Interview Questions
for Oracle, DBA, Developer Candidates
PL/SQL Questions:
1. Describe the
difference between a procedure, function and anonymous pl/sql block.
Level: Low
Expected answer :
Candidate should mention use of DECLARE statement, a function must return a
value while a procedure doesn’t have to.
2. What is a
mutating table error and how can you get around it?
Level: Intermediate
Expected answer:
This happens with triggers. It occurs because the trigger is trying to update a
row it is currently using. The usual fix involves either use of views or
temporary tables so the database is selecting from one while updating the
other.
3. Describe the use of %ROWTYPE and %TYPE in PL/SQL
Level: Low
Expected answer:
%ROWTYPE allows you to associate a variable with an entire table row. The %TYPE
associates a variable with a single column type.
4. What packages (if
any) has Oracle provided for use by developers?
Level: Intermediate
to high
Expected answer:
Oracle provides the DBMS_ series of packages. There are many which developers
should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK,
DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they
can mention a few of these and describe how they used them, even better. If they
include the SQL routines provided by Oracle, great, but not really what was
asked.
5. Describe the use of PL/SQL tables
Level: Intermediate
Expected answer:
PL/SQL tables are scalar arrays that can be referenced by a binary integer.
They can be used to hold values for use in later queries or calculations. In
Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.
6. When is a declare
statement needed?
Level: Low
The DECLARE
statement is used in PL/SQL anonymous blocks such as with stand alone,
non-stored PL/SQL procedures. It must come first in a PL/SQL stand-alone file
if it is used.
7. In what order should a open/fetch/loop set of commands
in a PL/SQL block be implemented
if you use the %NOTFOUND cursor variable in the exit when statement? Why?
Level: Intermediate
Expected answer:
OPEN then FETCH then LOOP followed by the exit when. If not specified in this
order will result in the final return being done twice because of the way the
%NOTFOUND is handled by PL/SQL.
8. What are SQLCODE and
SQLERRM and why are they important for PL/SQL developers?
Level: Intermediate
Expected answer:
SQLCODE returns the value of the error number for the last error encountered.
The SQLERRM returns the actual error message for the last error encountered. They
can be used in exception handling to report, or, store in an error log table,
the error that occurred in the code. These are especially useful for the WHEN
OTHERS exception.
9. How can you find
within a PL/SQL block, if a cursor is open?
Level: Low
Expected answer: Use
the %ISOPEN cursor status variable.
10. How can you
generate debugging output from PL/SQL?
Level:Intermediate
to high
Expected answer: Use
the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR
command, but this only shows errors. The DBMS_OUTPUT package can be used to
show intermediate results from loops and the status of variables as the
procedure is executed. The new package UTL_FILE can also be used.
11. What are the
types of triggers?
Level: Intermediate
to high
Expected Answer:
There are 12 types of triggers in PL/SQL that consist of combinations of the
BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words:
BEFORE ALL ROW
INSERT
AFTER ALL ROW INSERT
BEFORE INSERT
AFTER INSERT etc.
DBA
1. Give one method
for transferring a table from one schema to another:
Level: Intermediate
Expected Answer:
There are several possible methods, export-import, CREATE TABLE... AS SELECT,
or COPY.
2. What is the
purpose of the IMPORT option IGNORE? What is it’s default setting?
Level: Low
Expected Answer: The
IMPORT IGNORE option tells import to ignore "already exists" errors.
If it is not specified the tables that already exist will be skipped. If it is
specified, the error is ignored and the tables data will be inserted. The
default value is N.
3. You have a
rollback segment in a version 7.2 data base that has expanded beyond optimal,
how can it be restored to optimal?
Level: Low
Expected answer: Use
the ALTER TABLESPACE ..... SHRINK command.
4. If the DEFAULT and TEMPORARY tablespace clauses are
left out of a CREATE USER
command what happens? Is this bad or good? Why?
Level: Low
Expected answer: The
user is assigned the SYSTEM tablespace as a default and temporary tablespace.
This is bad because it causes user objects and temporary segments to be placed
into the SYSTEM tablespace resulting in fragmentation and improper table
placement (only data dictionary objects and the system rollback segment should
be in SYSTEM).
5. What are some of the Oracle provided packages that DBAs
should be aware of?
