A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together.
What is a View?
A view is a virtual table. Every view has a Query attached to it. (The Query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.)
What are Schema Objects?
Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links.
What are the components of Physical database structure of Oracle Database?
ORACLE database is comprised of three types of files. One or more Data files, two are more Redo Log files, and one or more Control files.
What are the components of Logical database structure of ORACLE database?
Tablespaces and the Database's Schema Objects.
What is SYSTEM tablespace and when is it Created?
Every ORACLE database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.
Explain the relationship among Database, Tablespace and Data file?
Each databases logically divided into one or more tablespaces One or more data files are explicitly created for each tablespace.
What is schema?
A schema is collection of database objects of a User.
Can objects of the same Schema reside in different tablespaces?
Yes.
Can a Tablespace hold objects from different Schemes?
Yes.
What is Table?
A table is the basic unit of data storage in an ORACLE database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.
Does View contain Data?
Views do not contain or store data.
Can a View based on another View?
Yes.
What are the advantages of Views?
Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table. Hide data complexity. Simplify commands for the user. Present the data in a different perspective from that of the base table. Store complex queries.
What is a Sequence?
A sequence generates a serial list of unique numbers for numerical columns of a database's tables.
What is a Synonym?
A synonym is an alias for a table, view, sequence or program unit.
What are the types of Synonyms?
There are two types of Synonyms Private and Public.
What is a Private Synonyms?
A Private Synonyms can be accessed only by the owner.
What is a Public Synonyms?
Public synonyms can be accessed by any user on the database.
What are synonyms used for?
Synonyms are used to : Mask the real name and owner of an object. Provide public access to an object Provide location transparency for tables, views or program units of a remote database. Simplify the SQL statements for database users.
What is an Index?
An Index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table.
How are Indexes Update?
Indexes are automatically maintained and used by ORACLE. Changes to table data are automatically incorporated into all relevant indexes.
What are Clusters?
Clusters are groups of one or more tables physically stores together to share common columns and are often used together.
What is cluster Key?
The related columns of the tables in a cluster is called the Cluster Key.
What is Index Cluster?
A Cluster with an index on the Cluster Key.
What is Hash Cluster?
A row is stored in a hash cluster based on the result of applying a hash function to the row's cluster key value. All rows with the same hash key value are stores together on disk.
When can Hash Cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.
What is Database Link?
A database link is a named object that describes a "path" from one database to another.
What are the types of Database Links?
Private Database Link, Public Database Link & Network Database Link.
What is Private Database Link?
Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner's views or procedures.
What is Public Database Link?
Public database link is created for the special user group PUBLIC. A public database link can be used when any user in the associated database specifies a global object name in a SQL statement or object definition.
What is Network Database link?
Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.
What is Data Block?
ORACLE database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.
How to define Data Block size?
A data block size is specified for each ORACLE database when the database is created. A database users and allocated free database space in ORACLE datablocks. Block size is specified in INIT.ORA file and cann't be changed latter.
What is Row Chaining?
In Circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs, the data for the row is stored in a chain of data block (one or more) reserved for that segment.
What is an Extent?
An Extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
What is a Segment?
A segment is a set of extents allocated for a certain logical structure.
What are the different types of Segments?
Data Segment, Index Segment, Rollback Segment and Temporary Segment.
What is a Data Segment?
Each Non-clustered table has a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment.
What is an Index Segment?
Each Index has an Index segment that stores all of its data.
What is Rollback Segment?
A Database contains one or more Rollback Segments to temporarily store "undo" information.
What are the uses of Rollback Segment?
Rollback Segments are used: To generate read-consistent database information during database recovery to rollback uncommitted transactions for users.
What is a Temporary Segment?
Temporary segments are created by ORACLE when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.
What is a Data File?
Every ORACLE database has one or more physical data files. A database's data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database.
What are the Characteristics of Data Files?
A data file can be associated with only one database. Once created a data file can't change size. One or more data files form a logical unit of database storage called a tablespace.
What is a Redo Log?
The set of Redo Log files for a database is collectively known as the database's redo log.
What is the function of Redo Log?
The Primary function of the redo log is to record all changes made to data.
What is the use of Redo Log Information?
The Information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files.
What does a Control file Contain?
A Control file records the physical structure of the database. It contains the following information.
Database Name Names and locations of a database's files and redolog files. Time stamp of database creation.
What is the use of Control File?
When an instance of an ORACLE database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery.
What is a Data Dictionary?
The data dictionary of an ORACLE database is a set of tables and views that are used as a read-only reference about the database. It stores information about both the logical and physical structure of the database, the valid users of an ORACLE database, integrity constraints defined for tables in the database and space allocated for
a schema object and how much of it is being used.
What is an Integrity Constrains?
An integrity constraint is a declarative way to define a business rule for a column of a table.
Can an Integrity Constraint be enforced on a table if some existing table data does not satisfy the constraint?
No.
Describe the different type of Integrity Constraints supported by ORACLE?
NOT NULL Constraint - Disallows NULLs in a table's column.
UNIQUE Constraint - Disallows duplicate values in a column or set of columns.
PRIMARY KEY Constraint - Disallows duplicate values and Nulls in a column or set of columns.
FOREIGN KEY Constrain - Require each value in a column or set of columns match a value in a related table's UNIQUE or PRIMARY KEY.
CHECK Constraint - Disallows values that do not satisfy the logical expression of the constraint.
What is difference between UNIQUE constraint and PRIMARY KEY constraint?
A column defined as UNIQUE can contain Nulls while a column defined as PRIMARY KEY can't contain Nulls.
Describe Referential Integrity?
A rule defined on a column (or set of columns) in one table that allows the insert or update of a row only if the value for the column or set of columns (the dependent value) matches a value in a column of a related table (the referenced value). It also specifies the type of data manipulation allowed on referenced data and the action to be performed on dependent data as a result of any action on referenced data.
What are the Referential actions supported by FOREIGN KEY integrity constraint?
UPDATE and DELETE Restrict - A referential integrity rule that disallows the update or deletion of referenced data. DELETE Cascade - When a referenced row is deleted all associated
dependent rows are deleted.
What is self-referential integrity constraint?
If a foreign key reference a parent key of the same table is called self-referential integrity constraint.
What are the Limitations of a CHECK Constraint?
The condition must be a Boolean expression evaluated using the values in the row being inserted or updated and can't contain sub queries, sequence, the SYSDATE,UID,USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.
What is the maximum number of CHECK constraints that can be defined on a column?
