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Here Are Some SQL Server DBA

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Here are some SQL Server DBA/Developer interview questions I faced myself personally and/or heard from people.

I will try to answer these questions briefly here, but be advised that these answers may not be complete and it will be better for you to go through text books, books online and other resources on the net. At the end of this article, I listed some useful books for aspiring DBAs and developers. Before you go for the interview, be prepared to explain the database design of one of your latest projects. Don't be surprised if the interviewer asks you to draw ER diagrams. Well, here are some questions for you. Hope this helps you prepare for your interview. Wish you all the best in your job hunt! Feel free to email me 'interview questions' that you've faced. Questions are categorized under the following sections, for your convenience: Database design SQL Server architecture Database administration Database programming (8 questions)
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Database design

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What is normalization? Explain different levels of normalization? Check out the article Q100139 from Microsoft knowledge base and of course, there's much more information available in the net. It'll be a good idea to get a hold of any RDBMS fundamentals text book, especially the one by C. J. Date. Most of the times, it will be okay if you can explain till third normal form. What is denormalization and when would you go for it? As the name indicates, denormalization is the reverse process of normalization. It's the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced. How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables? One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table. It will be a good idea to read up a database designing fundamentals text book.

What's the difference between a primary key and a unique key? Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only. What are user defined datatypes and when you should go for them? User defined datatypes let you extend the base SQL Server datatypes by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar(8). In this case you could create a user defined datatype called Flight_num_type of varchar(8) and use it across all your tables. See sp_addtype, sp_droptype in books online. What is bit datatype and what's the information that can be stored inside a bit column? Bit datatype is used to store boolean information like 1 or 0 (true or false). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0 and there was no support for NULL. But from SQL Server 7.0 onwards, bit datatype can represent a third state, which is NULL. Define candidate key, alternate key, composite key. A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys. A key formed by combining at least two or more columns is called composite key. What are defaults? Is there a column to which a default can't be bound? A default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can't have defaults bound to them. See CREATE DEFUALT in books online. Back to top SQL Server architecture What is a transaction and what are ACID properties? A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction. For more information and explanation of these properties, see SQL Server books online or any RDBMS fundamentals text book.

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Explain different isolation levels An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed, Repeatable Read, Serializable. See SQL Server books online for an explanation of the isolation levels. Be sure to read about SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation level at the connection level. CREATE INDEX myIndex ON myTable(myColumn) What type of Index will get created after executing the above statement? Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise. What's the maximum size of a row? 8060 bytes. Don't be surprised with questions like 'what is the maximum number of columns per table'. Check out SQL Server books online for the page titled: "Maximum Capacity Specifications". Explain Active/Active and Active/Passive cluster configurations Hopefully you have experience setting up cluster servers. But if you don't, at least be familiar with the way clustering works and the two clusterning configurations Active/Active and Active/Passive. SQL Server books online has enough information on this topic and there is a good white paper available on Microsoft site. Explain the architecture of SQL Server This is a very important question and you better be able to answer it if consider yourself a DBA. SQL Server books online is the best place to read about SQL Server architecture. Read up the chapter dedicated to SQL Server Architecture. What is lock escalation? Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it's dynamically managed by SQL Server. What's the difference between DELETE TABLE and TRUNCATE TABLE commands? DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it won't log the deletion of each row, instead it logs the deallocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back.

Explain the storage models of OLAP Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation. What are the new features introduced in SQL Server 2000 (or the latest release of SQL Server at the time of your interview)? What changed between the previous version of SQL Server and the current version? This question is generally asked to see how current is your knowledge. Generally there is a section in the beginning of the books online titled "What's New", which has all such information. Of course, reading just that is not enough, you should have tried those things to better answer the questions. Also check out the section titled "Backward Compatibility" in books online which talks about the changes that have taken place in the new version. What are constraints? Explain different types of constraints. Constraints enable the RDBMS enforce the integrity of the database automatically, without needing you to create triggers, rule or defaults. Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY For an explanation of these constraints see books online for the pages titled: "Constraints" and "CREATE TABLE", "ALTER TABLE" Whar is an index? What are the types of indexes? How many clustered indexes can be created on a table? I create a separate index on each column of a table. what are the advantages and disadvantages of this approach? Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker. Indexes are of two types. Clustered indexes and non-clustered indexes. When you craete a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Nonclustered indexes have their own storage separate from the table data storage. Nonclustered indexes are stored as B-tree structures (so do clustered indexes), with the leaf level nodes having the index key and it's row locater. The row located could be the RID or the Clustered index key, depending up on the absence or presence of clustered index on the table. If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same t ime, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used. Back to top Database administration