Level: Intermediate
to High
Expected answer:
Oracle provides a number of packages in the form of the DBMS_ packages owned by
the SYS user. The packages used by DBAs may include: DBMS_SHARED_POOL,
DBMS_UTILITY, DBMS_SQL, DBMS_DDL, DBMS_SESSION, DBMS_OUTPUT and DBMS_SNAPSHOT.
They may also try to answer with the UTL*.SQL or CAT*.SQL series of SQL
procedures. These can be viewed as extra credit but aren’t part of the answer.
6. What happens if
the constraint name is left out of a constraint clause?
Level: Low
Expected answer: The
Oracle system will use the default name of SYS_Cxxxx where xxxx is a system
generated number. This is bad since it makes tracking which table the
constraint belongs to or what the constraint does harder.
7. What happens if a table space clause is left off of a
primary key constraint clause?
Level: Low
Expected answer:
This results in the index that is automatically generated being placed in then
users default tablespace. Since this will usually be the same tablespace as the
table is being created in, this can cause serious performance problems.
8. What is the proper method for disabling and re-enabling
a primary key constraint?
Level: Intermediate
Expected answer: You
use the ALTER TABLE command for both. However, for the enable clause you must
specify the USING INDEX and TABLESPACE clause for primary keys.
9. What happens if a
primary key constraint is disabled and then enabled without fully specifying
the index clause?
Level: Intermediate
Expected answer: The
index is created in the user’s default tablespace and all sizing information is
lost. Oracle doesn’t store this information as a part of the constraint
definition, but only as part of the index definition, when the constraint was
disabled the index was dropped and the information is gone.
10. (On UNIX) when
should more than one DB writer process be used? How many should be used?
Level: High
Expected answer: If
the UNIX system being used is capable of asynchronous IO then only one is
required, if the system is not capable of asynchronous IO then up to twice the
number of disks used by Oracle number of DB writers should be specified by use
of the db_writers initialization parameter.
11. You are using
hot backup without being in archivelog mode, can you recover in the event of a
failure? Why or why not?
Level: High
Expected answer: You
can’t use hot backup without being in archivelog mode. So no, you couldn’t
recover.
12. What causes the
"snapshot too old" error? How can this be prevented or mitigated?
Level: Intermediate
Expected answer:
This is caused by large or long running transactions that have either wrapped
onto their own rollback space or have had another transaction write on part of
their rollback space. This can be prevented or mitigated by breaking the
transaction into a set of smaller transactions or increasing the size of the
rollback segments and their extents.
13. How can you tell
if a database object is invalid?
Level: Low
Expected answer: By
checking the status column of the DBA_, ALL_ or USER_OBJECTS views, depending
upon whether you own or only have permission on the view or are using a DBA
account.
14. A user is
getting an ORA-00942 error yet you know you have granted them permission on the
table, what else should you check?
Level: Low
Expected answer: You
need to check that the user has specified the full name of the object (select
empid from scott.emp; instead of select empid from emp;) or has a synonym that
points to the object (create synonym emp for scott.emp;)
15. A developer is
trying to create a view and the database won’t let him. He has the
"DEVELOPER" role, which has the "CREATE VIEW" system
privilege and SELECT grants on the tables he is using, what is the problem?
Level: Intermediate
Expected answer: You
need to verify the developer has direct grants on all tables used in the view.
You can’t create a stored object with grants given through views.
16. If you have an
example table, what is the best way to get sizing data for the production table
implementation?
Level: Intermediate
Expected answer: The
best way is to analyze the table and then use the data provided in the
DBA_TABLES view to get the average row length and other pertinent data for the
calculation. The quick and dirty way is to look at the number of blocks the
table is actually using and ratio the number of rows in the table to its number
of blocks against the number of expected rows.
17. How can you find
out how many users are currently logged into the database? How can you find
their operating system id?
Level: high
Expected answer:
There are several ways. One is to look at the v$session or v$process views.
Another way is to check the current_logins parameter in the v$sysstat view.
Another if you are on UNIX is to do a "ps -ef|grep oracle|wc -l’ command,
but this only works against a single instance installation.
18. A user selects
from a sequence and gets back two values, his select is:
SELECT pk_seq.nextval FROM dual;
What is the problem?
Level: Intermediate
Expected answer:
Somehow two values have been inserted into the dual table. This table is a
single row, single column table that should only have one value in it.
19. How can you
determine if an index needs to be dropped and rebuilt?