No Limit.
What constitute an ORACLE Instance?
SGA and ORACLE background processes constitute an ORACLE instance. (Or) Combination of memory structure and background process.
What is SGA?
The System Global Area (SGA) is a shared memory region allocated by ORACLE that contains data and control information for one ORACLE instance.
What are the components of SGA?
Database buffers, Redo Log Buffer the Shared Pool and Cursors.
What do Database Buffers contain?
Database buffers store the most recently used blocks of database data. It can also contain modified data that has not yet been permanently written to disk.
What do Redo Log Buffers contain?
Redo Log Buffer stores redo entries a log of changes made to the database.
What is Shared Pool?
Shared Pool is a portion of the SGA that contains shared memory constructs such as shared SQL areas.
What is Shared SQL Area?
A Shared SQL area is required to process every unique SQL statement submitted to a database and contains information such as the parse tree and execution plan for the corresponding statement.
What is Cursor?
A Cursor is a handle (a name or pointer) for the memory associated with a specific statement.
What is PGA?
Program Global Area (PGA) is a memory buffer that contains data and control information for a server process.
What is User Process?
A user process is created and maintained to execute the software code of an application program. It is a shadow process created automatically to facilitate communication between the user and the server process.
What is Server Process?
Server Process handles requests from connected user process. A server process is in charge of communicating with the user process and interacting with ORACLE carry out requests of the associated user process.
What are the two types of Server Configurations?
Dedicated Server Configuration and Multi-threaded Server Configuration.
What is Dedicated Server Configuration?
In a Dedicated Server Configuration a Server Process handles requests for a Single User Process.
What is a Multi-threaded Server Configuration?
In a Multi-threaded Server Configuration many user processes share a group of server process.
What is a Parallel Server option in ORACLE?
A configuration for loosely coupled systems where multiple instance share a single physical database is called Parallel Server.
Name the ORACLE Background Process?
DBWR - Database Writer.
LGWR - Log Writer
CKPT - Check Point
SMON - System Monitor
PMON - Process Monitor
ARCH - Archiver
RECO - Recover
Dnnn - Dispatcher, and
LCKn - Lock
Snnn - Server.
What Does DBWR do?
Database writer writes modified blocks from the database buffer cache to the data files.
When Does DBWR write to the database?
DBWR writes when more data needs to be read into the SGA and too few database buffers are free. The least recently used data is written to the data files first. DBWR also writes when Checkpoint occurs.
What does LGWR do?
Log Writer (LGWR) writes redo log entries generated in the redo log buffer of the SGA to on-line Redo Log File.
When does LGWR write to the database?
LGWR writes redo log entries into an on-line redo log file when transactions commit and the log buffer files are full.
What is the function of checkpoint (CKPT)?
The Checkpoint (CKPT) process is responsible for signaling DBWR at checkpoints and updating all the data files and control files of the database.
What are the functions of SMON?
System Monitor (SMON) performs instance recovery at instance start-up. In a multiple instance system (one that uses the Parallel Server), SMON of one instance can also perform instance recovery for other instance that have failed SMON also cleans up temporary segments that are no longer in use and recovers dead transactions skipped during crash and instance recovery because of file-read or off-line errors. These transactions are eventually recovered by SMON when the tablespace or file is brought back on-line SMON also coalesces free extents within the database to make free space contiguous and easier to allocate.
What are functions of PMON?
Process Monitor (PMON) performs process recovery when a user process fails PMON is responsible for cleaning up the cache and Freeing resources that the process was using PMON also checks on dispatcher and server processes and restarts them if they have failed.
What is the function of ARCH?
Archiver (ARCH) copies the on-line redo log files to archival storage when they are full. ARCH is active only when a database's redo log is used in ARCHIVELOG mode.
What is function of RECO?
RECOver (RECO) is used to resolve distributed transactions that are pending due to a network or system failure in a distributed database. At timed intervals, the local RECO attempts to connect to remote databases and automatically complete the commit or rollback of the local portion of any pending distributed transactions.
What is the function of Dispatcher (Dnnn)?
Dispatcher (Dnnn) process is responsible for routing requests from connected user processes to available shared server processes and returning the responses back to the appropriate user processes.
How many Dispatcher Processes are created?
At least one Dispatcher process is created for every communication protocol in use.
What is the function of Lock (LCKn) Process?
Lock (LCKn) are used for inter-instance locking when the ORACLE Parallel Server option is used.
What is the maximum number of Lock Processes used?
Though a single LCK process is sufficient for most Parallel Server systems upto Ten Locks (LCK0,....LCK9) are used for inter-instance locking.
Define Transaction?
A Transaction is a logical unit of work that comprises one or more SQL statements executed by a single user.
When does a Transaction end?
When it is committed or Roll backed.
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.
What does ROLLBACK do?
ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.
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.
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.
What is the function of Optimizer?
The goal of the optimizer is to choose the most efficient way to execute a SQL statement.
What is Execution Plan?
The combination of the steps the optimizer chooses to execute a statement is called an execution plan.
What are the different approaches used by Optimizer in choosing an execution plan?
Rule-based and Cost-based.
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.
What are the values that can be specified for OPTIMIZER MODE Parameter?
COST and RULE.
Will the Optimizer always use COST-based approach if OPTIMIZER_MODE is set to "Cost'?
Presence of statistics in the data dictionary for atleast one of the tables accessed by the SQL statements is necessary for the OPTIMIZER to use COST-based approach. Otherwise OPTIMIZER chooses RULE-
based approach.
What is the effect of setting the value of OPTIMIZER_MODE to ‘RULE’?
This value causes the optimizer to choose the rule based approach for all SQL statements issued to the instance regardless of the presence of statistics.
What are the values that can be specified for OPTIMIZER_GOAL parameter of the ALTER SESSION Command?
CHOOSE, ALL_ROWS, FIRST_ROWS and RULE.
What is the effect of setting the value "CHOOSE" for OPTIMIZER_GOAL, parameter of the ALTER SESSION Command?
The Optimizer chooses Cost based approach and optimizes with the goal of best throughput if statistics for at least one of the tables accessed by the SQL statement exist in the data dictionary. Otherwise the OPTIMIZER chooses RULE based approach.
What is the effect of setting the value "ALL_ROWS" for OPTIMIZER_GOAL parameter of the ALTER SESSION command?
This value causes the optimizer to the cost-based approach for all SQL statements in the session regardless of the presence of statistics and to optimize with a goal of best throughput.
