SQL Server Replication Interview Questions & Answers

Question – How many Replication Agents support SQL Server Replication and what are their roles?

Answer – There are total five Replication Agents in SQL Server Replication. Here is the list of all Replication Agents with their description.

1. Snapshot Agent – This agent is used by all types of replication whether its Snapshot Replication, Merge Replication or Transactional Replication. Snapshot Agent prepares initial data files of published objects, stores the snapshot files, and records information about synchronization in the distribution database. The Snapshot Agent runs at the Distributor.
2. Log Reader Agent – This replication agent is used by Transactional replication only. This Agent is very important to run transactional replication smoothly. Log Reader Agent replicates all transactions that are marked for replication from Publisher to Distributor. Every database that is marked for replication has its own log reader agent and it runs on distributor.
3. Distribution Agent – This agent is responsible to move transactions from distributor to subscriber. Distribution agent is used by Snapshot and Transactional Replication. This agent runs on distributor for PUSH subscription and runs on Subscriber for PULL subscription.
4. Merge Agent –The Merge Agent applies the initial snapshots to the Subscribers. It also merges incremental data changes that occurred at the Publisher or Subscribers after the initial snapshot was created, and detects and resolves any conflicts according to rules you configure. This agent also runs at either the Distributor for push subscriptions or the Subscriber for pull subscriptions.
5. Queue Reader Agent – The Queue Reader Agent is used with transactional replication with the queued updating option. The agent runs at the Distributor and moves changes made at the Subscriber back to the Publisher.

Question – What is PUSH and PULL subscription?

Answer – These are subscription modes that defines where should your distribution, merge and queue reader agent will run.

When you choose PUSH subscription, data will be replicated from Publisher to Subscriber. Subscriber will act as passive and will receive data based on publisher and distributor configurations. Distribution and Merge Agent will run on distributor in this type of subscription.

While in PULL subscription, subscriber play a key role. Subscriber can control when it will receive data from publisher in pull subscription. Distribution and Merge Agents will also be running on subscriber in PULL subscription.

Question – What is merge replication and how does it work?

Answer – Merge Replication a type of SQL Server Replication that can be used to synchronize data from publisher to subscriber or vice versa. Merge replication also starts with a snapshot of the publication database objects and data. Subsequent data changes and schema modifications made at the Publisher and Subscribers are tracked with triggers. The Subscriber synchronizes with the Publisher when connected to the network and exchanges all rows that have changed between the Publisher and Subscriber since the last time synchronization occurred. Merge replication is implemented by the SQL Server Snapshot Agent and Merge Agent. The Merge Agent applies the initial snapshots to the Subscribers. It also merges incremental data changes that occurred at the Publisher or Subscribers after the initial snapshot was created, and detects and resolves any conflicts according to rules you configure.

Question – How many Replication Agents are used in Merge Agent?

Answer – Merge Replication uses Snapshot Agent and Merge Agent to replicate the data.

Question – How Merge Replication tracks data changes post initial snapshot creation?

Answer – Merge replication tracks changes through triggers and metadata tables. Merge replication adds the column rowguid to every table, unless the table already has a column of data type uniqueidentifier with the ROWGUIDCOL property set. The newid() function is provided as a default for the rowguid column, however customers can provide a guid for each row if needed.

Question – What is the actual location where the distributor agent runs?

Answer – Distribution agent runs on distributor instance for PUSH subscription and runs on Subscriber for PULL subscription.

Question – Can we Rename Publication Database?

Answer – No, we cannot rename publication database directly but if we want to rename any publication database then we need to drop the publications, rename the database and re-configure replication all over again. We cannot directly rename publication database.

Question – What is the difference between Publisher and Publication?

Answer – Publisher is the database Instance where replicated databases are hosted. Publication is the set of database objects commonly known as articles that are marked for replication.

Question – Can we prevent DELETE commands to replicate on Subscriber that has executed on Publisher?

Answer – We can exclude DELETE commands to replicate on subscriber.

Question – Why Subscribers are not getting data from Publisher?

Answer – There are several reasons for data not being replicated to Subscribers. Some of the reasons are given below.

• Replication Agents jobs are not running or failing during execution.
• The table is filtered.
• Data might be deleted by a trigger.
• Replication of stored procedure execution for a transactional publication produces different results at the Subscriber.
• Data is deleted by a user, a replication script, or another application.