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What is RAID and what are different types of RAID configurations? RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance to database servers. There are six RAID levels 0 through 5 offering different levels of performance, fault tolerance. MSDN has some information about RAID levels and for detailed information, check out the RAID advisory board's homepage What are the steps you will take to improve performance of a poor performing query? This is a very open ended question and there could be a lot of reasons behind the poor performance of a query. But some general issues that you could talk about would be: No indexes, table scans, missing or out of date statistics, blocking, excess recompilations of stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much normalization, excess usage of cursors and temporary tables. Some of the tools/ways that help you troubleshooting performance problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance monitor, Graphical execution plan in Query Analyzer. Download the white paper on performance tuning SQL Server from Microsoft web site. Don't forget to check out sql-server-performance.com What are the steps you will take, if you are tasked with securing an SQL Server? Again this is another open ended question. Here are some things you could talk about: Preferring NT authentication, using server, databse and application roles to control access to the data, securing the physical database files using NTFS permissions, using an unguessable SA password, restricting physical access to the SQL Server, renaming the Administrator account on the SQL Server computer, disabling the Guest account, enabling auditing, using multiprotocol encryption, setting up SSL, setting up firewalls, isolating SQL Server from the web server etc. Read the white paper on SQL Server security from Microsoft website. Also check out My SQL Server security best practices What is a deadlock and what is a live lock? How will you go about resolving deadlocks? Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other's piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user's process. A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely. Check out SET DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL Server books online. Also check out the article Q169960 from Microsoft knowledge base. What is blocking and how would you troubleshoot it?

Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first. Read up the following topics in SQL Server books online: Understanding and avoiding blocking, Coding efficient transactions. Explain CREATE DATABASE syntax Many of us are used to craeting databases from the Enterprise Manager or by just issuing the command: CREATE DATABAE MyDB. But what if you have to create a database with two filegroups, one on drive C and the other on drive D with log on drive E with an initial size of 600 MB and with a growth factor of 15%? That's why being a DBA you should be familiar with the CREATE DATABASE syntax. Check out SQL Server books online for more information. How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode? SQL Server can be started from command line, using the SQLSERVR.EXE. This EXE has some very important parameters with which a DBA should be familiar with. -m is used for starting SQL Server in single user mode and -f is used to start the SQL Server in minimal confuguration mode. Check out SQL Server books online for more parameters and their explanations. As a part of your job, what are the DBCC commands that you commonly use for database maintenance? DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there are a whole load of DBCC commands which are very useful for DBAs. Check out SQL Server books online for more information. What are statistics, under what circumstances they go out of date, how do you update them? Statistics determine the selectivity of the indexes. If an indexed column has unique values then the selectivity of that index is more, as opposed to an index with nonunique values. Query optimizer uses these indexes in determining whether to choose an index or not while executing a query. Some situations under which you should update statistics: 1) If there is significant change in the key values in the index 2) If a large amount of data in an indexed column has been added, changed, or removed (that is, if the distribution of key values has changed), or the table has been truncated using the TRUNCATE TABLE statement and then repopulated 3) Database is upgraded from a previous version Look up SQL Server books online for the following commands: UPDATE STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROP STATISTICS, sp_autostats, sp_createstats, sp_updatestats What are the different ways of moving data/databases between servers and databases

in SQL Server? There are lots of options available, you have to choose your option depending upon your requirements. Some of the options you have are: BACKUP/RESTORE, dettaching and attaching databases, replication, DTS, BCP, logshipping, INSERT...SELECT, SELECT...INTO, creating INSERT scripts to generate data. Explian different types of BACKUPs avaialabe in SQL Server? Given a particular scenario, how would you go about choosing a backup plan? Types of backups you can create in SQL Sever 7.0+ are Full database backup, differential database backup, transaction log backup, filegroup backup. Check out the BACKUP and RESTORE commands in SQL Server books online. Be prepared to write the commands in your interview. Books online also has information on detailed backup/restore architecture and when one should go for a particular kind of backup. What is database replicaion? What are the different types of replication you can set up in SQL Server? Replication is the process of copying/moving data between databases on the same or different servers. SQL Server supports the following types of replication scenarios: Snapshot replication Transactional replication (with immediate updating subscribers, with queued updating subscribers) Merge replication