What is the effect of setting the value 'FIRST_ROWS' for OPTIMIZER_GOAL parameter of the ALTER SESSION command?
This value causes the optimizer to use the cost-based approach for all SQL statements in the session regardless of the presence of statistics and to optimize with a goal of best response time.
What is the effect of setting the 'RULE' for OPTIMIER_GOAL parameter of the ALTER SESSION Command?
This value causes the optimizer to choose the rule-based approach for all SQL statements in a session regardless of the presence of statistics.
What is RULE-based approach to optimization?
Choosing an executing plan based on the access paths available and the ranks of these access paths.
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.
What are the different types of PL/SQL program units that can be defined and stored in ORACLE database?
Procedures and Functions, Packages and Database Triggers.
What is a Procedure?
A Procedure consist of a set of SQL and PL/SQL statements that are grouped together as a unit to solve a specific problem or perform a set of related tasks.
What is difference between Procedures and Functions?
A Function returns a value to the caller where as a Procedure does not.
What is a Package?
A Package is a collection of related procedures, functions, variables and other package constructs together as a unit in the database.
What are the advantages of having a Package?
Increased functionality (for example, global package variables can be declared and used by any proecdure in the package) and performance (for example all objects of the package are parsed compiled, and loaded into memory once)
What is Database Trigger?
A Database Trigger is procedure (set of SQL and PL/SQL statements) that is automatically executed as a result of an insert in, update to, or delete from a table.
What are the uses of Database Trigger?
Database triggers can be used to automatic data generation, audit data modifications, enforce complex Integrity constraints, and customize complex security
authorizations.
What are the differences between Database Trigger and Integrity constraints?
A declarative integrity constraint is a statement about the database that is always true. A constraint applies to existing data in the table and any statement that manipulates the table.
A trigger does not apply to data loaded before the definition of the trigger; therefore, it does not guarantee all data in a table conforms to the rules established by an associated trigger.
A trigger can be used to enforce transitional constraints where as a declarative integrity constraint cannot be used.
DATABASE SECURITY
What are Roles?
Roles are named groups of related privileges that are granted to users or other roles.
What is the use of Roles?
REDUCED GRANTING OF PRIVILEGES - Rather than explicitly granting the same set of privileges to many users a database administrator can grant the privileges for a group of related users granted to a role and then grant only the role to each member of the group.
DYNAMIC PRIVILEGE MANAGEMENT - When the privileges of a group must change, only the privileges of the role need to be modified. The security domains of all users granted the group's role automatically reflect the changes made to the role.
SELECTIVE AVAILABILITY OF PRIVILEGES - The roles granted to a user can be selectively enable (available for use) or disabled (not available for use). This allows specific control of a user's privileges in any given situation.
APPLICATION AWARENESS - A database application can be designed to automatically enable and disable selective roles when a user attempts to use the application.
How to prevent unauthorized use of privileges granted to a Role?
By creating a Role with a password.
What is default tablespace?
The Tablespace to contain schema objects created without specifying a tablespace name.
What is Tablespace Quota?
The collective amount of disk space available to the objects in a schema on a particular tablespace.
What is a profile?
Each database user is assigned a Profile that specifies limitations on various system resources available to the user.
What are the system resources that can be controlled through Profile?
The number of concurrent sessions the user can establish the CPU processing time available to the user's session the CPU processing time available to a single call to ORACLE made by a SQL statement the amount of logical I/O available to the user's session the amout of logical I/O available to a single call to ORACLE made by a SQL statement the allowed amount of idle time for the user's session the allowed amount of connect time for the user's session.
What is Auditing?
Monitoring of user access to aid in the investigation of database use.
What are the different Levels of Auditing?
Statement Auditing, Privilege Auditing and Object Auditing.
What is Statement Auditing?
Statement auditing is the auditing of the powerful system privileges without regard to specifically named objects.
What is Privilege Auditing?
Privilege auditing is the auditing of the use of powerful system privileges without regard to specifically named objects.
What is Object Auditing?
Object auditing is the auditing of accesses to specific schema objects without regard to user.
DISTRIBUTED PROCESSING AND DISTRIBUTED DATABASES
What is Distributed database?
A distributed database is a network of databases managed by multiple database servers that appears to a user as single logical database. The data of all databases in the distributed database can be simultaneously accessed and modified.
What are the steps involved in Database Startup?
Start an instance, Mount the Database and Open the Database.
What are the steps involved in Database Shutdown?
Close the Database; Dismount the Database and Shutdown the Instance.
What is Restricted Mode of Instance Startup?
An instance can be started in (or later altered to be in) restricted mode so that when the database is open connections are limited only to those whose user accounts have been granted the RESTRICTED SESSION system privilege.
What are the different modes of mounting a Database with the Parallel Server?
Exclusive Mode If the first instance that mounts a database does so in exclusive mode, only that Instance can mount the database. Parallel Mode If the first instance that mounts a database is started in parallel mode, other instances that are started in parallel mode can also mount the database.
What is Two-Phase Commit?
Two-phase commit is mechanism that guarantees a distributed transaction either commits on all involved nodes or rolls back on all involved nodes to maintain data consistency across the global distributed database. It has two phases, a Prepare Phase and a Commit Phase.
Describe two phases of Two-phase commit?
Prepare phase - The global coordinator (initiating node) ask a participants to prepare (to promise to commit or rollback the transaction, even if there is a failure)
Commit - Phase - If all participants respond to the coordinator that they are prepared, the coordinator asks all nodes to commit the transaction, if all participants cannot prepare, the coordinator asks all nodes to roll back the transaction.
What is the mechanism provided by ORACLE for table replication?
Snapshots and SNAPSHOT Logs
What is a SNAPSHOT?
Snapshots are read-only copies of a master table located on a remote node which is periodically refreshed to reflect changes made to the master table.
What is a SNAPSHOT LOG?
A snapshot log is a table in the master database that is associated with the master table. ORACLE uses a snapshot log to track the rows that have been updated in the master table. Snapshot logs are used in updating the snapshots based on the master table.
What is Full Backup?
A full backup is an operating system backup of all data files, on-line redo log files and control file that constitute ORACLE database and the parameter.
Can Full Backup be performed when the database is open?
No.
What is Partial Backup?
A Partial Backup is any operating system backup short of a full backup, taken while the database is open or shut down.
What is On-line Redo Log?
