ORACLE FREQUENTLY ASKED
QUESTIONS - I
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?
There are tablespaces and
database's schema objects.
What is a tablespace?
A database is divided
into Logical Storage Unit called tablespaces. A tablespace is used to grouped
related logical structures together.
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.
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.
Can objects of the same
schema reside in different tablespaces?
Yes.
Can a tablespace hold
objects from different schemes?
Yes.
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.
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.)
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 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 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 a View based on
another View?
Yes.
Can a tablespace hold
objects from different Schemes?
Yes.
Can objects of the same
Schema reside in different tablespace?
Yes.
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.
Do View contain Data?
Views do not contain or store 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 are the type of
Synonyms?
There are two types of Synonyms Private and Public.
What is a Redo Log?
The set of Redo Log files YSDATE, UID, USER or USERENV SQL functions, or the
pseudo columns LEVEL or ROWNUM.
What is an Index Segment?
Each Index has an Index segment that stores all of its data.
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 are the different
type of Segments?
Data Segment, Index
Segment, Rollback Segment and Temporary Segment.
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 an Integrity
Constraints?
An integrity constraint is a declarative way to define a business rule for a
column of a table.
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.
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.
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 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.
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 an Oracle
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
synonym?
Only its owner can access a private synonym.
What is a public synonym?
Any database user can access a public synonym.
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.
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.
How are the index updated?
Indexes are automatically maintained and used by Oracle. Changes to table data
are automatically incorporated into all relevant indexes.
What is a
Tablespace?
A database is divided
into Logical Storage Unit called tablespace. A tablespace is used to grouped
related logical structures together
What is Rollback Segment?
A Database contains one or more Rollback Segments to temporarily store
"undo" information.
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.
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 can’t be changed latter.
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 redo log files.
Time stamp of database creation.
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.
What is Index Cluster?
A Cluster with an index on the Cluster Key
When does a Transaction end?
When it is committed or Rollbacked.
What is the effect of
setting the value "ALL_ROWS" for OPTIMIZER_GOAL parameter of the
ALTER SESSION command? What are the factors that affect OPTIMIZER in choosing
an Optimization approach?
Answer The OPTIMIZER_MODE initialization parameter Statistics in the Data
Dictionary the OPTIMIZER_GOAL parameter of the ALTER SESSION command hints in
the statement.
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.
How does one create a new
database? (For DBA)
One can create and modify Oracle databases using the Oracle "dbca"
(Database Configuration Assistant) utility. The dbca utility is located in the
$ORACLE_HOME/bin directory. The Oracle Universal Installer (oui) normally
starts it after installing the database server software.
One can also create databases manually using scripts. This option, however, is
falling out of fashion, as it is quite involved and error prone. Look at this
example for creating and Oracle 9i database:
CONNECT SYS AS SYSDBA
ALTER SYSTEM SET DB_CREATE_FILE_DEST='/u01/oradata/';
ALTER SYSTEM SET DB_CREATE_ONLINE_LOG_DEST_1='/u02/oradata/';
ALTER SYSTEM SET DB_CREATE_ONLINE_LOG_DEST_2='/u03/oradata/';
CREATE DATABASE;
What database block size
should I use? (For DBA)
Oracle recommends that your database block size match, or be multiples of your
operating system block size. One can use smaller block sizes, but the
performance cost is significant. Your choice should depend on the type of
application you are running. If you have many small transactions as with OLTP,
use a smaller block size. With fewer but larger transactions, as with a DSS
application, use a larger block size. If you are using a volume manager,
consider your "operating system block size" to be 8K. This is because
volume manager products use 8K blocks (and this is not configurable).
What are the different
approaches used by Optimizer in choosing an execution plan?
Rule-based and Cost-based.
What does ROLLBACK do?
ROLLBACK retracts any of
the changes resulting from the SQL statements in the transaction.
How does one coalesce
free space? (For DBA)
SMON coalesces free space (extents) into larger, contiguous extents every 2
hours and even then, only for a short period of time.
SMON will not coalesce free space if a tablespace's default storage parameter
"pctincrease" is set to 0. With Oracle 7.3 one can manually coalesce
a tablespace using the ALTER TABLESPACE ... COALESCE; command, until then use:
SQL> alter session set events 'immediate trace name coalesce level n';
Where 'n' is the tablespace number you get from SELECT TS#, NAME FROM SYS.TS$;
You can get status information about this process by selecting from the
SYS.DBA_FREE_SPACE_COALESCED dictionary view.
