SOFTWARE INTERVIEW TIPS

81. How will you monitor the space allocation?

By querying DBA_SEGMENT table/view

82. How will you monitor rollback segment status?

Querying the DBA_ROLLBACK_SEGS view
IN USE - Rollback Segment is on-line.
AVAILABLE - Rollback Segment available but not on-line.
OFF-LINE - Rollback Segment off-line
INVALID - Rollback Segment Dropped.
NEEDS RECOVERY - Contains data but need recovery or corrupted.
PARTLY AVAILABLE - Contains data from an unresolved transaction involving a
distributed database.

83. List the sequence of events when a large transaction that exceeds beyond its optimal value
when an entry wraps and causes the rollback segment to expand into another extend.

Transaction Begins.
An entry is made in the RES header for new transactions entry
Transaction acquires blocks in an extent of RBS
The entry attempts to wrap into second extent. None is available, so that the RBS must extent.
The RBS checks to see if it is part of its OPTIMAL size.
RBS chooses its oldest inactive segment.
Oldest inactive segment is eliminated.
RBS extents
The data dictionary tables for space management are updated.
Transaction Completes.

84. How can we plan storage for very large tables?

Limit the number of extents in the table
Separate table from its indexes.
Allocate sufficient temporary storage.
85. How will you estimate the space required by a non-clustered table?
Calculate the total header size
Calculate the available data space per data block
Calculate the combined column lengths of the average row
Calculate the total average row size.
Calculate the average number rows that can fit in a block
Calculate the number of blocks and bytes required for the table.
After arriving the calculation, add 10 % additional space to calculate the initial extent size for a working table.

86. It is possible to use raw devices as data files and what are the advantages over file system files?

Yes.
The advantages over file system files are that I/O will be improved because Oracle is bye-passing the kernel which writing into disk. Disk corruption will be very less.

87. What is a Control file?

Database's overall physical architecture is maintained in a file called control file. It will be used to maintain internal consistency and guide recovery operations. Multiple copies of control files are advisable.

88. How to implement the multiple control files for an existing database?

Shutdown the database
Copy one of the existing control file to new location
Edit Config ora file by adding new control filename
Restart the database.

89. What is redo log file mirroring? How can be achieved?

Process of having a copy of redo log files is called mirroring.
This can be achieved by creating group of log files together, so that LGWR will automatically writes them to all the members of the current on-line redo log group. If any one group fails then database automatically switch over to next group. It degrades performance.

90. What is advantage of having disk shadowing / mirroring?

Shadow set of disks save as a backup in the event of disk failure. In most operating systems if any disk failure occurs it automatically switchover to place of failed disk.
Improved performance because most OS support volume shadowing can direct file I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks.

91. What is use of rollback segments in Oracle database?

They allow the database to maintain read consistency between multiple transactions.

92. What is a rollback segment entry?

It is the set of before image data blocks that contain rows that are modified by a transaction.
Each rollback segment entry must be completed within one rollback segment.
A single rollback segment can have multiple rollback segment entries.

93. What is hit ratio?

It is a measure of well the data cache buffer is handling requests for data.
Hit Ratio = (Logical Reads - Physical Reads - Hits Misses)/ Logical Reads.

94. When will be a segment released?

When Segment is dropped.
When Shrink (RBS only)

When truncated (TRUNCATE used with drop storage option)

95. What are disadvantages of having raw devices?

We should depend on export/import utility for backup/recovery (fully reliable)
The tar command cannot be used for physical file backup, instead we can use dd command, which is less flexible and has limited recoveries.

96. List the factors that can affect the accuracy of the estimations?

- The space used transaction entries and deleted records, does not become free immediately after completion due to delayed cleanout.
- Trailing nulls and length bytes are not stored.
- Inserts of, updates to and deletes of rows as well as columns larger than a single data block, can cause fragmentation a chained row pieces.
Database Security & Administration

97. What is user Account in Oracle database?

A user account is not a physical structure in database but it is having important relationship to the objects in the database and will be having certain privileges.

98. How will you enforce security using stored procedures?

Don't grant user access directly to tables within the application.
Instead grant the ability to access the procedures that access the tables.
When procedure executed it will execute the privilege of procedures owner. Users cannot access tables except via the procedure.