The On-line Redo Log is a set of tow or more on-line redo files that record all committed changes made to the database. Whenever a transaction is committed, the corresponding redo entries temporarily stores in redo log buffers of the SGA are written to an on-line redo log file by the background process LGWR. The on-line redo log files are used in cyclical fashion.
What is Mirrored on-line Redo Log?
A mirrored on-line redo log consists of copies of on-line redo log files physically located on separate disks; changes made to one member of the group are made to all members.
What is Archived Redo Log?
Archived Redo Log consists of Redo Log files that have archived before being reused.
What are the advantages of operating a database in ARCHIVELOG mode over operating it in NO ARCHIVELOG mode?
Complete database recovery from disk failure is possible only in ARCHIVELOG mode. Online database backup is possible only in ARCHIVELOG mode.
What is Log Switch?
The point at which ORACLE ends writing to one online redo log file and begins writing to another is called a log switch.
What are the steps involved in Instance Recovery?
Rolling forward to recover data that has not been recorded in data files yet has been recorded in the on-line redo log, including the contents of rollback segments. Rolling back transactions that have been explicitly rolled back or have not been committed as indicated by the rollback segments regenerated in step a. Releasing any resources (locks) held by transactions in process at the time of the failure. Resolving any pending distributed transactions undergoing a two-phase commit at the time of the instance failure.
What is a Database instance?
A database instance (Server) is a set of memory structure and background processes that access a set of database files. The process can be shared by all users.
The memory structure that is used to store most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file.
What is Parallel Server?
Multiple instances accessing the same database (Only In Multi-CPU environments)
What is a Schema?
The set of objects owned by user account is called the schema.
What is an Index? How it is implemented in Oracle Database?
An index is a database structure used by the server to have direct access of a row in a table.
An index is automatically created when a unique of primary key constraint clause is specified in create table command.
What is cluster?
Group of tables physically stored together because they share common columns and are often used together is called Cluster.
What is a cluster Key?
The related columns of the tables are called the cluster key. The cluster key is indexed using a cluster index and its value is stored only once for multiple tables in the cluster.
What are the basic elements of Base configuration of an oracle Database?
It consists of one or more data files. one or more control files. Two or more redo log files. The Database contains multiple users/schemas one or more rollback segments one or more tablespaces Data dictionary tables User objects (table,indexes,views etc.,) The server that access the database consists of SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool) SMON (System MONito) PMON (Process MONitor) LGWR (LoG Write) DBWR (Data Base Write) ARCH (ARCHiver) CKPT (Check Point) RECO Dispatcher User Process with associated PGS
What is a deadlock?
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 these processes being killed externally. MEMORY MANAGEMENT
What is SGA? How it is different from 6.0 and 7.0?
The System Global Area in a Oracle database is the area in memory to facilitates the transfer of information between users. It holds the most recently requested structural information between users. It holds the most recently requested structural information about the database.
The structure is Database buffers, Dictionary cache, Redo Log Buffer and Shared SQL pool (ver 7.0 only) area.
What is a Shared SQL pool?
The data dictionary cache is stored in an area in SGA called the Shared SQL Pool. This will allow sharing of parsed SQL statements among concurrent users.
What is mean by Program Global Area (PGA)?
It is area in memory that is used by a Single Oracle User Process.
What is a data segment?
Data segment are the physical areas within a database block in which the data associated with tables and clusters are stored.
What are the factors causing the reparsing of SQL statements in SGA?
Due to insufficient Shared SQL pool size. Monitor the ratio of the reloads takes place while executing SQL statements. If the ratio is greater than 1 then increase the SHARED_POOL_SIZE. LOGICAL & PHYSICAL ARCHITECTURE OF DATABASE.
What is Database Buffers?
Database buffers are cache in the SGA used to hold the data blocks that are read from the data segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
What is dictionary cache?
Dictionary cache is information about the databse objects stored in a data dictionary table.
What is meant by recursive hints?
Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of Data Dictionary Cache.
What is meant by redo log buffer?
Changes made to entries are written to the on-line redo log files. So that they can be used in roll forward operations during database recoveries. Before writing them into the redo log files, they will first bring to redo log buffers in SGA and LGWR will write into files frequently. LOG_BUFFER parameter will decide the size.
How will you swap objects into a different table space for an existing database?
Export the user Perform import using the command imp system/manager file=export.dmp indexfile=newrite.sql.
This will create all definitions into newfile.sql. Drop necessary objects.Run the script newfile.sql after altering the tablespaces. Import from the backup for the necessary objects.
List the Optional Flexible Architecture (OFA) of Oracle database? How can we organize the tablespaces in Oracle database to have maximum performance?
SYSTEM - Data dictionary tables. DATA - Standard operational tables. DATA2- Static tables used for standard operations INDEXES - Indexes for Standard operational tables. INDEXES1 - Indexes of static tables used for standard operations. TOOLS - Tools table. TOOLS1 - Indexes for tools table. RBS - Standard Operations Rollback Segments, RBS1,RBS2 - Additional/Special Rollback segments. TEMP - Temporary purpose tablespace TEMP_USER - Temporary tablespace for users. USERS - User tablespace.
How will you force database to use particular rollback segment?
SET TRANSACTION USE ROLLBACK SEGMENT rbs_name.
What is meant by free extent?
A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free.
Which parameter in Storage clause will reduce no. of rows per block?
PCTFREE parameter. Row size also reduces no of rows per block.
What is the significance of having storage clause?
We can plan the storage for a table as how much initial extents are required, how much can be extended next, how much % should leave free for managing row updations etc.
How does Space allocation table place within a block?
Each block contains entries as follows Fixed block header Variable block header Row Header,row date (multiple rows may exists) PCTEREE (% of free space for row updation in future)
What is the role of PCTFREE parameter is Storage clause?
This is used to reserve certain amount of space in a block for expansion of rows.
What is the OPTIMAL parameter ?
It is used to set the optimal length of a rollback segment.
What is the functionality of SYSTEM table space?
To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage.
How will you create multiple rollback segments in a database?
Create a database which implicitly creates a SYSTEM Rollback Segment in a SYSTEM tablespace.
Create a Second Rollback Segment name R0 in the SYSTEM tablespace.
Make new rollback segment available (After shutdown, modify init.ora file and Start database)
Create other tablespaces (RBS) for rollback segments.
Deactivate Rollback Segment R0 and activate the newly created rollback segments.
How the space utilization takes place within rollback segments?
It will try to fit the transaction in a cyclic fashion to all existing extents. Once it found an extent is in use then it forced to acquire a new extent (No. of extents is based on the optimal size)
Why query fails sometimes?
