Tag Archives: Top performance tuning tools

SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) [Page – 5]

15 Friday May 2015

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers)

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Question 41. What is the best value for MAXDOP value?

Answer-

MAXDOP is Max degree of parallelism. This option controls the number of processors that are used for the execution of a query in a parallel plan.

One can use the max degree of parallelism option to limit the number of processors to use for parallel plan execution and to prevent run-away queries from impacting SQL Server performance by using all available CPUs.

The answer is: It depends. It depends on the hardware, the environment (OLTP vs. OLAP), and the load and so on.

The default value for MAXDOP is 0 (zero) and can be set or viewed using (sp_configure). A value of 0 means that SQL Server will use all processors if a query runs in parallel.

For details please visit –
http://www.mssqltips.com/sqlservertip/2650/what-maxdop-setting-should-be-used-for-sql-server/

Question 42. Which is better “Left Outer” Or “NOT EXIST”?

Answer-

In SQL Server, NOT EXISTS and NOT IN predicates are the best ways to search for missing values, as long as both columns in question are NOT NULL. They produce the safe efficient plans with some kind of an Anti-Join.

LEFT JOIN is less efficient, since it makes no attempt to skip the already matched values in the right table, returning all results and filtering them out instead.
http://explainextended.com/2009/09/15/not-in-vs-not-exists-vs-left-join-is-null-sql-server/

Question 43. How to find the statistics are outdated?

Answer –

You can check statistics by using below command –


DBCC SHOW_STATISTICS('TestRIDInxs','Ix_Index')

Table Name – TestRIDInxs
Index Name - 'Ix_Index

Output of the above query is given below-

Statistics

Question 44. How to find the query running on a given SPID?

Answer-

You can use below query to find query running on an active SPID. Number (52) is the SPID showing in the below example.


DECLARE @sqltext VARBINARY(128)
SELECT @sqltext = sql_handle
FROM sys.dm_exec_requests
WHERE session_id = 52
SELECT TEXT
FROM sys.dm_exec_sql_text(@sqltext);

Question 45. Is “TRUNCATE” DDL or DML command?

Answer- DDL

Question 46. I have written a recursive CTE, it’s been running infinitely and failed with an error. The requirement is that the recursion required for 1000 times. How could you be able to handle the situation?

Answer-

Well if you write a proper recursive CTE then it will not run indefinitely.

MAXRECURSION can be used to break the recursive CTE. MAXRECURSION can be used to prevent a poorly formed recursive CTE from entering into an infinite loop.

The following example intentionally creates an infinite loop and uses the MAXRECURSION hint to limit the number of recursion levels to ten.


;WITH CTE AS
(
	SELECT ParentID FROM Hierarchies WHERE ParentID = 1
	UNION ALL
	SELECT C.ParentID FROM Hierarchies H INNER JOIN CTE C
	ON C.ParentID = H.ParentID
)
SELECT * FROM CTE
OPTION (MAXRECURSION 10);
GO

You will also get this kind of message-

Msg 530, Level 16, State 1, Line 1
The statement terminated. The maximum recursion 10 has been exhausted before statement completion.

Question 47. Which is better a CTE or a subquery? Why?

Answer-

Both CTEs and Sub Queries have pretty much the same performance and function.

CTE’s have an advantage over using a subquery in that you can use recursion in a CTE.

The biggest advantage of using CTE is readability. CTEs can be reference multiple times in the same statement where as sub query cannot.

Question 48. Any alternative to triggers?

Answer –

There are some options available depending exclusively on the requirement. They are
Change Data Capture
Change Tracking
Using output clause (Write the data out to a specific Audit table )
Computed column

Question 49. Why resourceDB introduced?

Answer –

It is a read-only database that contains system objects that are included with SQL Server. SQL Server system objects, such as sys.objects, are physically persisted in the Resource database, but they logically appear in the sys schema of every database. The Resource database does not contain user data or user metadata.

Also Resource database makes upgrading to a new version of SQL Server an easier and faster procedure. In earlier versions of SQL Server, upgrading required dropping and creating system objects. Because the Resource database file contains all system objects, an upgrade is now accomplished simply by copying the single Resource database file to the local server.

For details please visit –

http://blogs.msdn.com/b/vsanil/archive/2012/11/02/resource-database-common-questions.aspx

Question 50. How to create linked server.

Answer –

Please use below T-SQL script or follow the screen shots


USE [master]
GO
EXEC MASTER.dbo.sp_addlinkedserver
    @server = N'SRVR002\ACCTG',
    @srvproduct=N'SQL Server' ;
GO

LinkedServer

SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) [Page -3]

15 Friday May 2015

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers)

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Question 21. What are the magic tables? Do we have “Updated” magic table?