Level: Intermediate
Expected answer: Run
the ANALYZE INDEX command on the index to validate its structure and then
calculate the ratio of LF_BLK_LEN/LF_BLK_LEN+BR_BLK_LEN and if it isn’t near
1.0 (i.e. greater than 0.7 or so) then the index should be rebuilt.
Or
if the ratio
BR_BLK_LEN/ LF_BLK_LEN+BR_BLK_LEN is nearing 0.3.
SQL/
SQLPlus
1. How can variables
be passed to a SQL routine?
Level: Low
Expected answer: By
use of the & symbol. For passing in variables the numbers 1-8 can be used
(&1, &2,...,&8) to pass the values after the command into the
SQLPLUS session. To be prompted for a specific variable, place the ampersanded
variable in the code itself:
"select * from
dba_tables where owner=&owner_name;" . Use of double ampersands tells
SQLPLUS to resubstitute the value for each subsequent use of the variable, a
single ampersand will cause a reprompt for the value unless an ACCEPT statement
is used to get the value from the user.
2. You want to
include a carriage return/linefeed in your output from a SQL script, how can
you do this?
Level: Intermediate
to high
Expected answer: The
best method is to use the CHR() function (CHR(10) is a return/linefeed) and the
concatenation function "||". Another method, although it is hard to
document and isn’t always portable is to use the return/linefeed as a part of a
quoted string.
3. How can you call
a PL/SQL procedure from SQL?
Level: Intermediate
Expected answer: By
use of the EXECUTE (short form EXEC) command.
4. How do you
execute a host operating system command from within SQL?
Level: Low
Expected answer: By
use of the exclamation point "!" (in UNIX and some other OS) or the
HOST (HO) command.
5. You want to use SQL to build SQL, what is this called
and give an example
Level: Intermediate
to high
Expected answer:
This is called dynamic SQL. An example would be:
set lines 90 pages 0
termout off feedback off verify off
spool drop_all.sql
select ‘drop user
‘||username||’ cascade;’ from dba_users
where username not
in ("SYS’,’SYSTEM’);
spool off
Essentially you are
looking to see that they know to include a command (in this case DROP
USER...CASCADE;) and that you need to concatenate using the ‘||’ the values
selected from the database.
6. What SQLPlus
command is used to format output from a select?
Level: low
Expected answer:
This is best done with the COLUMN command.
7. You want to group
the following set of select returns, what can you group on?
Max(sum_of_cost),
min(sum_of_cost), count(item_no), item_no
Level: Intermediate
Expected answer: The
only column that can be grouped on is the "item_no" column, the rest
have aggregate functions associated with them.
8. What special Oracle feature allows you to specify how
the cost based system treats a SQL statement?
Level: Intermediate
to high
Expected answer: The
COST based system allows the use of HINTs to control the optimizer path
selection. If they can give some example hints such as FIRST ROWS, ALL ROWS,
USING INDEX, STAR, even better.
9. You want to
determine the location of identical rows in a table before attempting to place
a unique index on the table, how can this be done?
Level: High
Expected answer: Oracle
tables always have one guaranteed unique column, the rowid column. If you use a
min/max function against your rowid and then select against the proposed
primary key you can squeeze out the rowids of the duplicate rows pretty quick.
For example:
select rowid from
emp e
where e.rowid >
(select min(x.rowid)
from emp x
where x.emp_no =
e.emp_no);
In the situation
where multiple columns make up the proposed key, they must all be used in the
where clause.
10. What is a
Cartesian product?
Level: Low
Expected answer: A
Cartesian product is the result of an unrestricted join of two or more tables.
The result set of a three table Cartesian product will have x * y * z number of
rows where x, y, z correspond to the number of rows in each table involved in
the join.
11. You are joining
a local and a remote table, the network manager complains about the traffic
involved, how can you reduce the network traffic?
Level: High
Expected answer:
Push the processing of the remote data to the remote instance by using a view to
pre-select the information for the join. This will result in only the data
required for the join being sent across.
12. What is the
default ordering of an ORDER BY clause in a SELECT statement?
Level: Low
Expected answer:
Ascending
13. What is tkprof
and how is it used?
Level: Intermediate
to high
Expected answer: The
tkprof tool is a tuning tool used to determine cpu and execution times for SQL
statements. You use it by first setting timed_statistics to true in the
initialization file and then turning on tracing for either the entire database
via the sql_trace parameter or for the session using the ALTER SESSION command.