Rollback segment dynamically extent to handle larger transactions entry loads.
A single transaction may wipeout all available free space in the Rollback Segment Tablespace. This prevents other user using Rollback segments.
How will you monitor the space allocation?
By querying DBA_SEGMENT table/view.
How will you monitor rollback segment status?
Querying the DBA_ROLLBACK_SEGS viewIN USE - Rollback Segment is on-line.AVAILABLE - Rollback Segment available but not on-line.OFF-LINE - Rollback Segment off-lineINVALID - Rollback Segment Dropped.NEEDS RECOVERY - Contains data but need recovery or corupted.PARTLY AVAILABLE - Contains data from an unresolved transaction involving a distributed database.
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 extentsThe Data dictionary table for space management are updated.
Transaction Completes.
How can we plan storage for very large tables?
Limit the number of extents in the tableSeparate Table from its indexes.Allocate sufficient temporary storage.
How will you estimate the space required by a non-clustered table?
Calculate the total header sizeCalculate the available dataspace per data blockCalculate the combined column lengths of the average rowCalculate the total average row size.Calculate the average number rows that can fit in a blockCalculate 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.
It is possible to use raw devices as data files and what are the advantages over system files?
Yes. The advantages over file system files. I/O will be improved because Oracle is bye-passing the kernel which writing into disk. Disk Corruption will be very less.
Oracle Interview questions
Oracle Concepts and Architecture Database Structures
1. What are the components of physical database structure of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two or more redo log files, and one or more control files.
2. What are the components of logical database structure of Oracle database?
There are tablespaces and database's schema objects.
3. What is a tablespace?
A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together.
4. What is SYSTEM tablespace and when is it created?
Every Oracle database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.
5. Explain the relationship among database, tablespace and data file.
Each databases logically divided into one or more tablespaces one or more data files are explicitly created for each tablespace.
6. What is schema?
A schema is collection of database objects of a user.
7. What are Schema Objects?
Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links.
8. Can objects of the same schema reside in different tablespaces?
Yes.
9. Can a tablespace hold objects from different schemes?
Yes.
10. What is Oracle table?
A table is the basic unit of data storage in an Oracle database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.
11. What is an Oracle view?
A view is a virtual table. Every view has a query attached to it. (The query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.)
12. Do a view contain data?
Views do not contain or store data.
13. Can a view based on another view?
Yes.
14. What are the advantages of views?
- Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.
- Hide data complexity.
- Simplify commands for the user.
- Present the data in a different perspective from that of the base table.
- Store complex queries.
15. What is an Oracle sequence?
A sequence generates a serial list of unique numbers for numerical columns of a database's tables.
16. What is a synonym?
A synonym is an alias for a table, view, sequence or program unit.
17. What are the types of synonyms?
There are two types of synonyms private and public.
18. What is a private synonym?
Only its owner can access a private synonym.
19. What is a public synonym?
Any database user can access a public synonym.
20. What are synonyms used for?
- Mask the real name and owner of an object.
- Provide public access to an object
- Provide location transparency for tables, views or program units of a remote database.
- Simplify the SQL statements for database users.
21. What is an Oracle index?
An index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table.
22. How are the indexes update?
Indexes are automatically maintained and used by Oracle. Changes to table data are automatically incorporated into all relevant indexes.
23. What are clusters?
Clusters are groups of one or more tables physically stores together to share common columns and are often used together.
24. What is cluster key?
The related columns of the tables in a cluster are called the cluster key.
25. What is index cluster?
A cluster with an index on the cluster key.
26. What is hash cluster?
A row is stored in a hash cluster based on the result of applying a hash function to the row's cluster key value. All rows with the same hash key value are stores together on disk.
27. When can hash cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.
28. What is database link?
A database link is a named object that describes a "path" from one database to another.
29. What are the types of database links?
Private database link, public database link & network database link.
30. What is private database link?
Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner's views or procedures.
31. What is public database link?
Public database link is created for the special user group PUBLIC. A public database link can be used when any user in the associated database specifies a global object name in a SQL statement or object definition.
32. What is network database link?
Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.
33. What is data block?
Oracle database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.
34. How to define data block size?
A data block size is specified for each Oracle database when the database is created. A database users and allocated free database space in Oracle data blocks. Block size is specified in init.ora file and cannot be changed latter.
35. What is row chaining?
In circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs, the data for the row is stored in a chain of data block (one or more) reserved for that segment.
36. What is an extent?
An extent is a specific number of contiguous data blocks, obtained in a single allocation and used to store a specific type of information.
37. What is a segment?
A segment is a set of extents allocated for a certain logical structure.
38. What are the different types of segments?
Data segment, index segment, rollback segment and temporary segment.
39. What is a data segment?
Each non-clustered table has a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment.
40. What is an index segment?
Each index has an index segment that stores all of its data.
41. What is rollback segment?
A database contains one or more rollback segments to temporarily store "undo" information.
42. What are the uses of rollback segment?
To generate read-consistent database information during database recovery and to rollback uncommitted transactions by the users.
43. What is a temporary segment?
Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.
44. What is a datafile?
Every Oracle database has one or more physical data files. A database's data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database.
45. What are the characteristics of data files?
A data file can be associated with only one database. Once created a data file can't change size. One or more data files form a logical unit of database storage called a tablespace.
46. What is a redo log?
The set of redo log files for a database is collectively known as the database redo log.
47. What is the function of redo log?
The primary function of the redo log is to record all changes made to data.
48. What is the use of redo log information?
The information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files.
49. What does a control file contains?
- Database name
- Names and locations of a database's files and redolog files.
- Time stamp of database creation.
50. What is the use of control file?
When an instance of an Oracle database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery.
Data Base Administration
51. What is a database instance? Explain.
A database instance (Server) is a set of memory structure and background processes that access a set of database files. The processes can be shared by all of the users. The memory structure that is used to store the most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file.
52. What is Parallel Server?
Multiple instances accessing the same database (only in multi-CPU environments)
53. What is a schema?
The set of objects owned by user account is called the schema.
54. What is an index? How it is implemented in Oracle database?
An index is a database structure used by the server to have direct access of a row in a table. An index is automatically created when a unique of primary key constraint clause is specified in create table command.
55. What are clusters?
Group of tables physically stored together because they share common columns and are often used together is called cluster.
56. What is a cluster key?
The related columns of the tables are called the cluster key. The cluster key is indexed using a cluster index and its value is stored only once for multiple tables in the cluster.