Question – How do you check Replication Latency?

Answer – We can check replication latency either by using Replication monitor or using system stored procedure called sp_replcounters. This stored procedure gives information about the transaction rate, latency, and first and last log sequence number (LSN) for each publication on a server. This is run on the publishing server. Similar information we can get in replication monitor as well.

Question – I want to add a column in a table that has been marked for replication. I don’t want to replicate this column to subscribers. How can we do that?

Answer – To add a new column to a table and not include that column in an existing publication, we need to disable the replication of schema changes and then execute ALTER TABLE <Table_Name> ADD <Column_Name> statement.

Question – How can we drop an article from publication?

Answer – We can drop any article from publication if needed. We need to launch Publication properties page then we need to select the publication from which we need to drop the identified article. To delete an article, clear the check box next to each article you want to delete then click OK.

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Read More:

• SQL Server DBA Interview Questions & Answers
• SQL Server Interview Questions & Answers on Indexes
• Always On Availability Group Interview Questions & Answers
• SQL Server Backup & Recovery Interview Questions
• SQL Server Architecture Interview Questions & Answers

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SQL Server Interview Questions and Answers on Alwayson

Question- How Always ON Availability Group is different from Database Mirroring?

Answer- Always On Availability Group is advanced version of Database Mirroring. We cannot configure database mirroring for more than one database per configuration whereas we can group multiple databases as one availability group and do the failover as one unit in AOAG.

Question- How many Replicas we can create in an AOAG Configuration?

Answer- We can create single primary replica and one to eight secondary replicas in AOAG Configuration.

Question-How many Availability groups we can create in an AOAG configuration?

Answer- The actual number of availability groups you can create on a Server depends on the hardware and workload, but there is no enforced limit.

Question-How many databases can be added in an AOAG configuration?

Answer- The actual number of databases you add to the AOAG depends on the available worker threads on the server, as such there is no enforced limit for databases as well.

Question- I have an AOAG configuration between an FCI and standalone server. Why AOAG is not able to perform automatic failover?

Answer- Automatic Failover is not supported for Availability groups that are configured on Failover Cluster Instances. That is why AOAG is not able to perform automatic failover on such configurations.

Question-What are basic prerequisites for a database to be a part of AOAG?

Answer-Below are the list of prerequisites for a database that needs to be added in Availability Group.

1. Database should be Online for Read-Write Operation.
2. Database must be in Full Recovery Model.
3. Database should be a user database because you cannot configure AOAG for system database.

Question-Can we configure database mirroring on a database that is part of Availability group?

Answer- No, we cannot configure database mirroring for an availability database.

Question- Can we configure Log shipping for a database that is part of Availability Group?

Answer-Yes, we can configure Log shipping for an availability database.

Question- Can we configure Replication for a database that is part of availability group?

Answer- Yes, we can configure replication of a AOAG database.

Question- Can we configure AOAG for master database or any system databases?

Answer- No, we cannot create AOAG for system database.

Question- What is difference between Synchronous-commit mode and Asynchronous-commit mode?

Answer- Under synchronous-commit mode, primary replica waits for a synchronous-commit secondary replica to acknowledge that it has finished hardening the log before committing transactions on primary replica.

Under asynchronous-commit mode, the primary replica commits transactions without waiting for acknowledgement that an asynchronous-commit secondary replica has hardened the log.

Question- What are the benefits of Readable Secondary Replicas?

Answer- Readable Secondary Replicas can be used for reporting purpose. We can divide some workload from primary replica and route all SELECT statements to run on readable secondary replicas. Even we can run database backups as well from these secondary replicas.

Question- How can we run backups from secondary replicas?

Answer- Performing backups on a secondary replica to offload the backup workload from the primary production server is a great benefit.  We can perform log backups and copy_only full backups from secondary replicas. We need to change Backup Preferences setting of AOAG configuration to run these backups from secondary replica. Read below attached article to understand AOAG Backup Preferences and how it works.

Question-If one of your availability database got corrupted or showing in suspect mode, will AOAG initiate failover to secondary replica?

Answer- No, failover will not be initiated due to any issue at database level.