Once the trace file is generated you run the tkprof tool against the trace file
and then look at the output from the tkprof tool. This can also be used to
generate explain plan output.
14. What is explain
plan and how is it used?
Level: Intermediate
to high
Expected answer: The
EXPLAIN PLAN command is a tool to tune SQL statements. To use it you must have
an explain_table generated in the user you are running the explain plan for.
This is created using the utlxplan.sql script. Once the explain plan table
exists you run the explain plan command giving as its argument the SQL
statement to be explained. The explain_plan table is then queried to see the
execution plan of the statement. Explain plans can also be run using tkprof.
15. How do you set
the number of lines on a page of output? The width?
Level: Low
Expected answer: The
SET command in SQLPLUS is used to control the number of lines generated per
page and the width of those lines, for example SET PAGESIZE 60 LINESIZE 80 will
generate reports that are 60 lines long with a line width of 80 characters. The
PAGESIZE and LINESIZE options can be shortened to PAGES and LINES.
16. How do you prevent
output from coming to the screen?
Level: Low
Expected answer: The
SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off
screen output. This option can be shortened to TERM.
17. How do you
prevent Oracle from giving you informational messages during and after a SQL
statement execution?
Level: Low
Expected answer: The
SET options FEEDBACK and VERIFY can be set to OFF.
18. How do you
generate file output from SQL?
Level: Low
Expected answer: By
use of the SPOOL command
Tuning Questions
1. A tablespace has
a table with 30 extents in it. Is this bad? Why or why not.
Level: Intermediate
Expected answer:
Multiple extents in and of themselves aren’t bad. However if you also have
chained rows this can hurt performance.
2. How do you set up
tablespaces during an Oracle installation?
Level: Low
Expected answer: You
should always attempt to use the Oracle Flexible Architecture standard or
another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK,
REDO LOG, DATA, TEMPORARY and INDEX segments.
3. You see multiple
fragments in the SYSTEM tablespace, what should you check first?
Level: Low
Expected answer:
Ensure that users don’t have the SYSTEM tablespace as their TEMPORARY or
DEFAULT tablespace assignment by checking the DBA_USERS view.
4. What are some
indications that you need to increase the SHARED_POOL_SIZE parameter?
Level: Intermediate
Expected answer:
Poor data dictionary or library cache hit ratios, getting error ORA-04031.
Another indication is steadily decreasing performance with all other tuning
parameters the same.
5. What is the
general guideline for sizing db_block_size and db_multi_block_read for an
application that does many full table scans?
Level: High
Expected answer:
Oracle almost always reads in 64k chunks. The two should have a product equal
to 64 or a multiple of 64.
6. What is the
fastest query method for a table?
Level: Intermediate
Expected answer:
Fetch by rowid
7. Explain the use
of TKPROF? What initialization parameter should be turned on to get full TKPROF
output?
Level: High
Expected answer: The
tkprof tool is a tuning tool used to determine cpu and execution times for SQL
statements. You use it by first setting timed_statistics to true in the
initialization file and then turning on tracing for either the entire database
via the sql_trace parameter or for the session using the ALTER SESSION command.
Once the trace file is generated you run the tkprof tool against the trace file
and then look at the output from the tkprof tool. This can also be used to
generate explain plan output.
8. When looking at
v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How
do you correct it?
Level: Intermediate
Expected answer: If
you get excessive disk sorts this is bad. This indicates you need to tune the
sort area parameters in the initialization files. The major sort are parameter
is the SORT_AREA_SIZe parameter.
9. When should you
increase copy latches? What parameters control copy latches?
Level: high
Expected answer:
When you get excessive contention for the copy latches as shown by the
"redo copy" latch hit ratio. You can increase copy latches via the
initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on
your system.
10. Where can you
get a list of all initialization parameters for your instance? How about an
indication if they are default settings or have been changed?
Level: Low
Expected answer: You
can look in the init.ora file for an indication of manually set
parameters. For all parameters, their value and whether or not the current
value is the default value, look in the v$parameter view.
11. Describe hit
ratio as it pertains to the database buffers. What is the difference between
instantaneous and cumulative hit ratio and which should be used for tuning?
