b) FALSE

View Answer

Answer: TRUE
Triggers define actions to be executed automatically when certain events occur and corresponding conditions are satisfied. Trigger works on Event-Condition-Action (ECA) model.
Triggers can be defined to run before or after the data manipulation language instructions. That is, actions can be triggered when events like insertion, deletion and updates happen.


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