Question- Can we configure AOAG of a database that is running in bulk logged recovery model?

Answer- No, database must be in full recovery model to configure AOAG.

Question- Can we redirect the read only transactions to secondary replica instead of primary replica?

Answer- Yes, we can specify the read_only intent in the connection string and add only secondary replicas to the read_only_routing list.

Question- What is read intent option?

Answer- If Read Intent option is set that means read-only connections are allowed to secondary databases on secondary replica.

Question- What is default settings for read intent or read access transactions?

Answer- By default both read-write and read-intent access are allowed to the primary replica and no connections are allowed to secondary replicas of an Always On availability group.

Question- Why is asynchronous commit mode is faster than synchronous commit mode?

Answer- Asynchronous-commit is faster because primary replica does not wait for any acknowledgement received from secondary replica to harden the logs there. Read More about these in attached article Always On Availability Group.

Question- Is it necessary to have shared storage for AOAG?

Answer- No, it is not necessary to have shared storage of AOAG.

Question- Can we configure AOAG without windows cluster?

Answer- We can create clusterless AOAG in SQL Server 2017 but rest of all previous version you must have windows cluster to create AOAG.

Question- Can we restore a database participating in Always on Availability Group?

Answer- No, we cannot restore a database that is participated in AOAG. Neither we can delete or drop these databases.

Question- How can we Restore a database that is participated in AOAG?

Answer- To Restore a database that is part of Availability Group, first we need to remove it from availability group. Once database will be removed from availability group, you can restore it and later you can add it to the same availability group. Read attached article to learn how to restore a database that is part of AOAG.

Question- What will be the difference between configuring AOAG in multi-subnet network and same subnet network?

Answer- When we need to configure AOAG between multi-subnet cluster, we need to have one IP from each subnet to configure Listener whereas we need only one IP for listener in same subnet.

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Read More Interview Questions:

• SQL Server Interview Q&A on Architecture
• SQL Server Interview Q&A on Indexes
• MSSQL Backup and Recovery Interview Q&A
• SQL Server General DBA Interview Q&A

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Should We Clear Wait Stats Data in SQL Server?

We do not recommend to clear the wait stats data frequently because all valuable information about wait statistics will be vanished if you clear them. Always try to capture wait statistics information before clearing wait stats. This way you would have both before and after data. You can use sys.dm_os_wait_stats DMV to get the SQL Server wait statistics information.

Generally, we reset or clear wait stats information to capture actual data during a given time duration because this DMV does not show only current data. The DMV sys.dm_os_wait_stats accumulates all the counts and wait times since last time SQL Server started or its stats got cleared by dbcc sqlperf command.

Here I will show you how to clear the SQL Server wait statistics and the difference between data captured before clearing the wait stats and after clearing the wait stats.

Get Wait Statistics Data

The DMV sys.dm_os_wait_stats provides essential metrics for diagnosing SQL Server performance problems. This DMV captures all the details like waiting requests counts, signal waits and waiting time since last time SQL Server instance got started or last time you have cleared your wait stats using DBCC SQLPERF command. Run below command to get performance wait stats details of your SQL Server instance.

--Get Wait Statistics using DMV sys.dm_os_wait_stats
SELECT * FROM sys.dm_os_wait_stats
Order by wait_time_ms desc

You can see the details about all the wait types on your SQL Server instance. You can also see the total wait time in milliseconds. This data is very important if you are dealing with any performance issue but as i discussed above details like total wait time, total waiting tasks counts are the accumulated/incremental values.

Reset or CLEAR Wait Statistics

Now you have captured wait stats information since your SQL Server was started. If you want to get only current waiting task counts or waiting time then you need to reset or clear wait statistics data using dbcc sqlperf command. Run below command to clear all wait stats data on your SQL Server Instance.

DBCC SQLPERF(“sys.dm_os_wait_stats”, CLEAR)

You can see I have successfully executed this command in below screenshot.

Now we will again run same command to get the wait statistics data. I ran it and I get below information that is showing different than the one showing before clearing wait stats.

You can see how you will lose valuable information if you will clear your wait stats without capture its earlier data. This way you can also better analyze your database wait types, waiting tasks to reach on any conclusion to fix the issue.