How does one prevent
tablespace fragmentation? (For DBA)
Always set PCTINCREASE to 0 or 100.
Bizarre values for PCTINCREASE will contribute to fragmentation. For example if
you set PCTINCREASE to 1 you will see that your extents are going to have weird
and wacky sizes: 100K, 100K, 101K, 102K, etc. Such extents of bizarre size are
rarely re-used in their entirety. PCTINCREASE of 0 or 100 gives you nice round
extent sizes that can easily be reused. E.g... 100K, 100K, 200K, 400K, etc.
Use the same extent size for all the segments in a given tablespace. Locally
Managed tablespaces (available from 8i onwards) with uniform extent sizes
virtually eliminates any tablespace fragmentation. Note that the number of
extents per segment does not cause any performance issue anymore, unless they
run into thousands and thousands where additional I/O may be required to fetch
the additional blocks where extent maps of the segment are stored.
Where can one find the
high water mark for a table? (For DBA)
There is no single system table, which contains the high water mark (HWM) for a
table. A table's HWM can be calculated using the results from the following SQL
statements:
SELECT BLOCKS
FROM DBA_SEGMENTS
WHERE OWNER=UPPER(owner) AND SEGMENT_NAME = UPPER(table);
ANALYZE TABLE owner.table ESTIMATE STATISTICS;
SELECT EMPTY_BLOCKS
FROM DBA_TABLES
WHERE OWNER=UPPER(owner) AND SEGMENT_NAME = UPPER(table);
Thus, the tables' HWM = (query result 1) - (query result 2) - 1
NOTE: You can also use the DBMS_SPACE package and calculate the HWM =
TOTAL_BLOCKS - UNUSED_BLOCKS - 1.
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 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.
How are extents allocated
to a segment? (For DBA)
Oracle8 and above rounds off extents to a multiple of 5 blocks when more than 5
blocks are requested. If one requests 16K or 2 blocks (assuming an 8K block
size), Oracle doesn't round it up to 5 blocks, but it allocates 2 blocks or 16K
as requested. If one asks for 8 blocks, Oracle will round it up to 10 blocks.
Space allocation also depends upon the size of contiguous free space available.
If one asks for 8 blocks and Oracle finds a contiguous free space that is
exactly 8 blocks, it would give it you. If it were 9 blocks, Oracle would also
give it to you. Clearly Oracle doesn't always round extents to a multiple of 5
blocks.
The exception to this rule is locally managed tablespaces. If a tablespace is
created with local extent management and the extent size is 64K, then Oracle
allocates 64K or 8 blocks assuming 8K-block size. Oracle doesn't round it up to
the multiple of 5 when a tablespace is locally managed.
Can one rename a database
user (schema)? (For DBA)
No, this is listed as Enhancement Request 158508. Workaround:
Do a user-level export of user A
create new user B
Import system/manager fromuser=A touser=B
Drop user A
Define Transaction?
A Transaction is a
logical unit of work that comprises one or more SQL statements executed by a
single user.
What is Read-Only Transaction?
A Read-Only transaction ensures that the results of each query executed in the
transaction are consistent with respect to the same point in time.
What is a deadlock?
Explain.
Two processes waiting to update the rows of a table which are locked by the
other process then deadlock arises. In a database environment this will often
happen because of not issuing proper row lock commands. Poor design of
front-end application may cause this situation and the performance of server
will reduce drastically.
These locks will be released automatically when a commit/rollback operation
performed or any one of this processes being killed externally.
What is a Schema?
The set of objects owned by user account is called the schema.
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 is Parallel Server?
Multiple instances accessing the same database (Only in Multi-CPU environments)
What are the basic
element 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 (LoG Write)
DBWR (Data Base Write)
ARCH (ARCHiver)
CKPT (Check Point)
RECO
Dispatcher
User Process with associated PGS
What is clusters?
Group of tables physically stored together because they share common columns
and are often used together is called Cluster.
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 (Ver 7.0)
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 process can be shared by all users. The memory structure that are 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 the use of
ANALYZE command?