Level: Intermediate
Expected answer: The
hit ratio is a measure of how many times the database was able to read a value
from the buffers verses how many times it had to re-read a data value from the
disks. A value greater than 80-90% is good, less could indicate problems. If
you simply take the ratio of existing parameters this will be a cumulative
value since the database started. If you do a comparison between pairs of
readings based on some arbitrary time span, this is the instantaneous ratio for
that time span. Generally speaking an instantaneous reading gives more valuable
data since it will tell you what your instance is doing for the time it was
generated over.
12. Discuss row
chaining, how does it happen? How can you reduce it? How do you correct it?
Level: high
Expected answer: Row
chaining occurs when a VARCHAR2 value is updated and the length of the new
value is longer than the old value and won’t fit in the remaining block space.
This results in the row chaining to another block. It can be reduced by setting
the storage parameters on the table to appropriate values. It can be corrected
by export and import of the effected table.
13. When looking at
the estat events report you see that you are getting busy buffer waits. Is this
bad? How can you find what is causing it?
Level: high
Expected answer:
Buffer busy waits could indicate contention in redo, rollback or data blocks.
You need to check the v$waitstat view to see what areas are causing the
problem. The value of the "count" column tells where the problem is,
the "class" column tells you with what. UNDO is rollback segments,
DATA is data base buffers.
14. If you see
contention for library caches how can you fix it?
Level: Intermediate
Expected answer:
Increase the size of the shared pool.
15. If you see
statistics that deal with "undo" what are they really talking about?
Level: Intermediate
Expected answer:
Rollback segments and associated structures.
16. If a tablespace
has a default pctincrease of zero what will this cause (in relationship to the
smon process)?
Level: High
Expected answer: The
SMON process won’t automatically coalesce its free space fragments.
17. If a tablespace
shows excessive fragmentation what are some methods to defragment the
tablespace? (7.1,7.2 and 7.3 only)
Level: High
Expected answer: In
Oracle 7.0 to 7.2 The use of the 'alter session set events 'immediate trace
name coalesce level ts#';’ command is the easiest way to defragment contiguous
free space fragmentation. The ts# parameter corresponds to the ts# value found
in the ts$ SYS table. In version 7.3 the ‘alter tablespace
coalesce;’ is best. If the free space isn’t contiguous then export, drop and
import of the tablespace contents may be the only way to reclaim non-contiguous
free space.
18. How can you tell
if a tablespace has excessive fragmentation?
Level: Intermediate
If a select against
the dba_free_space table shows that the count of a tablespaces extents is
greater than the count of its data files, then it is fragmented.
19. You see the following on a status report:
redo log space requests 23
redo log space wait time 0
Is this something to worry about? What if redo log space
wait time is high? How can you fix this?
Level: Intermediate
Expected answer:
Since the wait time is zero, no. If the wait time was high it might indicate a
need for more or larger redo logs.
20. What can cause a
high value for recursive calls? How can this be fixed?
Level: High
Expected answer: A
high value for recursive calls is cause by improper cursor usage, excessive
dynamic space management actions, and or excessive statement re-parses. You
need to determine the cause and correct it By either relinking applications to
hold cursors, use proper space management techniques (proper storage and
sizing) or ensure repeat queries are placed in packages for proper reuse.
21. If you see a pin
hit ratio of less than 0.8 in the estat library cache report is this a problem?
If so, how do you fix it?
Level: Intermediate
Expected answer:
This indicate that the shared pool may be too small. Increase the shared pool
size.
22. If you see the
value for reloads is high in the estat library cache report is this a matter
for concern?
Level: Intermediate
Expected answer:
Yes, you should strive for zero reloads if possible. If you see excessive
reloads then increase the size of the shared pool.
23. You look at the
dba_rollback_segs view and see that there is a large number of shrinks and they
are of relatively small size, is this a problem? How can it be fixed if it is a
problem?
Level: High
Expected answer: A large
number of small shrinks indicates a need to increase the size of the rollback
segment extents. Ideally you should have no shrinks or a small number of large
shrinks. To fix this just increase the size of the extents and adjust optimal
accordingly.
24. You look at the
dba_rollback_segs view and see that you have a large number of wraps is this a
problem?
Level: High
Expected answer: A
large number of wraps indicates that your extent size for your rollback
segments are probably too small. Increase the size of your extents to reduce
the number of wraps. You can look at the average transaction size in the same
view to get the information on transaction size.
