What’s the difference between latch & enque?
1. What is a latch?
Latches are low level serialization mechanisms used to protect shared
data structures in the SGA. The implementation of latches is operating
system dependent, particularly in regard to whether a process will wait
for a latch and for how long.
A latch is a type of a lock that can be very quickly acquired and freed.
Latches are typically used to prevent more than one process from
executing the same piece of code at a given time. Associated with each
latch is a cleanup procedure that will be called if a process dies while
holding the latch. Latches have an associated level that is used to
prevent deadlocks. Once a process acquires a latch at a certain level it
cannot subsequently acquire a latch at a level that is equal to or less
than that level (unless it acquires it nowait).
2. Latches vs Enqueues
Enqueues are another type of locking mechanism used in Oracle.
An enqueue is a more sophisticated mechanism which permits several concurrent
processes to have varying degree of sharing of "known" resources. Any object
which can be concurrently used, can be protected with enqueues. A good example
is of locks on tables. We allow varying levels of sharing on tables e.g.
two processes can lock a table in share mode or in share update mode etc.
One difference is that the enqueue is obtained using an OS specific
locking mechanism. An enqueue allows the user to store a value in the lock,
i.e the mode in which we are requesting it. The OS lock manager keeps track
of the resources locked. If a process cannot be granted the lock because it
is incompatible with the mode requested and the lock is requested with wait,
the OS puts the requesting process on a wait queue which is serviced in FIFO.
Another difference between latches and enqueues is that
in latches there is no ordered queue of waiters like in enqueues. Latch
waiters may either use timers to wakeup and retry or spin (only in
multiprocessors). Since all waiters are concurrently retrying (depending on
the scheduler), anyone might get the latch and conceivably the first one to
try might be the last one to get.
3. When do we need to obtain a latch?
A process acquires a latch when working with a structure in the SGA
(System Global Area). It continues to hold the latch for the period
of time it works with the structure. The latch is dropped when the
process is finished with the structure. Each latch protects a different
set of data, identified by the name of the latch.
Oracle uses atomic instructions like "test and set" for operating on latches.
Processes waiting to execute a part of code for which a latch has
already been obtained by some other process will wait until the
latch is released. Examples are redo allocation latches, copy
latches, archive control latch etc. The basic idea is to block concurrent
access to shared data structures. Since the instructions to
set and free latches are atomic, the OS guarantees that only one process gets
it. Since it is only one instruction, it is quite fast. Latches are held
for short periods of time and provide a mechanism for cleanup in case
a holder dies abnormally while holding it. This cleaning is done using
the services of PMON.
4. Latches request modes?
Latches request can be made in two modes: "willing-to-wait" or "no wait". Normally,
latches will be requested in "willing-to-wait" mode. A request in "willing-to-wait" mode
will loop, wait, and request again until the latch is obtained. In "no wait" mode the process
request the latch. If one is not available, instead of waiting, another one is requested. Only
when all fail does the server process have to wait.
Examples of "willing-to-wait" latches are: shared pool and library cache latches
A example of "no wait" latches is the redo copy latch.
5. What causes latch contention?
If a required latch is busy, the process requesting it spins, tries again
and if still not available, spins again. The loop is repeated up to a maximum
number of times determined by the initialization parameter _SPIN_COUNT.
If after this entire loop, the latch is still not available, the process must yield
the CPU and go to sleep. Initially is sleeps for one centisecond. This time is
doubled in every subsequent sleep.
This causes a slowdown to occur and results in additional CPU usage,
until a latch is available. The CPU usage is a consequence of the
"spinning" of the process. "Spinning" means that the process continues to
look for the availability of the latch after certain intervals of time,
during which it sleeps.
6. How to identify contention for internal latches?
Relevant data dictionary views to query
---------------------------------------------
V$LATCH
V$LATCHHOLDER
V$LATCHNAME
Each row in the V$LATCH table contains statistics for a different type
of latch. The columns of the table reflect activity for different types
of latch requests. The distinction between these types of requests is
whether the requesting process continues to request a latch if it
is unavailable:
willing-to-wait If the latch requested with a willing-to-wait
request is not available, the requesting process
waits a short time and requests the latch again.