See SQL Server books online for indepth coverage on replication. Be prepared to explain how different replication agents function, what are the main system tables used in replication etc. How to determine the service pack currently installed on SQL Server? The global variable @@Version stores the build number of the sqlservr.exe, which is used to determine the service pack installed. To know more about this process visit SQL Server service packs and versions. Back to top Database programming

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What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors? Cursors allow row-by-row prcessing of the resultsets. Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information. Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one rowundtrip,

however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. Here is an example: If you have to give a flat hike to your employees using the following criteria: Salary between 30000 and 40000 -- 5000 hike Salary between 40000 and 55000 -- 7000 hike Salary between 55000 and 65000 -- 9000 hike In this situation many developers tend to use a cursor, determine each employee's salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below: UPDATE tbl_emp SET salary = CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000 WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000 WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000 END Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don't have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row. For examples of using WHILE loop for row by row processing, check out the 'My code library' section of my site or search for WHILE. Write down the general syntax for a SELECT statements covering all the options. Here's the basic syntax: (Also checkout SELECT in books online for advanced syntax). SELECT select_list [INTO new_table_] FROM table_source [WHERE search_condition] [GROUP BY group_by_expression] [HAVING search_condition] [ORDER BY order_expression [ASC | DESC] ] What is a join and explain different types of joins. Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table. Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. For more information see pages from books online titled: "Join Fundamentals" and "Using Joins". Can you have a nested transaction?

Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT What is an extended stored procedure? Can you instantiate a COM object by using TSQL? An extended stored procedure is a function within a DLL (written in a programming language like C, C++ using Open Data Services (ODS) API) that can be called from TSQL, just the way we call normal stored procedures using the EXEC statement. See books online to learn how to create extended stored procedures and how to add them to SQL Server. Yes, you can instantiate a COM (written in languages like VB, VC++) object from T-SQL by using sp_OACreate stored procedure. Also see books online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, sp_OADestroy. For an example of creating a COM object in VB and calling it from T-SQL, see 'My code library' section of this site. What is the system function to get the current user's user id? USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME(). What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand? Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table. In SQL Server 6.5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 7.0 there's no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder Triggers can't be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined. Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster. Till SQL Server 7.0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers. Also check out books online for 'inserted table', 'deleted table' and COLUMNS_UPDATED() There is a trigger defined for INSERT operations on a table, in an OLTP system. The trigger is written to instantiate a COM object and pass the newly insterted rows to it for some custom processing. What do you think of this implementation? Can this be implemented better?

Instantiating COM objects is a time consuming process and since you are doing it from within a trigger, it slows down the data insertion process. Same is the case with sending emails from triggers. This scenario can be better implemented by logging all the necessary data into a separate table, and have a job which periodically checks this table and does the needful. What is a self join? Explain it with an example. Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example: Employees table which contains rows for normal employees as well as managers. So, to find out the managers of all the employees, you need a self join. CREATE TABLE emp ( empid int, mgrid int, empname char(10) ) INSERT INSERT INSERT INSERT INSERT emp emp emp emp emp SELECT SELECT SELECT SELECT SELECT 1,2,'Vyas' 2,3,'Mohan' 3,NULL,'Shobha' 4,2,'Shridhar' 5,2,'Sourabh'

SELECT t1.empname [Employee], t2.empname [Manager] FROM emp t1, emp t2 WHERE t1.mgrid = t2.empid Here's an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses) SELECT t1.empname [Employee], COALESCE(t2.empname, 'No manager') [Manager] FROM emp t1 LEFT OUTER JOIN emp t2 ON t1.mgrid = t2.empid

A "sort merge" join is performed by sorting the two data sets to be joined according to the join keys and then merging them together. The merge is very cheap, but the sort can be prohibitively expensive especially if the sort spills to disk. The cost of the sort can be lowered if one of the data sets can be accessed in sorted order via an index, although accessing a high proportion of blocks of a table via an index scan can also be very expensive in comparison to a full table scan. A hash join is performed by hashing one data set into memory based on join columns and reading the other one and probing the hash table for matches. The hash join is very low cost when the hash table can be held entirely in memory, with the total cost amounting to very

little more than the cost of reading the data sets. The cost rises if the hash table has to be spilled to disk in a one-pass sort, and rises considerably for a multipass sort. You should note that hash joins can only be used for equi-joins, but merge joins are more flexible. In general, if you are joining large amounts of data in an equi-join then a hash join is going to be a better bet.