25. In a system with an average of 40 concurrent users you
get the following from a query on rollback extents:
ROLLBACK CUR EXTENTS
--------------------- --------------------------
R01 11
R02 8
R03 12
R04 9
SYSTEM 4
You have room for each to grow by 20 more extents each. Is
there a problem? Should you take any action?
Level: Intermediate
Expected answer: No
there is not a problem. You have 40 extents showing and an average of 40
concurrent users. Since there is plenty of room to grow no action is needed.
26. You see multiple
extents in the temporary tablespace. Is this a problem?
Level: Intermediate
Expected answer: As
long as they are all the same size this isn’t a problem. In fact, it can even
improve performance since Oracle won’t have to create a new extent when a user
needs one.
Installation/Configuration
1. Define OFA.
Level: Low
Expected answer: OFA
stands for Optimal Flexible Architecture. It is a method of placing directories
and files in an Oracle system so that you get the maximum flexibility for
future tuning and file placement.
2. How do you set up
your tablespace on installation?
Level: Low
Expected answer: The
answer here should show an understanding of separation of redo and rollback,
data and indexes and isolation os SYSTEM tables from other tables. An example
would be to specify that at least 7 disks should be used for an Oracle installation
so that you can place SYSTEM tablespace on one, redo logs on two (mirrored redo
logs) the TEMPORARY tablespace on another, ROLLBACK tablespace on another and
still have two for DATA and INDEXES. They should indicate how they will handle
archive logs and exports as well. As long as they have a logical plan for
combining or further separation more or less disks can be specified.
3. What should be
done prior to installing Oracle (for the OS and the disks)?
Level: Low
Expected Answer:
adjust kernel parameters or OS tuning parameters in accordance with
installation guide. Be sure enough contiguous disk space is available.
4. You have
installed Oracle and you are now setting up the actual instance. You have been
waiting an hour for the initialization script to finish, what should you check
first to determine if there is a problem?
Level: Intermediate
to high
Expected Answer:
Check to make sure that the archiver isn’t stuck. If archive logging is turned
on during install a large number of logs will be created. This can fill up your
archive log destination causing Oracle to stop to wait for more space.
5. When configuring
SQLNET on the server what files must be set up?
Level: Intermediate
Expected answer:
INITIALIZATION file, TNSNAMES.ORA file, SQLNET.ORA file
6. When configuring
SQLNET on the client what files need to be set up?
Level: Intermediate
Expected answer:
SQLNET.ORA, TNSNAMES.ORA
7. What must be
installed with ODBC on the client in order for it to work with Oracle?
Level: Intermediate
Expected answer: SQLNET
and PROTOCOL (for example: TCPIP adapter) layers of the transport programs.
8. You have just
started a new instance with a large SGA on a busy existing server. Performance
is terrible, what should you check for?
Level: Intermediate
Expected answer: The
first thing to check with a large SGA is that it isn’t being swapped out.
9. What OS user
should be used for the first part of an Oracle installation (on UNIX)?
Level: low
Expected answer: You
must use root first.
10. When should the
default values for Oracle initialization parameters be used as is?
Level: Low
Expected answer:
Never
11. How many control
files should you have? Where should they be located?
Level: Low
Expected answer: At
least 2 on separate disk spindles. Be sure they say on separate disks, not just
file systems.
12. How many redo
logs should you have and how should they be configured for maximum
recoverability?
Level: Intermediate
Expected answer: You
should have at least three groups of two redo logs with the two logs each on a
separate disk spindle (mirrored by Oracle). The redo logs should not be on raw
devices on UNIX if it can be avoided.
13. You have a
simple application with no "hot" tables (i.e. uniform IO and access
requirements). How many disks should you have assuming standard layout for
SYSTEM, USER, TEMP and ROLLBACK tablespaces?
Expected answer: At
least 7, see disk configuration answer above.
Data
Modeler
1. Describe third
normal form?
Level: Low
Expected answer:
Something like: In third normal form all attributes in an entity are related to
the primary key and only to the primary key
2. Is the following
statement true or false:
"All relational databases must be in third normal
form"
Why or why not?
Level: Intermediate
Expected answer:
False. While 3NF is good for logical design most databases, if they have more
than just a few tables, will not perform well using full 3NF. Usually some
entities will be denormalized in the logical to physical transfer process.