The process continues waiting and requesting until
the latch is available.
no wait If the latch requested with an immediate request is
not available, the requesting process does not
wait, but continues processing.
Oracle 10 DBA Interview questions and answers
1. Is the following SQL statement syntactically correct? If not, please rewrite it correctly.
SELECT col1 FROM tableA WHERE NOT IN (SELECT col1 FROM tableB);
Ans. SQL is incorrect.
Correct SQL : SELECT col1 FROM tableA WHERE col1 NOT IN (SELECT col1 FROM tableB);
2. What is a more efficient way to write this query, to archive the same set?
Ans: SELECT col1 from tableA minus SELECT col1 from tableB
3.How would you determine that the new query is more efficient than the original query?
Ans: Run explain plan on both query and see the result .
4.How can we find the location of the database trace files from within the data dictionary?
Ans: Generally trace file on the database server machine is located in one of two locations:
1. If you are using a dedicated server connection, the trace file will be generated in the directory specified by
the USER_DUMP_DEST parameter.
2.If you are using a shared server connection, the trace file will be generated in the directory specified by the
BACKGROUND_DUMP_DEST parameter.
you can run sqlplus>SHOW PARAMETER DUMP_DEST
or
select name, value
from v$parameter
where name like '%dump_dest%'
5. What is the correct syntax for a UNIX endless WHILE loop?
while :
do
commands
done
6. Write the SQL statement that will return the name and size of the largest datafile in the database.
SQL> select name,bytes from v$datafile where bytes=(select max(bytes) from v$datafile);
7. What are the proper steps to changing the Oracle database block size?
cold backup all data files and backup controlfile to trace, recreate your database
with the new block size using the backup control file, and restore. It may be easier
with rman. You can not change datbase block size on fly.
8. Using awk, write a script to print the 3rd field of every line.
Ans:
awk '{print }'
awk '{print $3}
awk '{print $3}
9.Under what conditions, is a nested loop better than a merge join?
Ans:
Optimizer uses nested loop when we are joining tables containing small number of rows with an efficient driving
condition.
It is important to have an index on column of inner join table as this table is probed every time for a new value
from outer table.
Optimizer may not use nested loop in case:
1. No of rows of both the table is quite high
2. Inner query always results in same set of records
3. The access path of inner table is independent of data coming from outer table.
merge join is used to join two independent data sources. They perform better than nested loop when the volume of
data is big in tables
but not as good as hash joins in general.
10.Which database views would you use to ascertain the number of commits a user's session has performed?
Joining V$SESSTAT ,V$STATNAME
select * from V$SESSTAT a ,V$STATNAME b where b.CLASS=a.STATISTIC# and b.NAME='user commits' and a.sid=
11.What does the #!bin/ksh at the beginning of a shell script do? Why should it be there?
Ans: On the first line of an interpreter script, the "#!", is the name of a program which should be used to
interpret the contents of the file.
For instance, if the first line contains "#! /bin/ksh", then the contents of the file are executed as a korn shell
script.
12.What command is used to find the status of Oracle 10g Clusterware (CRS) and the various components it manages
(ONS, VIP, listener, instances, etc.)?
Ans:
$ocrcheck
13.Describe a scenario in which a vendor clusterware is required, in addition to the Oracle 10g Clusterware.
If you choose external redundancy for the OCR and voting disk, then to enable redundancy, the disk subsystem must be configurable for RAID mirroring/vendor clusterware. Otherwise, your system may be vulnerable because the OCR and voting disk are single points of failure.
14.How would you find the interconnect IP address from any node within an Oracle 10g RAC configuration?
using oifcfg command.