Related Articles

• Understanding SQL Server Wait types
• Identify Disk Bottleneck in SQL Server using Perfmon Disk Counters
• Understanding Resource_Semaphore wait type

Now you understand that we should not reset or clear wait stats frequently because wait stats data is very crucial to deal with any performance issue. You can reset or clear the wait stats but make sure to capture the data before clearing the wait stats. I hope you like this article. Please follow our Facebook page and Twitter handle to get latest updates.

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Related Articles:

• SQL Server Interview Q&A on Alwayson Availability Group
• SQL Server DBA Interview Questions & Answers
• SQL Server Backup & Recovery Interview Questions & Answers
• SQL Server Architecture Interview Questions & Answers
• Index Rebuild vs Index Reorganize Operation

SQL Server Interview Questions & Answers on Indexes

Question – How data stores in Indexes?

Answer – Data stores in indexes or in tables on a series of 8 kb data pages. Indexes Data on these data pages are organized in a B-Tree structure that supports fast retrieval of the rows, based on their index key values. Each page in an index B-tree is called an index node. The top node of the B-tree is called the root node. The bottom level of nodes in the index is called the leaf nodes. The leaf nodes of a nonclustered index is made up of index pages instead of data pages whereas in a clustered index, the leaf nodes contain the data pages.

Question – What is Fill Factor and what should be the perfect value for it?

Answer – A fill factor value determines the percentage of space on each leaf-level page to be filled with data, reserving the remainder on each page as free space for future growth. Fill factor values 0 and 100 are the same in all respects. The fill-factor option is provided for fine-tuning index data storage and performance. Although, there is no defined value that we can say is perfect value for fill factor. You can set it to somewhere around 80% and monitor fragmentation over time. Then, you can tweak its value up or down depending on how fragmented the indexes get. Read more about fill factor and its best value.

Question – Explain Page Split and whether it is good for SQL Server or bad?

Answer – Whenever you update existing rows with the data that is bigger in size to save on their data page then Page Split operation occur to make space for this new update. Page split reduces performance so we can say page split is not at all a good thing for our database.

Question – How page split is bad for performance?

Answer – Page split is a resource intensive operation and causes fragmentation that leads to deficient performance in terms of increased I/O operations. We should take it seriously if there are frequent page splits occur and consider reducing its occurrences

Question – What is Difference between clustered and nonclustered Indexes?

Answer – Clustered indexes sort and store the data rows in the table or view based on their key values. There can be only one clustered index per table, because the data rows themselves can be sorted in only one order.

Nonclustered indexes have the same B-tree structure as clustered indexes. The data rows of the underlying table are not sorted and stored in order based on their non clustered keys. The leaf layer of a nonclustered index is made up of index pages instead of data pages. You can create up to 999 nonclustered indexes on a table. 

Question – Does indexes always improve database performance?

Answer – NO, sometimes indexes can reduce your database performance if you are running a bulk data upload or your application has a nature of frequent data Insert operations. Indexes are very useful if you are accessing or reading the data.

Question – What is filter index?

Answer –  An optimized nonclustered index, especially suited to cover queries that select from a well-defined subset of data. It uses a filter predicate to index a portion of rows in the table. A well-designed filtered index can improve query performance, reduce index maintenance costs, and reduce index storage costs compared with full-table indexes.  

Question – What is covering index?

Answer – A covering index is one which can satisfy all requested columns in a query without performing a further lookup into the clustered index.

Question – Explain column store index?

Answer – An in-memory columnstore index stores and manages data by using column-based data storage and column-based query processing.

Columnstore indexes work well for data warehousing workloads that primarily perform bulk loads and read-only queries. 

Question – Why can’t a table have more than one clustered indexes?

Answer – Clustered indexes sort and store the data rows in the table or view based on their key values so you can’t store data in table based on two index key that’s why you can create only one cluster index on a table.

Question -What is Heap and when it is useful from clustered table?

Answer – Heap is a table without having cluster indexes. Heaps are generally useful if you are running huge bulk data load or frequent DML operations.

Question – How do you find about missing indexes that you need to create to improve database performance?

Answer – We can run SQL Server database Tuning Advisor that will suggests us about all the missing indexes that we should create to improve db performance. We can also use DMVs related to missing indexes sys.dm_db_missing_index_details, sys.dm_db_missing_index_groups and sys.dm_db_missing_index_columns to get these details.