3. What is an ERD?
Level: Low
Expected answer: An
ERD is an Entity-Relationship-Diagram. It is used to show the entities and
relationships for a database logical model.
4. Why are recursive
relationships bad? How do you resolve them?
Level: Intermediate
A recursive
relationship (one where a table relates to itself) is bad when it is a hard
relationship (i.e. neither side is a "may" both are "must")
as this can result in it not being possible to put in a top or perhaps a bottom
of the table (for example in the EMPLOYEE table you couldn’t put in the PRESIDENT
of the company because he has no boss, or the junior janitor because he has no
subordinates). These type of relationships are usually resolved by adding a
small intersection entity.
5. What does a hard
one-to-one relationship mean (one where the relationship on both ends is
"must")?
Level: Low to
intermediate
Expected answer:
This means the two entities should probably be made into one entity.
6. How should a
many-to-many relationship be handled?
Level: Intermediate
Expected answer: By
adding an intersection entity table
7. What is an
artificial (derived) primary key? When should an artificial (or derived)
primary key be used?
Level: Intermediate
Expected answer: A
derived key comes from a sequence. Usually it is used when a concatenated key
becomes too cumbersome to use as a foreign key.
8. When should you
consider denormalization?
Level: Intermediate
Expected answer:
Whenever performance analysis indicates it would be beneficial to do so without
compromising data integrity.
UNIX
1. How can you
determine the space left in a file system?
Level: Low
Expected answer:
There are several commands to do this: du, df, or bdf
2. How can you
determine the number of SQLNET users logged in to the UNIX system?
Level: Intermediate
Expected answer:
SQLNET users will show up with a process unique name that begins with
oracle, if you do a ps -ef|grep oracle|wc -l you can get
a count of the number of users.
3. What command is
used to type files to the screen?
Level: Low
Expected answer:
cat, more, pg
4. What command is
used to remove a file?
Level: Low
Expected answer: rm
5. Can you remove an
open file under UNIX?
Level: Low
Expected answer: yes
6. How do you create
a decision tree in a shell script?
Level: intermediate
Expected answer:
depending on shell, usually a case-esac or an if-endif or fi structure
7. What is the
purpose of the grep command?
Level: Low
Expected answer:
grep is a string search command that parses the specified string from the
specified file or files
8. The system has a
program that always includes the word nocomp in its name, how can you determine
the number of processes that are using this program?
Level: intermediate
Expected answer: ps
-ef|grep *nocomp*|wc -l
9. What is an inode?
Level: Intermediate
Expected answer: an
inode is a file status indicator. It is stored in both disk and memory and
tracts file status. There is one inode for each file on the system.
10. The system
administrator tells you that the system hasn’t been rebooted in 6 months,
should he be proud of this?
Level: High
Expected answer:
Maybe. Some UNIX systems don’t clean up well after themselves. Inode problems
and dead user processes can accumulate causing possible performance and
corruption problems. Most UNIX systems should have a scheduled periodic reboot
so file systems can be checked and cleaned and dead or zombie processes cleared
out.
11. What is
redirection and how is it used?
Level: Intermediate
Expected answer:
redirection is the process by which input or output to or from a process is
redirected to another process. This can be done using the pipe symbol
"|", the greater than symbol ">" or the "tee"
command. This is one of the strengths of UNIX allowing the output from one
command to be redirected directly into the input of another command.
12. How can you find
dead processes?
Level: Intermediate
Expected answer: ps
-ef|grep zombie -- or -- who -d depending on the system.
13. How can you find
all the processes on your system?
Level: Low
Expected answer: Use
the ps command
14. How can you find
your id on a system?
Level: Low
Expected answer: Use
the "who am i" command.
15. What is the
finger command?
Level: Low
Expected answer: The
finger command uses data in the passwd file to give information on system
users.
16. What is the
easiest method to create a file on UNIX?
Level: Low
Expected answer: Use
the touch command
17. What does
>> do?
Level: Intermediate
Expected answer: The
">>" redirection symbol appends the output from the command
specified into the file specified. The file must already have been created.
18. If you aren’t
sure what command does a particular UNIX function what is the best way to
determine the command?
Expected answer: The
UNIX man -k command will search the man pages for the value
specified. Review the results from the command to find the command of interest.
Oracle Troubleshooting
1. How can you
determine if an Oracle instance is up from the operating system level?