se the oifcfg -help command to display online help for OIFCFG. The elements of OIFCFG commands, some of which are
optional depending on the command, are:
*nodename—Name of the Oracle Clusterware node as listed in the output from the olsnodes command
*if_name—Name by which the interface is configured in the system
*subnet—Subnet address of the interface
*if_type—Type of interface: public or cluster_interconnect
You can use OIFCFG to list the interface names and the subnets of all of the interfaces available on the local node
by executing the iflist keyword as shown in this example:
oifcfg iflist
hme0 139.185.141.0
qfe0 204.152.65.16
You can also retrieve specific OIFCFG information with a getif command using the following syntax:
oifcfg getif [ [-global | -node nodename] [-if if_name[/subnet]] [-type if_type] ]
To store a new interface use the setif keyword. For example, to store the interface hme0, with the subnet
139.185.141.0, as a global interface (to be used as an interconnect for all of the RAC instances in your cluster),
you would use the command:
oifcfg setif -global hme0/139.185.141.0:cluster_interconnect
For a cluster interconnect that exists between only two nodes, for example rac1 and rac2, you could create the cms0
interface with the following commands, assuming 139.185.142.0 is the subnet addresses for the interconnect on rac1
and rac2 respectively:
oifcfg setif -global cms0/139.185.142.0:cluster_interconnect
Use the OIFCFG delif command to delete the stored configuration for global or node-specific interfaces. A specific
node-specific or global interface can be deleted by supplying the interface name, with an optional subnet, on the
command line. Without the -node or -global options, the delif keyword deletes either the given interface or all of
the global and node-specific interfaces on all of the nodes in the cluster. For example, the following command
deletes the global interface named qfe0 for the subnet 204.152.65.0:
oifcfg delif -global qfe0/204.152.65.0
On the other hand, the next command deletes all of the global interfaces stored with OIFCFG:
oifcfg delif -global
15.What is the Purpose of the voting disk in Oracle 10g Clusterware?
Voting disk record node membership information. Oracle Clusterware uses the voting disk to determine which instances are members of a cluster. The voting disk must reside on a shared disk. For high availability, Oracle recommends that you have a minimum of three voting disks. If you configure a single voting disk, then you should use external mirroring to provide redundancy. You can have up to 32 voting disks in your cluster.
16.What is the purpose of the OCR in Oracle 10g Clusterware?
Ans: Oracle Cluster Registry (OCR) is a component in 10g RAC used to store the cluster configuration information. It is a shared disk component, typically located in a shared raw volume that must be accessible to all nodes in the cluster.
The daemon OCSSd manages the configuration info in OCR and maintains the changes to cluster in the registry.
17. In Oracle Streams archived log downstream capture, which database view can be used to determine which archived
logs are no longer needed by the capture process?
Ans: V$ARCHIVE_DEST_STATUS
SELECT col1 FROM tableA WHERE NOT IN (SELECT col1 FROM tableB);
Ans. SQL is incorrect.
Correct SQL : SELECT col1 FROM tableA WHERE col1 NOT IN (SELECT col1 FROM tableB);
2. What is a more efficient way to write this query, to archive the same set?
Ans: SELECT col1 from tableA minus SELECT col1 from tableB
3.How would you determine that the new query is more efficient than the original query?
Ans: Run explain plan on both query and see the result .
4.How can we find the location of the database trace files from within the data dictionary?
Ans: Generally trace file on the database server machine is located in one of two locations:
1. If you are using a dedicated server connection, the trace file will be generated in the directory specified by
the USER_DUMP_DEST parameter.
2.If you are using a shared server connection, the trace file will be generated in the directory specified by the
BACKGROUND_DUMP_DEST parameter.
you can run sqlplus>SHOW PARAMETER DUMP_DEST
or
select name, value
from v$parameter
where name like '%dump_dest%'
5. What is the correct syntax for a UNIX endless WHILE loop?
while :
do
commands
done
6. Write the SQL statement that will return the name and size of the largest datafile in the database.
SQL> select name,bytes from v$datafile where bytes=(select max(bytes) from v$datafile);
7. What are the proper steps to changing the Oracle database block size?
cold backup all data files and backup controlfile to trace, recreate your database
with the new block size using the backup control file, and restore. It may be easier
with rman. You can not change datbase block size on fly.
8. Using awk, write a script to print the 3rd field of every line.
Ans:
awk
awk '{print $3}
awk '{print $3}
9.Under what conditions, is a nested loop better than a merge join?
Ans:
Optimizer uses nested loop when we are joining tables containing small number of rows with an efficient driving
condition.
It is important to have an index on column of inner join table as this table is probed every time for a new value
from outer table.