Oracle
Oracle Database for DBA and Developers

Oracle RAC Interview questions and answers


What are Oracle Clusterware processes for 10g on Unix and Linux Cluster Synchronization Services (ocssd) Manages cluster node membership and runs as the oracle user; failure of this process results in cluster restart. Cluster Ready Services (crsd) The crs process manages cluster resources (which could be a database, an instance, a service, a Listener, a virtual IP (VIP) address, an application process, and so on) based on the resource's configuration information that is stored in the OCR. This includes start, stop, monitor and failover operations. This process runs as the root user Event manager daemon (evmd) A background process that publishes events that crs creates. Process Monitor Daemon (OPROCD) This process monitor the cluster and provide I/O fencing. OPROCD performs its check, stops running, and if the wake up is beyond the expected time, then OPROCD resets the processor and reboots the node. An OPROCD failure results in Oracle Clusterware restarting the node. OPROCD uses the hangcheck timer on Linux platforms. RACG (racgmain, racgimon) Extends clusterware to support Oracle-specific requirements and complex resources. Runs server callout scripts when FAN events occur. What are Oracle database background processes specific to RAC LMSGlobal Cache Service Process LMDGlobal Enqueue Service Daemon LMONGlobal Enqueue Service Monitor

LCK0Instance Enqueue Process To ensure that each Oracle RAC database instance obtains the block that it needs to satisfy a query or transaction, Oracle RAC instances use two processes, the Global Cache Se0rvice (GCS) and the Global Enqueue Service (GES). The GCS and GES maintain records of the statuses of each data file and each cached block using a Global Resource Directory (GRD). The GRD contents are distributed across all of the active instances. What are Oracle Clusterware Components Voting Disk Oracle RAC uses the voting disk to manage cluster membership by way of a health check and arbitrates cluster ownership among the instances in case of network failures. The voting disk must reside on shared disk. Oracle Cluster Registry (OCR) Maintains cluster configuration information as well as configuration information about any cluster database within the cluster. The OCR must reside on shared disk that is accessible by all of the nodes in your cluster How do you troubleshoot node reboot Please check metalink ... Note 265769.1 Troubleshooting CRS Reboots Note.559365.1 Using Diagwait as a diagnostic to get more information for diagnosing Oracle Clusterware Node evictions. How do you backup the OCR There is an automatic backup mechanism for OCR. The default location is : $ORA_CRS_HOME\cdata\"clustername"\ To display backups : #ocrconfig -showbackup To restore a backup : #ocrconfig -restore With Oracle RAC 10g Release 2 or later, you can also use the export command: #ocrconfig -export -s online, and use -import option to restore the contents back. With Oracle RAC 11g Release 1, you can do a manaual backup of the OCR with the command: # ocrconfig -manualbackup How do you backup voting disk #dd if=voting_disk_name of=backup_file_name

How do I identify the voting disk location #crsctl query css votedisk How do I identify the OCR file location check /var/opt/oracle/ocr.loc or /etc/ocr.loc ( depends upon platform) or #ocrcheck Is ssh required for normal Oracle RAC operation ? "ssh" are not required for normal Oracle RAC operation. However "ssh" should be enabled for Oracle RAC and patchset installation. What is SCAN? Single Client Access Name (SCAN) is s a new Oracle Real Application Clusters (RAC) 11g Release 2 feature that provides a single name for clients to access an Oracle Database running in a cluster. The benefit is clients using SCAN do not need to change if you add or remove nodes in the cluster. What is the purpose of Private Interconnect ? Clusterware uses the private interconnect for cluster synchronization (network heartbeat) and daemon communication between the clustered nodes. This communication is based on the TCP protocol. RAC uses the interconnect for cache fusion (UDP) and inter-process communication (TCP). Cache Fusion is the remote memory mapping of Oracle buffers, shared between the caches of participating nodes in the cluster. Why do we have a Virtual IP (VIP) in Oracle RAC? Without using VIPs or FAN, clients connected to a node that died will often wait for a TCP timeout period (which can be up to 10 min) before getting an error. As a result, you don't really have a good HA solution without using VIPs. When a node fails, the VIP associated with it is automatically failed over to some other node and new node re-arps the world indicating a new MAC address for the IP. Subsequent packets sent to the VIP go to the new node, which will send error RST packets back to the clients. This results in the clients getting errors immediately. What do you do if you see GC CR BLOCK LOST in top 5 Timed Events in AWR Report?