To perform one of these function on an index, table, or cluster:
- To collect statistics about object used by the optimizer and store them in
the data dictionary.
- To delete statistics about the object used by object from the data
dictionary.
- To validate the structure of the object.
- To identify migrated and chained rows of the table or cluster.
What is default tablespace?
The Tablespace to contain schema objects created without specifying a
tablespace name.
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 amount
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 Tablespace Quota?
The collective amount of disk space available to the objects in a schema on a
particular tablespace.
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 are the database administrator’s
utilities available?
SQL * DBA - This allows DBA to monitor and control an ORACLE database. SQL *
Loader - It loads data from standard operating system files (Flat files) into
ORACLE database tables. Export (EXP) and Import (imp) utilities allow you to
move existing data in ORACLE format to and from ORACLE database.
How can you enable
automatic archiving?
Shut the database
Backup the database
Modify/Include LOG_ARCHIVE_START_TRUE in init.ora file.
Start up the database.
What are roles? How can
we implement roles?
Roles are the easiest way
to grant and manage common privileges needed by different groups of database
users. Creating roles and assigning provides to roles. Assign each role to group
of users. This will simplify the job of assigning privileges to individual
users.
What are Roles?
Roles are named groups of
related privileges that are granted to users or other roles.
What are 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.
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.
What is Auditing?
Monitoring of user access to aid in the investigation of database use.
How does one see the
uptime for a database? (For DBA)
Look at the following SQL
query:
SELECT to_char (startup_time,'DD-MON-YYYY HH24: MI: SS') "DB Startup
Time"
FROM sys.v_$instance;
Marco Bergman provided the following alternative solution:
SELECT to_char (logon_time,'Dy dd Mon HH24: MI: SS') "DB Startup
Time"
FROM sys.v_$session
WHERE Sid=1 /* this is pmon */
/
Users still running on Oracle 7 can try one of the following queries:
Column STARTED format a18 head 'STARTUP TIME'
Select C.INSTANCE,
to_date (JUL.VALUE, 'J')
|| to_char (floor (SEC.VALUE/3600), '09')
|| ':'
-- || Substr (to_char (mod (SEC.VALUE/60, 60), '09'), 2, 2)
|| Substr (to_char (floor (mod (SEC.VALUE/60, 60)), '09'), 2, 2)
|| '.'
|| Substr (to_char (mod (SEC.VALUE, 60), '09'), 2, 2) STARTED
from SYS.V_$INSTANCE JUL,
SYS.V_$INSTANCE SEC,
SYS.V_$THREAD C
Where JUL.KEY like '%JULIAN%'
and SEC.KEY like '%SECOND%';
Select to_date (JUL.VALUE, 'J')
|| to_char (to_date (SEC.VALUE, 'SSSSS'), ' HH24:MI:SS') STARTED
from SYS.V_$INSTANCE JUL,
SYS.V_$INSTANCE SEC
where JUL.KEY like '%JULIAN%'
and SEC.KEY like '%SECOND%';
select to_char (to_date (JUL.VALUE, 'J') + (SEC.VALUE/86400), -Return a DATE
'DD-MON-YY HH24:MI:SS') STARTED
from V$INSTANCE JUL,
V$INSTANCE SEC
where JUL.KEY like '%JULIAN%'
and SEC.KEY like '%SECOND%';
Where are my TEMPFILES, I
don't see them in V$DATAFILE or DBA_DATA_FILE? (For DBA)
Temp files, unlike normal data files, are not listed in v$datafile or
dba_data_files. Instead query v$tempfile or dba_temp_files:
SELECT * FROM v$tempfile;
SELECT * FROM dba_temp_files;
How do I find used/free
space in a TEMPORARY tablespace? (For DBA)
Unlike normal tablespaces, true temporary tablespace information is not listed
in DBA_FREE_SPACE. Instead use the V$TEMP_SPACE_HEADER view:
SELECT tablespace_name, SUM (bytes used), SUM (bytes free)
FROM V$temp_space_header
GROUP BY tablespace_name;
What is a profile?
Each database user is assigned a Profile that specifies limitations on various
system resources available to the user.
How will you enforce
security using stored procedures?
Don't grant user access directly to tables within the application. Instead
grant the ability to access the procedures that access the tables. When
procedure executed it will execute the privilege of procedures owner. Users
cannot access tables except via the procedure.