Level: Low
Expected answer:
There are several base Oracle processes that will be running on multi-user
operating systems, these will be smon, pmon, dbwr and lgwr. Any answer that has
them using their operating system process showing feature to check for these is
acceptable. For example, on UNIX a ps -ef|grep dbwr will show what instances
are up.
2. Users from the PC
clients are getting messages indicating :
Level: Low
ORA-06114: (Cnct
err, can't get err txt. See Servr Msgs & Codes Manual)
What could the
problem be?
Expected answer: The
instance name is probably incorrect in their connection string.
3. Users from the PC
clients are getting the following error stack:
Level: Low
ERROR: ORA-01034: ORACLE not available
ORA-07318: smsget: open error when opening sgadef.dbf
file.
HP-UX Error: 2: No such file or directory
What is the probable cause?
Expected answer: The
Oracle instance is shutdown that they are trying to access, restart the
instance.
4. How can you
determine if the SQLNET process is running for SQLNET V1? How about V2?
Level: Low
Expected answer: For
SQLNET V1 check for the existence of the orasrv process. You can use the
command "tcpctl status" to get a full status of the V1 TCPIP server,
other protocols have similar command formats. For SQLNET V2 check for the
presence of the LISTENER process(s) or you can issue the command "lsnrctl
status".
5. What file will
give you Oracle instance status information? Where is it located?
Level: Low
Expected answer: The
alert.ora log. It is located in the directory specified by the
background_dump_dest parameter in the v$parameter table.
6. Users aren’t
being allowed on the system. The following message is received:
Level: Intermediate
ORA-00257 archiver
is stuck. Connect internal only, until freed
What is the problem?
Expected answer: The
archive destination is probably full, backup the archive logs and remove them
and the archiver will re-start.
7. Where would you
look to find out if a redo log was corrupted assuming you are using Oracle
mirrored redo logs?
Level: Intermediate
Expected answer:
There is no message that comes to the SQLDBA or SRVMGR programs during startup
in this situation, you must check the alert.log file for this
information.
8. You attempt to
add a datafile and get:
Level: Intermediate
ORA-01118: cannot
add anymore datafiles: limit of 40 exceeded
What is the problem
and how can you fix it?
Expected answer:
When the database was created the db_files parameter in the initialization file
was set to 40. You can shutdown and reset this to a higher value, up to the
value of MAX_DATAFILES as specified at database creation. If the MAX_DATAFILES
is set to low, you will have to rebuild the control file to increase it before
proceeding.
9. You look at your
fragmentation report and see that smon hasn’t coalesced any of you tablespaces,
even though you know several have large chunks of contiguous free extents. What
is the problem?
Level: High
Expected answer:
Check the dba_tablespaces view for the value of pct_increase for the
tablespaces. If pct_increase is zero, smon will not coalesce their free space.
10. Your users get
the following error:
Level: Intermediate
ORA-00055 maximum
number of DML locks exceeded
What is the problem
and how do you fix it?
Expected answer: The
number of DML Locks is set by the initialization parameter DML_LOCKS. If this
value is set to low (which it is by default) you will get this error. Increase
the value of DML_LOCKS. If you are sure that this is just a temporary problem,
you can have them wait and then try again later and the error should clear.
11. You get a call
from you backup DBA while you are on vacation. He has corrupted all of the
control files while playing with the ALTER DATABASE BACKUP CONTROLFILE command.
What do you do?
Level: High
Expected answer: As
long as all datafiles are safe and he was successful with the BACKUP
controlfile command you can do the following:
CONNECT INTERNAL
STARTUP MOUNT
(Take any read-only
tablespaces offline before next step ALTER DATABASE DATAFILE .... OFFLINE;)
RECOVER DATABASE
USING BACKUP CONTROLFILE
ALTER DATABASE OPEN
RESETLOGS;
(bring read-only
tablespaces back online)
Shutdown and backup
the system, then restart
If they have a
recent output file from the ALTER DATABASE BACKUP CONTROL FILE TO TRACE;
command, they can use that to recover as well.
If no backup of the
control file is available then the following will be required:
CONNECT INTERNAL
STARTUP NOMOUNT
CREATE CONTROL FILE
.....;
However, they will
need to know all of the datafiles, logfiles, and settings for MAXLOGFILES,
MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES for the database to use the command.
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