57. What are the basic elements of base configuration of an Oracle database?
It consists of:-
one or more data files.
one or more control files.
two or more redo log files.
The Database contains
multiple users/schemas
one or more rollback segments
one or more tablespaces
Data dictionary tables
User objects (table,indexes,views etc.,)
The server that access the database consists of
SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool)
SMON (System MONitor)
PMON (Process MONitor)
LGWR R (LoG Write)
DBWR (Data Base Writer)
ARCH (ARCHiver)
CKPT (Check Point)
RECO
Dispatcher
User Process with associated PGS.
58. What is a deadlock? Explain.
Two processes waiting to update the rows of a table, which are locked by other processes then deadlock arises. In a database environment this will often happen because of not issuing the 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.
Memory Management
59. What is SGA?
The System Global Area in an Oracle database is the area in memory to facilitate the transfer of information between users. It holds the most recently requested structural information between users. It holds the most recently requested structural information about the database. The structure is database buffers, dictionary cache, redo log buffer and shared pool area.
60. What is a shared pool?
The data dictionary cache is stored in an area in SGA called the shared pool. This will
allow sharing of parsed SQL statements among concurrent users.
61. What is mean by Program Global Area (PGA)?
It is area in memory that is used by a single Oracle user process.
62. What is a data segment?
Data segment are the physical areas within a database block in which the data associated with tables and clusters are stored.
63. What are the factors causing the reparsing of SQL statements in SGA?
Due to insufficient shared pool size.
Monitor the ratio of the reloads takes place while executing SQL statements. If the ratio is greater than 1 then increase the SHARED_POOL_SIZE.
Database Logical & Physical Architecture
64. What is Database Buffers?
Database buffers are cache in the SGA used to hold the data blocks that are read from the data segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
65. What is dictionary cache?
Dictionary cache is information about the database objects stored in a data dictionary table.
66. What is meant by recursive hints?
Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of data dictionary cache.
67. What is redo log buffer?
Changes made to the records are written to the on-line redo log files. So that they can be used in roll forward operations during database recoveries. Before writing them into the redo log files, they will first brought to redo log buffers in SGA and LGWR will write into files frequently. LOG_BUFFER parameter will decide the size.
68. How will you swap objects into a different table space for an existing database?
- Export the user
- Perform import using the command imp system/manager file=export.dmp indexfile=newrite.sql. This will create all definitions into newfile.sql.
- Drop necessary objects.
- Run the script newfile.sql after altering the tablespaces.
- Import from the backup for the necessary objects.
69. List the Optional Flexible Architecture (OFA) of Oracle database? How can we
organize the tablespaces in Oracle database to have maximum performance?
SYSTEM - Data dictionary tables.
DATA - Standard operational tables.
DATA2- Static tables used for standard operations
INDEXES - Indexes for Standard operational tables.
INDEXES1 - Indexes of static tables used for standard operations.
TOOLS - Tools table.
TOOLS1 - Indexes for tools table.
RBS - Standard Operations Rollback Segments,
RBS1,RBS2 - Additional/Special Rollback segments.
TEMP - Temporary purpose tablespace
TEMP_USER - Temporary tablespace for users.
USERS - User tablespace.
70. How will you force database to use particular rollback segment?
SET TRANSACTION USE ROLLBACK SEGMENT rbs_name.
71. What is meant by free extent?
A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free.
72. Which parameter in Storage clause will reduce number of rows per block?
PCTFREE parameter. Row size also reduces no of rows per block.
73. What is the significance of having storage clause?
We can plan the storage for a table as how much initial extents are required, how much can be extended next, how much % should leave free for managing row updating, etc.
74. How does Space allocation table place within a block?
Each block contains entries as follows
Fixed block header
Variable block header
Row Header, row date (multiple rows may exists)
PCTEREE (% of free space for row updating in future)
75. What is the role of PCTFREE parameter is storage clause?
This is used to reserve certain amount of space in a block for expansion of rows.
76. What is the OPTIMAL parameter?
It is used to set the optimal length of a rollback segment.
77. What is the functionality of SYSTEM table space?
To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage.
78. How will you create multiple rollback segments in a database?
- Create a database, which implicitly creates a SYSTEM rollback segment in a SYSTEM tablespace.
- Create a second rollback segment name R0 in the SYSTEM tablespace.
- Make new rollback segment available (after shutdown, modify init.ora file and start database)
- Create other tablespaces (RBS) for rollback segments.
- Deactivate rollback segment R0 and activate the newly created rollback segments.
79. How the space utilization takes place within rollback segments?
It will try to fit the transaction in a cyclic fashion to all existing extents. Once it found an extent is in use then it forced to acquire a new extent (number of extents is based on the optimal size)
80. Why query fails sometimes?
Rollback segment dynamically extent to handle larger transactions entry loads. A single transaction may wipeout all available free space in the rollback segment tablespace. This prevents other user using rollback segments.
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 tables?
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 controlfile 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 mth 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 nth 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.
125. What are the advantages of VIEW?
- 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.
The interview process can be quite stressful. Here is the first part of a two part series on helping you answer those tough questions that you might experience in your quest for an Oracle DBA position.
Ever since I wrote the past article on the Oracle Technical Interview, I have been bombarded with e-mails asking for help on getting through the interview questions that I presented. Most of you I have answered, others I was reluctant to post all of the answers so that you could begin your own quest for the answers. Now, however, I have decided to post the answers knowing that we can all benefit from them. If there are any questions here that you still need clarification on, please e-mail me and I will do my best to further explain the answer I have given. Please remember that as you go through the article, it is not enough to know the answer to a particular question, you must try and put yourself in an interview situation and experience answering the question for yourself. Therefore, after you have gone through the questions and answers read the question yourself and then answer it with your own words. As always, good luck, and cheers.
Personal
This part of the interview question is not to be regarded as insignificant. If the interviewer asks you these questions take it as a sign that they are interested in you, your qualities, and how you interact with people throughout the day. Take it as an opportunity to prove that you have been around the block a few times, are willing to work with other people, and enjoy the job you do. Many times people see DBA types as stuffy and pointed, not willing to work with others, and only concerned with the database and its day-to-day operational needs. Put aside the needs of the database and talk about how you work with people and the different departments in the organization and are concerned with providing them with top notch database services.