How can one see who is
using a temporary segment? (For DBA)
For every user using temporary space, there is an entry in SYS.V$_LOCK with
type 'TS'.
All temporary segments are named 'ffff.bbbb' where 'ffff' is the file it is in
and 'bbbb' is first block of the segment. If your temporary tablespace is set
to TEMPORARY, all sorts are done in one large temporary segment. For usage
stats, see SYS.V_$SORT_SEGMENT
From Oracle 8.0, one can just query SYS.v$sort_usage. Look at these examples:
select s.username, u."USER", u.tablespace, u.contents, u.extents,
u.blocks
from sys.v_$session s, sys.v_$sort_usage u
where s.addr = u.session_addr
/
select s.osuser, s.process, s.username, s.serial#,
Sum (u.blocks)*vp.value/1024 sort_size
from sys.v_$session s, sys.v_$sort_usage u, sys.v_$parameter VP
where s.saddr = u.session_addr
and vp.name = 'db_block_size'
and s.osuser like '&1'
group by s.osuser, s.process, s.username, s.serial#, vp.value
/
How does one get the view
definition of fixed views/tables?
Query v$fixed_view_definition. Example: SELECT * FROM v$fixed_view_definition
WHERE view_name='V$SESSION';
What are the dictionary
tables used to monitor a database spaces?
DBA_FREE_SPACE
DBA_SEGMENTS
DBA_DATA_FILES.
How can we specify the
Archived log file name format and destination?
By setting the following values in init.ora file. LOG_ARCHIVE_FORMAT = arch
%S/s/T/tarc (%S - Log sequence number and is zero left paded, %s - Log sequence
number not padded. %T - Thread number lef-zero-paded and %t - Thread number not
padded). The file name created is arch 0001 are if %S is used. LOG_ARCHIVE_DEST
= path.
What is user Account in
Oracle database?
An 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.
When will the data in the
snapshot log be used?
We must be able to create an after row trigger on table (i.e., it should be not
be already available) after giving table privileges. We cannot specify snapshot
log name because oracle uses the name of the master table in the name of the
database objects that support its snapshot log. The master table name should be
less than or equal to 23 characters. (The table name created will be
MLOGS_tablename, and trigger name will be TLOGS name).
What dynamic data
replication?
Updating or Inserting records in remote database through database triggers. It
may fail if remote database is having any problem.
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 phase, a
Prepare Phase and a Commit Phase.
How can you Enforce
Referential Integrity in snapshots?
Time the references to occur when master tables are not in use. Perform the
reference the manually immediately locking the master tables. We can join
tables in snapshots by creating a complex snapshots that will based on the
master tables.
What is a SQL * NET?
SQL *NET is ORACLE's mechanism for interfacing with the communication protocols
used by the networks that facilitate distributed processing and distributed
databases. It is used in Clint-Server and Server-Server communications.
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 the mechanism
provided by ORACLE for table replication?
Snapshots and SNAPSHOT LOGs
What is snapshots?
Snapshot is an object used to dynamically replicate data between distribute
database at specified time intervals. In ver 7.0 they are read only.
What are the various type
of snapshots?
Simple and Complex.
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 snapshot log?
It is a table that maintains a record of modifications to the master table in a
snapshot. It is stored in the same database as master table and is only
available for simple snapshots. It should be created before creating snapshots.
What are the benefits of
distributed options in databases?
Database on other servers can be updated and those transactions can be grouped
together with others in a logical unit.
Database uses a two phase commit.
What are the options
available to refresh snapshots?
COMPLETE - Tables are completely regenerated using the snapshots query and the
master tables every time the snapshot referenced.
FAST - If simple snapshot used then a snapshot log can be used to send the
changes to the snapshot tables.
FORCE - Default value. If possible it performs a FAST refresh; otherwise it
will perform a complete refresh.
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 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.
How can we reduce the
network traffic?
- Replication of data in distributed environment.
- Using snapshots to replicate data.
- Using remote procedure calls.
Differentiate simple and
complex, snapshots?
- A simple snapshot is based on a query that does not contains GROUP BY
clauses, CONNECT BY clauses, JOINs, sub-query or snapshot of operations.
- A complex snapshots contain at least any one of the above.