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SQL Server DBA Interview Questions and Answers

Question- What is your troubleshooting strategy? Suppose a user reported an issue, how will you start to fix the issue?

Answer- A good troubleshooters are those who understand the in-depth information about the product\features where issue arises. To fix any issue, we should follow below points.

1. First we should understand the issue/error codes, ask questions to the user about when and how they get error.
2. If we are not getting anything from step 2,  we need to read error log file, Job history log, and Event log files based on the nature of the issue.
3. Look into SQL Server transactions and analyze the wait types and other parameters like CPU, IO, Memory if it is related to performance issue.
4. If we are still not getting any solution, try to search the error on social sites or take help from your seniors because you are not the only one who has received this error.

Question- Tell me in which sequence SQL Server databases come ONLINE after starting or restarting a SQL Server Instance.

Answer- There is no defined sequence for user databases to come online. Although, below is the sequence in which system databases come online. User databases can come online anytime after master database become accessible.

1. Master
2. MSDB
3. ResourceDB (Although this database is not visible in SSMS)
4. Model
5. Tempdb

Question- What should be the optimum AutoGrowth size for SQL Server databases?

Answer- Optimum Autogrowth size of any database file is based on the data growth. We should analyze the data growth and set autogrowth size accordingly. Ideally, Microsoft suggests to keep one-eighth of the size of data file. Read attached article to Understand database Autogrowth in SQL Server.

Question- How will you check the total number of autogrowth events occurred on your database?

Answer- You can get it by running Disk Usage dashboard report of any database. There is a section of all autogrowth and auto shrink events occurred on the database. Have a look in the attached article where i have shown this dashboard report to see number of autogrowth events for a database.

Question-What will be the impact on database if you have default value of Autogrowth or you keep small value of autogrowth size?

Answer- If we go with default value of Autogrowth or any amount that is less in size and not per your database growth pattern will lead to the performance issue. Read attached article to know why performance issue will be occurred if you have inappropriate value set to autogrowth.

Questions- What is compatibility level and how it impacts SQL Server?

Answer- Compatibility level shows the SQL Server version on which your database is running. This is used to support the backward compatibility. You can run your databases to the lower version of SQL Server having higher version of SQL Server installed on your machine by setting compatibility level of the corresponding SQL Server version.

Question- What do you think, should we enable Auto_Shrink option for a database?

Answer- We should never enable database auto shrink property for any database. Read this article to understand why we should not enable auto shrink for any database.

Question- What would be your options if your database files are full with maximum disk capacity?

Answer- We have below options in case disk space is full where data files are placed.

1. Add Additional Disk Space to the same drive.
2. Add additional data file to the different disk where you have enough space or you can add new disk as well to create this data file.
3. Move the data file to another drive that has enough space.
4. Delete\Purge unused data from your database to make some space.

We can choose any option depending on the requirement.

 Question- What will you do if your log file drive is full during data load?

Answer- We have below options in case you are in middle of data load.

1. Run Transaction log backup frequently.
2. Add another log file in another disk where is enough space to grow the log.
3. You can enable Auto shrink during data load if you are running with SIMPLE recovery model.

Question – How many SQL Server Instances we can install on a cluster Instance?

Answer- We can install 25 instance on clustered environment and maximum 50 instances on standalone instances.

Question- What is the difference between updating statistics using sp_updatestats and UPDATE STATISTICS?

Answer- sp_updatestats updates only the statistics that require updating based on the rowmodctr information in the sys.sysindexes catalog view, thus avoiding unnecessary updates of statistics on unchanged rows.

While going with UPDATE STATISTICS you have quite lot of options to use for stats update. This command scan all tables and data based on your scan value during updating statistics. This can take lot of time and performed intensive IO operations.

Question- Do we need to update Statistics after index rebuild operation?

Answer- No, Update stats is not required after index rebuild operations because statistics got updated during index rebuild operation.

Read More SQL Server DBA Interview Questions:

• SQL Server Interview Q&A on Alwayson Availability Group
• SQL Server Backup & Recovery Interview Questions
• SQL Server Interview Questions & Answers on Indexes
• SQL Server Architecture Interview Questions & Answers

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