99. What are the dictionary tables used to monitor a database space?

DBA_FREE_SPACE
DBA_SEGMENTS
DBA_DATA_FILES.
SQL*Plus Statements

100. What are the types of SQL statement?

Data Definition Language: CREATE, ALTER, DROP, TRUNCATE, REVOKE, NO AUDIT & COMMIT.
Data Manipulation Language: INSERT, UPDATE, DELETE, LOCK TABLE, EXPLAIN PLAN & SELECT.
Transactional Control: COMMIT & ROLLBACK
Session Control: ALTERSESSION & SET ROLE
System Control: ALTER SYSTEM.

101. What is a transaction?

Transaction is logical unit between two commits and commit and rollback.

102. What is difference between TRUNCATE & DELETE?

TRUNCATE commits after deleting entire table i.e., cannot be rolled back.
Database triggers do not fire on TRUNCATE
DELETE allows the filtered deletion. Deleted records can be rolled back or committed.
Database triggers fire on DELETE.

103. What is a join? Explain the different types of joins?

Join is a query, which retrieves related columns or rows from multiple tables.
Self Join - Joining the table with itself.
Equi Join - Joining two tables by equating two common columns.
Non-Equi Join - Joining two tables by equating two common columns.
Outer Join - Joining two tables in such a way that query can also retrieve rows that do not have corresponding join value in the other table.

104. What is the sub-query?

Sub-query is a query whose return values are used in filtering conditions of the main query.

105. What is correlated sub-query?

Correlated sub-query is a sub-query, which has reference to the main query.

106. Explain CONNECT BY PRIOR?

Retrieves rows in hierarchical order eg.
select empno, ename from emp where.

107. Difference between SUBSTR and INSTR?

INSTR (String1, String2 (n, (m)),
INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1.
SUBSTR (String1 n, m)
SUBSTR returns a character string of size m in string1, starting from n-th position of string1.

108. Explain UNION, MINUS, UNION ALL and INTERSECT?

INTERSECT - returns all distinct rows selected by both queries.
MINUS - returns all distinct rows selected by the first query but not by the second.
UNION - returns all distinct rows selected by either query
UNION ALL - returns all rows selected by either query, including all duplicates.

109. What is ROWID?

ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID.

110. What is the fastest way of accessing a row in a table?

Using ROWID.
CONSTRAINTS

111. What is an integrity constraint?

Integrity constraint is a rule that restricts values to a column in a table.

112. What is referential integrity constraint?

Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.

113. What is the usage of SAVEPOINTS?

SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.

114. What is ON DELETE CASCADE?

When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
115. What are the data types allowed in a table?

CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW.

116. What is difference between CHAR and VARCHAR2? What is the maximum SIZE allowed for each type?

CHAR pads blank spaces to the maximum length.
VARCHAR2 does not pad blank spaces.

For CHAR the maximum length is 255 and 2000 for VARCHAR2.

117. How many LONG columns are allowed in a table? Is it possible to use LONG columns in WHERE clause or ORDER BY?

Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.

118. What are the pre-requisites to modify datatype of a column and to add a column with NOT NULL constraint?

- To modify the datatype of a column the column must be empty.
- To add a column with NOT NULL constrain, the table must be empty.

119. Where the integrity constraints are stored in data dictionary?

The integrity constraints are stored in USER_CONSTRAINTS.

120. How will you activate/deactivate integrity constraints?

The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT.

121. If unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE?

It won't, Because SYSDATE format contains time attached with it.

122. What is a database link?

Database link is a named path through which a remote database can be accessed.

123. How to access the current value and next value from a sequence? Is it possible to access the current value in a session before accessing next value?

Sequence name CURRVAL, sequence name NEXTVAL. It is not possible. Only if you access next value in the session, current value can be accessed.

124. What is CYCLE/NO CYCLE in a Sequence?

CYCLE specifies that the sequence continue to generate values after reaching either maximum or minimum value. After pan-ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum.
NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value.EW?

- To protect some of the columns of a table from other users.
- To hide complexity of a query.
- To hide complexity of calculations.

126. Can a view be updated/inserted/deleted? If Yes - under what conditions?

A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible.

127. If a view on a single base table is manipulated will the changes be reflected on the base table?

If changes are made to the tables and these tables are the base tables of a view, then the changes will be reference on the view.