1. What DBA activities did you to do today?
Wow, this is a loaded question and almost begs for you to answer it with "What DBA activities do you LIKE to do on a daily basis?." And that is how I would answer this question. Again, do not get caught up in the "typical" day-to-day operational issues of database administration. Sure, you can talk about the index you rebuilt, the monitoring of system and session waits that were occurring, or the space you added to a data file, these are all good and great and you should convey that you understand the day-to-day operational issues. What you should also throw into this answer are the meetings that you attend to provide direction in the database arena, the people that you meet and talk with daily to answer adhoc questions about database use, the modeling of business needs within the database, and the extra time you spend early in the morning or late at night to get the job done. Just because the question stipulates "today" do not take "today" to mean "today." Make sure you wrap up a few good days into "today" and talk about them. This question also begs you to ask the question of "What typical DBA activities are performed day to day within X Corporation?"
2. What is your typical day like?
If you spend enough time on question 1, this question will never be asked. It is really a continuation of question 1 to try and get you to open up and talk about the type of things you like to do. Personally, I would continue with the theme of question 1 if you are cut short or this question is asked later in the interview process. Just note that this question is not all geared toward the day-to-day operational issues you experience as a DBA. This question also gives you the opportunity to see if they want to know about you as an individual. Since the question did not stipulate "on the job" I would throw in a few items like, I get up at 5:00am to get into work and get some quiet time to read up on new trends or you help coach your son/daughter's soccer team. Just test the waters to what is acceptable. If the interviewer starts to pull you back to "job" related issues, do not go to personal. Also, if you go to the office of the interviewer please notice the surroundings, if there are pictures of his/her family, it is probably a good idea to venture down the personal path. If there is a fly-fishing picture on the wall, do not say you like deep-sea fishing. You get the picture.
3. What other parts of your organization do you interact with and how?
Again, if you have exhausted question 1 and 2 you may never get to this question. But if you have been apprehensive to opening up and explaining yourself, take note that you may have an issue and the interviewer might also be already getting tired of the interview process. If you get to this question consider yourself in trouble. You really need to forget all your hang-ups and start explaining what it is that you like to do as a DBA, and why you want to work for this particular company. You are going to have to reel this interviewer back into the interview process or you might not get to the true technical question part of the interview.
4. Do you consider yourself a development DBA or a production DBA and why?
I take this as a trick question and explain it that way. Never in my database carrier have I distinguished between "development" and "production." Just ask your development staff or VP of engineering how much time and money is lost if development systems are down. Explain to the interviewer that both systems are equally important to the operation of the company and both should be considered as production systems because there are people relying on them and money is lost if either one of them is down. Ok you may be saying, and I know you are, that we lose more money if the production system is down. Ok, convey that to the interviewer and you won't get anyone to disagree with you unless your company sells software or there are million dollar deals on the table that are expecting the next release of your product or service.
5. Are you a nuts-n-bolts DBA or a tools-n-props DBA
This question begs for me to give definition around the terms I basically group DBAs into. These are not good or bad groups but something I like to think about when talking to DBAs. A nuts-n-bolts DBA is the type that likes to figure out every little item about how the database works. He/she is a DBA who typically hates a GUI environment and prefers the command line to execute commands and accomplish tasks. A nuts-n-bolts DBA like to feel in control of the database and only feels comfortable at the command line and vi as an editor. The tools-n-props DBA is mostly the opposite of a nuts-n-bolts DBA, they like the feel of a GUI, the ease at which things can be accomplished without knowing much about the database. They want to get the job done with the least amount of intervention from having to figure out what everything is doing behind the scenes. Now the answer, I would explain myself as a combination of the two. I, having been in this business for over 20 years, have grown up in a command line era where the GUIs never seemed to work. There was high complexity in systems and not much good documentation on how things worked. Thus, I had to learn everything about most aspects of the database environment I was working in and thus became a nuts-n-bolts DBA. I was a true command line and vi bigot. Times have changed and the GUIs are very reliable, understand the environment they are installed on, and can generally get the job done quicker for individuals new to database administration. I too am slowly slipping over to the dark side of GUI administration. If you find yourself as a tools-n-props DBA, try to convey that you are aware of some tasks that require you to be a nuts-n-bolts DBA.
Technical - Oracle
This is the part you have all been waiting on. Please if you have just skipped to this section, go back to the personal section and read it. There is much to be gained by the personal section and conveying to your interviewer who you are and how you tick from day to day. Also, the answers I am giving here are off the cuff and are not intended to be the definitive answer to these questions. There are many aspects to these questions that just cannot be answered here and honestly, you will not have time to explain any of these questions fully in the interview process. It is up to you to make sure your interviewer understands that you understand the question and have given enough information that they know you understand the concept.
1. Explain the difference between a hot backup and a cold backup and the benefits associated with each.
A hot backup is basically taking a backup of the database while it is still up and running and it must be in archive log mode. A cold backup is taking a backup of the database while it is shut down and does not require being in archive log mode. The benefit of taking a hot backup is that the database is still available for use while the backup is occurring and you can recover the database to any point in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process. In addition, since you are taking cold backups the database does not require being in archive log mode and thus there will be a slight performance gain as the database is not cutting archive logs to disk.
2. You have just had to restore from backup and do not have any control files. How would you go about bringing up this database?
I would create a text based backup control file, stipulating where on disk all the data files where and then issue the recover command with the using backup control file clause.
3. How do you switch from an init.ora file to a spfile?
Issue the create spfile from pfile command.
4. Explain the difference between a data block, an extent and a segment.
A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object.
5. Give two examples of how you might determine the structure of the table DEPT.
Use the describe command or use the dbms_metadata.get_ddl package.
6. Where would you look for errors from the database engine?
In the alert log.
7. Compare and contrast TRUNCATE and DELETE for a table.
Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces a now rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback and thus take longer to complete.
8. Give the reasoning behind using an index.
Faster access to data blocks in a table.
9. Give the two types of tables involved in producing a star schema and the type of data they hold.
Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables.
10. . What type of index should you use on a fact table?
A Bitmap index.
11. Give two examples of referential integrity constraints.
A primary key and a foreign key.
12. A table is classified as a parent table and you want to drop and re-create it. How would you do this without affecting the children tables?
Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.
13. Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each.
ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point in time. NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly.
14. What command would you use to create a backup control file?
Alter database backup control file to trace.
15. Give the stages of instance startup to a usable state where normal users may access it.
STARTUP NOMOUNT - Instance startup
STARTUP MOUNT - The database is mounted
STARTUP OPEN - The database is opened
16. What column differentiates the V$ views to the GV$ views and how?
The INST_ID column which indicates the instance in a RAC environment the information came from.