This is most likely due to a fault in interconnect network. Check netstat -s if you see "fragments dropped" or "packet reassemblies failed" , Work with your system administrator find the fault with network. How many nodes are supported in a RAC Database? 10g Release 2, support 100 nodes in a cluster using Oracle Clusterware, and 100 instances in a RAC database. Srvctl cannot start instance, I get the following error PRKP-1001 CRS-0215, however sqlplus can start it on both nodes? How do you identify the problem? Set the environmental variable SRVM_TRACE to true.. And start the instance with srvctl. Now you will get detailed error stack. what is the purpose of the ONS daemon? The Oracle Notification Service (ONS) daemon is an daemon started by the CRS clusterware as part of the nodeapps. There is one ons daemon started per clustered node. The Oracle Notification Service daemon receive a subset of published clusterware events via the local evmd and racgimon clusterware daemons and forward those events to application subscribers and to the local listeners. This in order to facilitate: a. the FAN or Fast Application Notification feature or allowing applications to respond to database state changes. b. the 10gR2 Load Balancing Advisory, the feature that permit load balancing accross different rac nodes dependent of the load on the different nodes. The rdbms MMON is creating an advisory for distribution of work every 30seconds and forward it via racgimon and ONS to listeners and applications. 1. Can Oracle's Data Guard be used on Standard Edition, and if so how? How can you test that the standby database is in sync? Oracle's Data Guard technology is a layer of software and automation built on top of the standby database facility. In Oracle Standard Edition it is possible to be a standby database, and update it *manually*. Roughly, put your production database in archivelog mode. Create a hotbackup of the database and move it to the standby machine. Then create a standby controlfile on the production machine, and ship that file, along with all the archived redolog files to the standby server. Once you have all these files assembled, place them in their proper locations, recover the standby database, and you're ready to roll. From this point on, you must manually ship, and manually apply those archived redologs to stay in sync with production.

To test your standby database, make a change to a table on the production server, and commit the change. Then manually switch a logfile so those changes are archived. Manually ship the newest archived redolog file, and manually apply it on the standby database. Then open your standby database in read-only mode, and select from your changed table to verify those changes are available. Once you're done, shutdown your standby and startup again in standby mode. 2. What is the difference between Active Dataguard, and the Logical Standby implementation of 10g dataguard? Active dataguard is mostly about the physical standby. Use physical standby for testing without compromising protection of the production system. You can open the physical standby read/write - do some destructive things in it (drop tables, change data, whatever - run a test - perhaps with real application testing). While this is happening, redo is still streaming from production, if production fails - you are covered. Use physical standby for reporting while in managed recovery mode. Since physical standby supports all of the datatypes - and logical standby does not (11g added broader support, but not 100%) - there are times when logical standby isnt sufficient. It also permits fast incremental backups when offloading backups to a physical standby database. 3. What is a Dataguard? Oracle Dataguard is a disaster recovery solution from Oracle Corporation that has been utilized in the industry extensively at times of Primary site failure, failover, switchover scenarios. 4. What are the uses of Oracle Data Guard? a) Oracle Data Guard ensures high availability, data protection, and disaster recovery for enterprise data. b) Data Guard provides a comprehensive set of services that create, maintain, manage, and monitor one or more standby databases to enable production Oracle databases to survive disasters and data corruptions. c) With Data Guard, administrators can optionally improve production database performance by offloading resource-intensive backup and reporting operations to standby systems. 5. What is Redo Transport Services?

It control the automated transfer of redo data from the production database to one or more archival destinations. Redo transport services perform the following tasks: a) Transmit redo data from the primary system to the standby systems in the configuration. b) Manage the process of resolving any gaps in the archived redo log files due to a network failure. c) Automatically detect missing or corrupted archived redo log files on a standby system and automatically retrieve replacement archived redo log files from the primary database or another standby database. 6. What is apply services? Apply redo data on the standby database to maintain transactional synchronization with the primary database. Redo data can be applied either from archived redo log files, or, if realtime apply is enabled, directly from the standby redo log files as they are being filled, without requiring the redo data to be archived first at the standby database. It also allows read-only access to the data. 7. What is difference between physical and standby databases? The main difference between physical and logical standby databases is the manner in which apply services apply the archived redo data: a) For physical standby databases, Data Guard uses Redo Apply technology, which applies redo data on the standby database using standard recovery techniques of an Oracle database. b) For logical standby databases, Data Guard uses SQL Apply technology, which first transforms the received redo data into SQL statements and then executes the generated SQL statements on the logical standby database. 8. What is Data Guard Broker? Data guard Broker manage primary and standby databases using the SQL command-line interfaces or the Data Guard broker interfaces, including a command-line interface (DGMGRL) and a graphical user interface that is integrated in Oracle Enterprise Manager. It can be used to perform: a) Create and enable Data Guard configurations, including setting up redo transport services and apply services b) Manage an entire Data Guard configuration from any system in the configuration

