RDBMS CONCEPTS

•         What are database files, control files and log files. How many of these files should a database have at least? Why?

Database Files

The database files hold the actual data and are typically the largest in size. Depending on their sizes, the tables (and other objects) for all the user accounts can go in one database file—but that's not an ideal situation because it does not make the database structure very flexible for controlling access to storage for different users, putting the database on different disk drives, or backing up and restoring just part of the database.

You must have at least one database file but usually, more than one files are used. In terms of accessing and using the data in the tables and other objects, the number (or location) of the files is immaterial.

The database files are fixed in size and never grow bigger than the size at which they were created

Control Files

The control files and redo logs support the rest of the architecture. Any database must have at least one control file, although you typically have more than one to guard against loss. The control file records the name of the database, the date and time it was created, the location of the database and redo logs, and the synchronization information to ensure that all three sets of files are always in step. Every time you add a new database or redo log file to the database, the information is recorded in the control files.

Redo Logs

Any database must have at least two redo logs. These are the journals for the database; the redo logs record all changes to the user objects or system objects. If any type of failure occurs, the changes recorded in the redo logs can be used to bring the database to a consistent state without losing any committed transactions. In the case of non-data loss failure, Oracle can apply the information in the redo logs automatically without intervention from the DBA.

The redo log files are fixed in size and never grow dynamically from the size at which they were created.

•          What is ROWID?

            The ROWID is a unique database-wide physical address for every row on every table. Once assigned (when the row is first inserted into the database), it never changes until the row is deleted or the table is dropped.

The ROWID consists of the following three components, the combination of which uniquely identifies the physical storage location of the row.

Ø  Oracle database file number, which contains the block with the rows

Ø  Oracle block address, which contains the row

Ø  The row within the block (because each block can hold many rows)

The ROWID is used internally in indexes as a quick means of retrieving rows with a particular key value. Application developers also use it in SQL statements as a quick way to access a row once they know the ROWID

•             What is Oracle Block? Can two Oracle Blocks have the same address?

            Oracle "formats" the database files into a number of Oracle blocks when they are first created—making it easier for the RDBMS software to manage the files and easier to read data into the memory areas.

The block size should be a multiple of the operating system block size. Regardless of the block size, the entire block is not available for holding data; Oracle takes up some space to manage the contents of the block. This block header has a minimum size, but it can grow.

These Oracle blocks are the smallest unit of storage. Increasing the Oracle block size can improve performance, but it should be done only when the database is first created.

Each Oracle block is numbered sequentially for each database file starting at 1. Two blocks can have the same block address if they are in different database files.

•           What is database Trigger?

            A database trigger is a PL/SQL block that can defined to automatically execute for insert, update, and delete statements against a table. The trigger can e defined to execute once for the entire statement or once for every row that is inserted, updated, or deleted. For any one table, there are twelve events for which you can define database triggers. A database trigger can call database procedures that are also written in PL/SQL.

•            Name two utilities that Oracle provides, which are use for backup and recovery.

            Along with the RDBMS software, Oracle provides two utilities that you can use to back up and restore the database. These utilities are Export and Import.

The Export utility dumps the definitions and data for the specified part of the database to an operating system binary file. The Import utility reads the file produced by an export, recreates the definitions of objects, and inserts the data

If Export and Import are used as a means of backing up and recovering the database, all the changes made to the database cannot be recovered since the export was performed. The best you can do is recover the database to the time when the export was last performed.

•            What are stored-procedures? And what are the advantages of using them.

Stored procedures are database objects that perform a user defined operation. A stored procedure can have a set of compound SQL statements. A stored procedure executes the SQL commands and returns the result to the client. Stored procedures are used to reduce network traffic.

•              How are exceptions handled in PL/SQL? Give some of the internal exceptions' name

            PL/SQL exception handling is a mechanism for dealing with run-time errors encountered during procedure execution. Use of this mechanism enables execution to continue if the error is not severe enough to cause procedure termination.

The exception handler must be defined within a subprogram specification. Errors cause the program to raise an exception with a transfer of control to the exception-handler block. After the exception handler executes, control returns to the block in which the handler was defined. If there are no more executable statements in the block, control returns to the caller.

User-Defined Exceptions

PL/SQL enables the user to define exception handlers in the declarations area of subprogram specifications. User accomplishes this by naming an exception as in the following example:

                                ot_failure  EXCEPTION;

In this case, the exception name is ot_failure. Code associated with this handler is written in the EXCEPTION specification area as follows:

EXCEPTION

      when OT_FAILURE then

        out_status_code := g_out_status_code;

        out_msg         := g_out_msg;

The following is an example of a subprogram exception:

EXCEPTION

      when NO_DATA_FOUND then

        g_out_status_code := 'FAIL';

        RAISE ot_failure;

Within this exception is the RAISE statement that transfers control back to the ot_failure exception handler. This technique of raising the exception is used to invoke all user-defined exceptions.

System-Defined Exceptions

Exceptions internal to PL/SQL are raised automatically upon error. NO_DATA_FOUND is a system-defined exception. Table below gives a complete list of internal exceptions.

PL/SQL internal exceptions.