Optimizer may not use nested loop in case:
1. No of rows of both the table is quite high
2. Inner query always results in same set of records
3. The access path of inner table is independent of data coming from outer table.
merge join is used to join two independent data sources. They perform better than nested loop when the volume of
data is big in tables
but not as good as hash joins in general.
10.Which database views would you use to ascertain the number of commits a user's session has performed?
Joining V$SESSTAT ,V$STATNAME
select * from V$SESSTAT a ,V$STATNAME b where b.CLASS=a.STATISTIC# and b.NAME='user commits' and a.sid=
11.What does the #!bin/ksh at the beginning of a shell script do? Why should it be there?
Ans: On the first line of an interpreter script, the "#!", is the name of a program which should be used to
interpret the contents of the file.
For instance, if the first line contains "#! /bin/ksh", then the contents of the file are executed as a korn shell
script.
12.What command is used to find the status of Oracle 10g Clusterware (CRS) and the various components it manages
(ONS, VIP, listener, instances, etc.)?
Ans:
$ocrcheck
13.Describe a scenario in which a vendor clusterware is required, in addition to the Oracle 10g Clusterware.
If you choose external redundancy for the OCR and voting disk, then to enable redundancy, the disk subsystem must be configurable for RAID mirroring/vendor clusterware. Otherwise, your system may be vulnerable because the OCR and voting disk are single points of failure.
14.How would you find the interconnect IP address from any node within an Oracle 10g RAC configuration?
using oifcfg command.
se the oifcfg -help command to display online help for OIFCFG. The elements of OIFCFG commands, some of which are
optional depending on the command, are:
*nodename—Name of the Oracle Clusterware node as listed in the output from the olsnodes command
*if_name—Name by which the interface is configured in the system
*subnet—Subnet address of the interface
*if_type—Type of interface: public or cluster_interconnect
You can use OIFCFG to list the interface names and the subnets of all of the interfaces available on the local node
by executing the iflist keyword as shown in this example:
oifcfg iflist
hme0 139.185.141.0
qfe0 204.152.65.16
You can also retrieve specific OIFCFG information with a getif command using the following syntax:
oifcfg getif [ [-global | -node nodename] [-if if_name[/subnet]] [-type if_type] ]
To store a new interface use the setif keyword. For example, to store the interface hme0, with the subnet
139.185.141.0, as a global interface (to be used as an interconnect for all of the RAC instances in your cluster),
you would use the command:
oifcfg setif -global hme0/139.185.141.0:cluster_interconnect
For a cluster interconnect that exists between only two nodes, for example rac1 and rac2, you could create the cms0
interface with the following commands, assuming 139.185.142.0 is the subnet addresses for the interconnect on rac1
and rac2 respectively:
oifcfg setif -global cms0/139.185.142.0:cluster_interconnect
Use the OIFCFG delif command to delete the stored configuration for global or node-specific interfaces. A specific
node-specific or global interface can be deleted by supplying the interface name, with an optional subnet, on the
command line. Without the -node or -global options, the delif keyword deletes either the given interface or all of
the global and node-specific interfaces on all of the nodes in the cluster. For example, the following command
deletes the global interface named qfe0 for the subnet 204.152.65.0:
oifcfg delif -global qfe0/204.152.65.0
On the other hand, the next command deletes all of the global interfaces stored with OIFCFG:
oifcfg delif -global
15.What is the Purpose of the voting disk in Oracle 10g Clusterware?
Voting disk record node membership information. Oracle Clusterware uses the voting disk to determine which instances are members of a cluster. The voting disk must reside on a shared disk. For high availability, Oracle recommends that you have a minimum of three voting disks. If you configure a single voting disk, then you should use external mirroring to provide redundancy. You can have up to 32 voting disks in your cluster.
16.What is the purpose of the OCR in Oracle 10g Clusterware?
Ans: Oracle Cluster Registry (OCR) is a component in 10g RAC used to store the cluster configuration information. It is a shared disk component, typically located in a shared raw volume that must be accessible to all nodes in the cluster.
The daemon OCSSd manages the configuration info in OCR and maintains the changes to cluster in the registry.
17. In Oracle Streams archived log downstream capture, which database view can be used to determine which archived
logs are no longer needed by the capture process?
Ans: V$ARCHIVE_DEST_STATUS
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