c) Manage and monitor Data Guard configurations that contain Oracle RAC primary or standby databases d) Simplify switchovers and failovers by allowing you to invoke them using either a single key click in Oracle Enterprise Manager or a single command in the DGMGRL commandline interface. e) Enable fast-start failover to fail over automatically when the primary database becomes unavailable. When fast-start failover is enabled, the Data Guard broker determines if a failover is necessary and initiates the failover to the specified target standby database automatically, with no need for DBA intervention. 9. What are the Data guard Protection modes and summarize each? Maximum availability : This protection mode provides the highest level of data protection that is possible without compromising the availability of a primary database. Transactions do not commit until all redo data needed to recover those transactions has been written to the online redo log and to at least one standby database. Maximum performance : This is the default protection mode. It provides the highest level of data protection that is possible without affecting the performance of a primary database. This is accomplished by allowing transactions to commit as soon as all redo data generated by those transactions has been written to the online log. Maximum protection : This protection mode ensures that no data loss will occur if the primary database fails. To provide this level of protection, the redo data needed to recover a transaction must be written to both the online redo log and to at least one standby database before the transaction commits. To ensure that data loss cannot occur, the primary database will shut down, rather than continue processing transactions. 10. If you didn't have access to the standby database and you wanted to find out what error has occurred in a data guard configuration, what view would you check in the primary database to check the error message? You can check the v$dataguard_status view. Select message from v$dataguard_status; 11. In Oracle 11g, what command in RMAN can you use to create the standby database while the target database is active? Oracle 11g has made it extremely simple to set up a standby database environment because Recovery Manager (RMAN) now supports the ability to clone the existing primary

database directly to the intended standby database site over the network via the DUPLICATE DATABASE command set while the target database is active. RMAN automatically generates a conversion script in memory on the primary site and uses that script to manage the cloning operation on the standby site with virtually no DBA intervention required. You can execute this in a run block in RMAN: duplicate target database for standby dorecover from active database; 12. What additional standby database mode does Oracle 11g offer? Oracle 11g has introduced the Oracle Snapshot Standby Database. In Snapshot Standby Database a physical standby database can easily open in read-write mode and again you can convert it back to the physical standby database. This is suitable for test and development environments and also maintains protection by continuing to receive data from the production database and archiving it for later use. 13. In Oracle 11g how can speed up backups on the standby database? In Oracle 11g, block change tracking is now supported in the standby database. 14. With the availability of Active Data Guard, what role does SQL Apply (logical standby) continue to play? Use SQL Apply for the following requirements: (a) when you require read-write access to a synchronized standby database but do not modify primary data, (b) when you wish to add local tables to the standby database that can also be updated, or (c) when you wish to create additional indexes to optimize read performance. The ability to handle local writes makes SQL Apply better suited to packaged reporting applications that often require write access to local tables that exist only at the target database. SQL Apply also provides rolling upgrade capability for patchsets and major database releases. This rolling upgrade functionality can also be used by physical standby databases beginning with Oracle 11g using Transient Logical Standby. 15. Why would I use Active Data Guard and not simply use SQL Apply (logical standby) that is included with Data Guard 11g? If read-only access satisfies the requirement - Active Data Guard is a closer fit for the requirement, and therefore is much easier to implement than any other approach. Active Data Guard supports all datatypes and is very simple to implement. An Active Data Guard replica can also easily support additional uses - offloading backups from the primary database, serve as an open read-write test system during off-peak hours (Snapshot Standby), and provide an exact copy of the production database for disaster recovery - fully utilizing standby servers, storage and software while in standby role.