17. How would you go about generating an EXPLAIN plan?
Create a plan table with utlxplan.sql.
Use the explain plan set statement_id = 'tst1' into plan_table for a SQL statement
Look at the explain plan with utlxplp.sql or utlxpls.sql
18. How would you go about increasing the buffer cache hit ratio?
Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command.
19. Explain an ORA-01555
You get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message.
20. Explain the difference between $ORACLE_HOME and $ORACLE_BASE.
ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside.
Well, we have gone through the first 25 questions as I would answer them during an interview. Please feel free to add your personal experiences to the answers as it will always improve the process and add your particular touch. As always remember these are "core" DBA questions and not necessarily related to the Oracle options that you may encounter in some interviews. Take a close look at the requirements for any job and try to come up with questions that the interviewer may ask. Next time we will tackle the rest of the questions. Until then, good luck with the process.
21. How would you determine the time zone under which a database was operating?
select DBTIMEZONE from dual;
22. Explain the use of setting GLOBAL_NAMES equal to TRUE.
Setting GLOBAL_NAMES dictates how you might connect to a database. This variable is either TRUE or FALSE and if it is set to TRUE it enforces database links to have the same name as the remote database to which they are linking.
23. What command would you use to encrypt a PL/SQL application?
WRAP
24. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.
A function and procedure are the same in that they are intended to be a collection of PL/SQL code that carries a single task. While a procedure does not have to return any values to the calling application, a function will return a single value. A package on the other hand is a collection of functions and procedures that are grouped together based on their commonality to a business function or application.
25. Explain the use of table functions.
Table functions are designed to return a set of rows through PL/SQL logic but are intended to be used as a normal table or view in a SQL statement. They are also used to pipeline information in an ETL process.
26. Name three advisory statistics you can collect.
Buffer Cache Advice, Segment Level Statistics, & Timed Statistics
27. Where in the Oracle directory tree structure are audit traces placed?
In unix $ORACLE_HOME/rdbms/audit, in Windows the event viewer
28. Explain materialized views and how they are used.
Materialized views are objects that are reduced sets of information that have been summarized, grouped, or aggregated from base tables. They are typically used in data warehouse or decision support systems.
29. When a user process fails, what background process cleans up after it?
PMON
30. What background process refreshes materialized views?
The Job Queue Processes.
31. How would you determine what sessions are connected and what resources they are waiting for?
Use of V$SESSION and V$SESSION_WAIT
32. Describe what redo logs are.
Redo logs are logical and physical structures that are designed to hold all the changes made to a database and are intended to aid in the recovery of a database.
33. How would you force a log switch?
ALTER SYSTEM SWITCH LOGFILE;
34. Give two methods you could use to determine what DDL changes have been made.
You could use Logminer or Streams
35. What does coalescing a tablespace do?
Coalescing is only valid for dictionary-managed tablespaces and de-fragments space by combining neighboring free extents into large single extents.
36. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?
A temporary tablespace is used for temporary objects such as sort structures while permanent tablespaces are used to store those objects meant to be used as the true objects of the database.
37. Name a tablespace automatically created when you create a database.
The SYSTEM tablespace.
38. When creating a user, what permissions must you grant to allow them to connect to the database?
Grant the CONNECT to the user.
39. How do you add a data file to a tablespace?
ALTER TABLESPACE
40. How do you resize a data file?
ALTER DATABASE DATAFILE
41. What view would you use to look at the size of a data file?
DBA_DATA_FILES
42. What view would you use to determine free space in a tablespace?
DBA_FREE_SPACE
43. How would you determine who has added a row to a table?
Turn on fine grain auditing for the table.
44. How can you rebuild an index?
ALTER INDEX
45. Explain what partitioning is and what its benefit is.
Partitioning is a method of taking large tables and indexes and splitting them into smaller, more manageable pieces.
46. You have just compiled a PL/SQL package but got errors, how would you view the errors?
SHOW ERRORS
47. How can you gather statistics on a table?
The ANALYZE command.
48. How can you enable a trace for a session?
Use the DBMS_SESSION.SET_SQL_TRACE or
Use ALTER SESSION SET SQL_TRACE = TRUE;
49. What is the difference between the SQL*Loader and IMPORT utilities?
These two Oracle utilities are used for loading data into the database. The difference is that the import utility relies on the data being produced by another Oracle utility EXPORT while the SQL*Loader utility allows data to be loaded that has been produced by other utilities from different data sources just so long as it conforms to ASCII formatted or delimited files.
50. Name two files used for network connection to a database.
TNSNAMES.ORA and SQLNET.ORA
Technical - UNIX
Every DBA should know something about the operating system that the database will be running on. The questions here are related to UNIX but you should equally be able to answer questions related to common Windows environments.
1. How do you list the files in an UNIX directory while also showing hidden files?
ls -ltra
2. How do you execute a UNIX command in the background?
Use the "&"
3. What UNIX command will control the default file permissions when files are created?
Umask
4. Explain the read, write, and execute permissions on a UNIX directory.
Read allows you to see and list the directory contents.
Write allows you to create, edit and delete files and subdirectories in the directory.
Execute gives you the previous read/write permissions plus allows you to change into the directory and execute programs or shells from the directory.
5. the difference between a soft link and a hard link?
A symbolic (soft) linked file and the targeted file can be located on the same or different file system while for a hard link they must be located on the same file system.
6. Give the command to display space usage on the UNIX file system.
df -lk
7. Explain iostat, vmstat and netstat.
Iostat reports on terminal, disk and tape I/O activity.
Vmstat reports on virtual memory statistics for processes, disk, tape and CPU activity.
Netstat reports on the contents of network data structures.
8. How would you change all occurrences of a value using VI?
Use :%s/
9. Give two UNIX kernel parameters that effect an Oracle install
SHMMAX & SHMMNI
10. Briefly, how do you install Oracle software on UNIX.
Basically, set up disks, kernel parameters, and run orainst.
I hope that these interview questions were not too hard. Remember these are "core" DBA questions and not necessarily related to the Oracle options that you may encounter in some interviews. Take a close look at the requirements for any job and try to extract questions that interviewers may ask from manuals and real life experiences. For instance, if they are looking for a DBA to run their databases in RAC environments, you should try to determine what hardware and software they are using BEFORE you get to the interview. This would allow you to brush up on particular environments and not be caught off-guard. Good luck!
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