Exception Name	Oracle Error
CURSOR_ALREADY_OPEN	ORA-06511
DUP_VAL_ON_INDEX	ORA-00001
INVALID_CURSOR	ORA-01001
INVALID_NUMBER	ORA-01722
LOGIN_DENIED	ORA-01017
NO_DATA_FOUND	ORA-01403
NOT_LOGGED_ON	ORA-01012
PROGRAM_ERROR	ORA-06501
STORAGE_ERROR	ORA-06500
TIMEOUT_ON_RESOURCE	ORA-00051
TOO_MANY_ROWS	ORA-01422
TRANSACTION_BACKED_OUT	ORA-00061
VALUE_ERROR	ORA-06502
ZERO_DIVIDE	ORA-01476

In addition to this list of exceptions, there is a catch-all exception named OTHERS that traps all errors for which specific error handling has not been established.

•            Does PL/SQL support "overloading"? Explain

            The concept of overloading in PL/SQL relates to the idea that you can define procedures and functions with the same name. PL/SQL does not look only at the referenced name, however, to resolve a procedure or function call. The count and data types of formal parameters are also considered.

PL/SQL also attempts to resolve any procedure or function calls in locally defined packages before looking at globally defined packages or internal functions. To further ensure calling the proper procedure, you can use the dot notation. Prefacing a procedure or function name with the package name fully qualifies any procedure or function reference.

•            Tables derived from the ERD

            a) Are totally unnormalised

            b) Are always in 1NF

            c) Can be further denormalised

            d) May have multi-valued attributes

 (b) Are always in 1NF

•           Spurious tuples may occur due to

       i. Bad normalization 

      ii. Theta joins

     iii. Updating tables from join

            a) i & ii                                    b) ii & iii

            c) i & iii                                   d) ii & iii

 (a) i & iii  because  theta joins are joins made on keys that are not primary keys.

•            A B C is a set of attributes. The functional dependency is as follows

      AB  ->   B

      AC ->    C

         C ->    B

            a) is in 1NF

            b) is in 2NF

            c) is in 3NF

            d) is in BCNF

 (a) is in 1NF since (AC)⁺ = { A, B, C} hence AC is the primary key. Since         C  ->     B is a FD given, where neither C is a Key nor B is a prime attribute, this it is not in 3NF. Further B is not functionally dependent on key AC thus it is not in 2NF.  Thus the given FDs is in 1NF.

•            In mapping of ERD to DFD

            a) entities in ERD should correspond to an existing entity/store in DFD

            b) entity in DFD is converted to attributes of an entity in ERD

            c) relations in ERD has 1 to 1 correspondence to processes in DFD

            d) relationships in ERD has 1 to 1 correspondence to flows in DFD

 (a) entities in ERD should correspond to an existing entity/store in DFD

•            A dominant entity is the entity

            a) on the N side in a 1 : N relationship

            b) on the 1 side in a 1 : N relationship

            c) on either side in a 1 : 1 relationship

            d) nothing to do with 1 : 1 or 1 : N relationship

(b) on the 1 side in a 1 : N relationship

•            Select  'NORTH', CUSTOMER From CUST_DTLS Where REGION = 'N' Order By

CUSTOMER Union Select  'EAST', CUSTOMER From CUST_DTLS Where REGION = 'E' Order By CUSTOMER

The above is

            a) Not an error

            b) Error - the string in single quotes 'NORTH' and 'SOUTH'

            c) Error - the string should be in double quotes

            d) Error -  ORDER BY clause

(d)  Error - the ORDER BY clause. Since ORDER BY clause cannot be used in UNIONS

•            What is Storage Manager? 

It is a program module that provides the interface between the low-level data stored in database, application programs and queries submitted to the system.   

 

•              What is Buffer Manager?

It is a program module, which is responsible for fetching data from disk storage into main memory and deciding what data to be cache in memory. 

•              What is Transaction Manager?

It is a program module, which ensures that database, remains in a consistent state despite system failures and concurrent transaction execution proceeds without conflicting.

•              What is File Manager?

It is a program module, which manages the allocation of space on disk storage and data structure used to represent information stored on a disk.  

•            What is Authorization and Integrity manager?

It is the program module, which tests for the satisfaction of integrity constraint and checks the authority of user to access data.   

•            What are stand-alone procedures?

Procedures that are not part of a package are known as stand-alone because they independently defined. A good example of a stand-alone procedure is one written in a SQL*Forms application. These types of procedures are not available for reference from other Oracle tools. Another limitation of stand-alone procedures is that they are compiled at run time, which slows execution.

•             What are cursors give different types of cursors.

PL/SQL uses cursors for all database information accesses statements. The language supports the use two types of cursors

Ø  Implicit

Ø  Explicit

•         What is cold backup and hot backup (in case of Oracle)?

Ø  Cold Backup:

It is copying the three sets of files (database files, redo logs, and control file) when the instance is shut down. This is a straight file copy, usually from the disk directly to tape. You must shut down the instance to guarantee a consistent copy.

If a cold backup is performed, the only option available in the event of data file loss is restoring all the files from the latest backup. All work performed on the database since the last backup is lost.

Ø  Hot Backup:

Some sites (such as worldwide airline reservations systems) cannot shut down the database while making a backup copy of the files. The cold backup is not an available option.

So different means of backing up database must be used — the hot backup. Issue a SQL command to indicate to Oracle, on a tablespace-by-tablespace basis, that the files of the tablespace are to backed up. The users can continue to make full use of the files, including making changes to the data. Once the user has indicated that he/she wants to back up the tablespace files, he/she can use the operating system to copy those files to the desired backup destination.

The database must be running in ARCHIVELOG mode for the hot backup option.

If a data loss failure does occur, the lost database files can be restored using the hot backup and the online and offline redo logs created since the backup was done. The database is restored to the most consistent state without any loss of committed transactions.