16. Why do I need the Oracle 11g Active Data Guard Option? Previous capabilities did not allow Redo Apply to be active while a physical standby database was open read-only, and did not enable RMAN block change tracking on the standby database. This resulted in (a) read-only access to data that was frozen as of the time that the standby database was opened read-only, (b) failover and switchover operations that could take longer to complete due to the backlog of redo data that would need to be applied, and (c) incremental backups that could take up to 20x longer to complete - even on a database with a moderate rate of change. Previous capabilities are still included with Oracle Data Guard 11g, no additional license is required to use previous capabilities. 17. If you wanted to upgrade your current 10g physical standby data guard configuration to 11g, can you upgrade the standby to 11g first then upgrade the primary ? Yes, in Oracle 11g, you can temporarily convert the physical standby database to a logical standby database to perform a rolling upgrade. When you issue the convert command you need to keep the identity: alter database recover logical standby keep identity; 18. If you have a low-bandwidth WAN network, what can you do to improve the Oracle 11g data guard configuration in a GAP detected situation? Oracle 11g introduces the capability to compress redo log data as it transports over the network to the standby database. It can be enabled using the compression parameter. Compression becomes enabled only when a gap exists and the standby database needs to catch up to the primary database. alter system set log_archive_dest_1='SERVICE=DBA11GDR COMPRESSION=ENABLE'; 19. In an Oracle 11g Logical Standby Data Guard configuration, how can you tell the dbms_scheduler to only run jobs in primary database? Oracle 11g, logical standby now provides support for DBMS_SCHEDULER. It is capable of running jobs in both primary and logical standby database. You can use the DBMS_SCHEDULER.SET_ATTRIBUTE procedure to set the database_role. You can specify that the jobs can run only when operating in that particular database role. 20. How can you control when an archive log can be deleted in the standby database in oracle 11g ? In Oracle 11g, you can control it by using the log_auto_delete initialization parameter. The

log_auto_delete parameter must be coupled with the log_auto_del_retention_target parameter to specify the number of minutes an archivelog is maintained until it is purged. Default is 24 hours. For archivelog retention to be effective, the log_auto_delete parameter must be set to true. 21. Can Oracle Data Guard be used with Standard Edition of Oracle ? Yes and No. The automated features of Data Guard are not available in the standard edition of Oracle. You can still however, perform log shipping manually and write scripts to manually perform the steps. If you are on unix platform, you can write shell scripts that identify the logs and then use the scp or sftp command to ship it to the standby server. Then on the standby server, identify which logs have not been applied and apply/recover them maually and remove them once applied.

EXPLAIN PLAN Usage


When an SQL statement is passed to the server the Cost Based Optimizer (CBO) uses database statistics to create an execution plan which it uses to navigate through the data. Once you've highlighted a problem query the first thing you should do is EXPLAIN the statement to check the execution plan that the CBO has created. This will often reveal that the query is not using the relevant indexes, or indexes to support the query are missing. Interpretation of the execution plan is beyond the scope of this article.

Plan Table AUTOTRACE - The Easy Option? EXPLAIN PLAN Statement ID

Related articles.

DBMS_XPLAN : Display Oracle Execution Plans

Plan Table
The explain plan process stores data in the PLAN_TABLE. This table can be located in the current schema or a shared schema and is created using in SQL*Plus as follows.
-- Creating a shared PLAN_TABLE prior to 11g SQL> CONN sys/password AS SYSDBA Connected SQL> @$ORACLE_HOME/rdbms/admin/utlxplan.sql SQL> GRANT ALL ON sys.plan_table TO public; SQL> CREATE PUBLIC SYNONYM plan_table FOR sys.plan_table;

In Oracle 11g a shared PLAN_TABLE is created by default, but you can still create a local version of the table using the "utlxplan.sql" script.

AUTOTRACE - The Easy Option?


Switching on the AUTOTRACE parameter in SQL*Plus causes an explain to be performed on every query.
SQL> SQL> 2 3 4 SET AUTOTRACE ON SELECT * FROM emp e, dept d WHERE e.deptno = d.deptno AND e.ename = 'SMITH'; ENAME JOB MGR HIREDATE SAL

EMPNO COMM ------------------DEPTNO ---------7369 20

---------- --------- ---------- --------- ---------DEPTNO ---------SMITH 20 DNAME LOC -------------- ------------CLERK 7902 17-DEC-80 RESEARCH DALLAS

800

Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE 1 0 NESTED LOOPS 2 1 TABLE ACCESS (FULL) OF 'EMP' 3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT' 4 3 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)

Statistics ---------------------------------------------------------81 recursive calls 4 db block gets 27 consistent gets 0 physical reads 0 redo size 941 bytes sent via SQL*Net to client 425 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL>

This is a relatively easy way to get the execution plan but there is an issue. In order to get the execution plan the statement must be run to completion. If the query is particularly

inefficient and/or returns many rows, this may take a considerable time. At first glance, using the TRACEONLY option of AUTOTRACE seems to remove this issue, but this option merely suppresses the output of the query data, it doesn't prevent the statement being run. As such, long running queries will still take a long time to complete, but they will not present their data. The following example show this in practice.
CREATE OR REPLACE FUNCTION pause_for_secs(p_seconds IN NUMBER) RETURN NUMBER A BEGIN DBMS_LOCK.sleep(p_seconds); RETURN p_seconds; END; / Function created. SQL> SET TIMING ON SQL> SET AUTOTRACE ON SQL> SELECT pause_for_secs(10) FROM DUAL; PAUSE_FOR_SECS(10) -----------------10 1 row selected. Elapsed: 00:00:10.28 Execution Plan ---------------------------------------------------------Plan hash value: 1550022268 -----------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -----------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | | 2 (0)| 00:00:01 | | 1 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 | -----------------------------------------------------------------------Statistics ---------------------------------------------------------189 recursive calls 0 db block gets 102 consistent gets 0 physical reads 0 redo size 331 bytes sent via SQL*Net to client 332 bytes received via SQL*Net from client

4 13 0 1

SQL*Net roundtrips to/from client sorts (memory) sorts (disk) rows processed

SQL> SET AUTOTRACE TRACEONLY SQL> SELECT pause_for_secs(10) FROM DUAL; 1 row selected. Elapsed: 00:00:10.26 Execution Plan ---------------------------------------------------------Plan hash value: 1550022268 -----------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -----------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | | 2 (0)| 00:00:01 | | 1 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 | -----------------------------------------------------------------------Statistics ---------------------------------------------------------0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 331 bytes sent via SQL*Net to client 332 bytes received via SQL*Net from client 4 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL>

The query takes the same time to return (about 10 seconds) whether the TRACEONLY option is used or not. If the TRACEONLY option prevented the query running, you would expect it to return instantly, like an EXPLAIN PLAN. The solution to this is to use the TRACEONLY EXPLAIN option, which only performs the EXPLAIN PLAN, rather than running the statement.

EXPLAIN PLAN
The EXPLAIN PLAN method doesn't require the query to be run, greatly reducing the time it takes to get an execution plan for long-running queries compared to AUTOTRACE. First the query must be explained.
SQL> 2 3 4 5 EXPLAIN PLAN FOR SELECT * FROM emp e, dept d WHERE e.deptno = d.deptno AND e.ename = 'SMITH';

Explained. SQL>

Then the execution plan displayed.


SQL> @$ORACLE_HOME/rdbms/admin/utlxpls.sql Plan Table ------------------------------------------------------------------------------| Operation | Name | Rows | Bytes| Cost | Pstart| Pstop | ------------------------------------------------------------------------------| SELECT STATEMENT | | | | | | | | NESTED LOOPS | | | | | | | | TABLE ACCESS FULL |EMP | | | | | | | TABLE ACCESS BY INDEX RO|DEPT | | | | | | | INDEX UNIQUE SCAN |PK_DEPT | | | | | | ------------------------------------------------------------------------------8 rows selected. SQL>

For parallel queries use the "utlxplp.sql" script instead of "utlxpls.sql". From Oracle 9i onward, you should display execution plans using the DBMS_XPLAN package.

Statement ID
If multiple people are accessing the same plan table, or you would like to keep a history of the execution plans you should use the STATEMENT_ID. This associates a user specified ID with each plan which can be used when retrieving the data.
SQL> 2 3 4 5 EXPLAIN PLAN SET STATEMENT_ID = 'TIM' FOR SELECT * FROM emp e, dept d WHERE e.deptno = d.deptno AND e.ename = 'SMITH';

Explained. SQL> @explain.sql TIM PLAN OBJECT_NAME BYTES COST PARTITION_START PARTITION_STOP -------------------------------------- ------------------- ----- --------------- --------------Select Statement 57 4 1.1 Nested Loops 57 4 2.1 Table Access (Full) EMP 37 3 2.2 Table Access (By Index Rowid) DEPT 20 1 3.1 Index (Unique Scan) PK_DEPT 0 5 rows selected. SQL> OBJECT_TYPE ---------------

TABLE TABLE INDEX (UNIQUE)

By default the Oracle scripts do not accept a statement_id parameter. You can easily modify the scripts or you can use the script listed under DBA Scripts on this site.

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