Answer-

No we don’t have UPDATED magic table.
The ‘magic tables’ are the INSERTED and DELETED tables, as well as the update() and columns_updated() functions, and are used to determine the changes resulting from DML statements.
• For an INSERT statement, the INSERTED table will contain the inserted rows.
• For an UPDATE statement, the INSERTED table will contain the rows after an update, and the DELETED table will contain the rows before an update.
• For a DELETE statement, the DELETED table will contain the rows to be deleted.

Question 22. What is a filtered index?

Answer-

Consider filter index as — > INDEX with a Where Clause

It uses a filter predicate to index a portion of rows in the table. A filtered index can improve query performance as well as reduce index maintenance and storage costs compared with full-table indexes.

Important points about filtered indexes are –
• They can be created only as NonClustered Index
• They can be used on Views only if they are persisted views.
• They cannot be created on full-text Indexes.

Please check out the sample example below –


CREATE NONCLUSTERED INDEX Ix_NCI ON TestRIDInxs(ID)
WHERE Title= 20

Question 23. Basic difference between stored procedure and user defined function?

Answer-

There are many differences between functions and stored procedures. Some of them are given below-

Functions will allow only Select statement, it will not allow us to use insert, update and delete statement.
Procedures can have select statements as well as DML statements such as insert, update, delete.

Transactions are not allowed within functions. Can use transactions within Stored procedures.

For details please visit –

https://msbiskills.com/2010/08/12/differences-between-stored-procedure-and-function-in-sql-server/

Question 24. See we have a simple query that’s calling a static function, like “Select * from employee where joiningdate < getstaticdate()”? Does it call function for every time or only for matched rows? How you tune this query?

Answer-

We can do something like below-


DECLARE @t AS DATETIME = GETDATE()
SELECT * FROM TestRIDInxs WHERE joiningDate <= @t

Question 25. Why should we use CTE?

Answer-

A CTE can be used:
• For recursion
• Substitute for a view when the general use of a view is not required; that is, you do not have to store the definition in metadata.
• Reference the resulting non large table multiple times in the same statement.

Question 26. What is the difference between sub query and correlated query

Answer-

Subquery: – The inner query is executed only once. The inner query will get executed first and the output of the inner query used by the outer query. The inner query is not dependent on outer query.

Please check out the example below-


SELECT * FROM Visits WHERE CustomerId IN (SELECT CustomerId FROM Visits WHERE VisitDate = '2013-09-03 00:00:00.000')

Correlated subquery: – The outer query will get executed first and for every row of outer query, inner query will get executed. So the inner query will get executed as many times as number of rows in result of the outer query. The outer query output can use the inner query output for comparison. This means inner query and outer query dependent on each other

Please check out the famous example below- ( 2nd Highest Salary )


SELECT * FROM NthHighest N WHERE 1 = (SELECT DISTINCT(COUNT(*)) FROM NthHighest m WHERE n.Salary < m.Salary )

Question 27 – How do you retrieve random 5 rows from a table

Answer –


SELECT TOP 5 * FROM [Table_Name] ORDER BY NEWID()

Question 28 – What exactly you check in the query execution plan window?

Answer –

Query Execution Plans describe the steps and the order used to access data in the database. Normally we check the execution plan for missing indexes, if any. We also check the execution plans if the query or stored procedure is working slowly. If working slowly we need to check for Red flags and see how we can remove them.

You can use execution plan to check out following things –

• Missing Indexes
• Missing Statistics
• Red Flags
• How data is being retrieved (Which path , what all indexes are used)

Question 29 – What is the output of SELECT Len(1234.56)

Answer – 7 (In this case it will consider all the characters including.)

Question 30 – Where in MS SQL Server is ’100’ equal to ‘0’?

Ans – Fill Factor is the answer. Fill-factor settings of 0 and 100 are equal. If fill-factor is set to 100 or 0, the SQL Server Engine fills pages to their capacity while creating indexes.
Server default Value = 0.

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) [Page -2]

14 Thursday May 2015

Posted by in Download SQL Interview Q's, SQL Server Interview Questions

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers)

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Question 11. What do you mean by Cardinality in SQL Server?

Answer-

Cardinality refers to the uniqueness of data values contained in a particular column (attribute) of a database table. The lower the cardinality, the more duplicated elements in a column

There are 3 types of cardinality:

High-cardinality, Normal-cardinality, and Low-cardinality

For details please refer – https://msbiskills.com/2015/04/08/cardinality-in-sql/

_______________________________________________________________

Question 12. What are Histogram and Density Vector?

Answer- 

Density is the ratio of unique values with in the given column or a set of columns. Density measure the uniqueness of column or selectivity of column. Density can have value between 0 and 1. If the column has density value 1, it means all the records have same value in that column and less selectivity. Higher the density lowers the selectivity. If the column has density value 0.003, that means there are 1/0.003=333 distinct values in that column.

Density  = 1 / (Number of distinct values for a column or set of column)

--

DBCC SHOW_STATISTICS('TestRIDInxs','Ix_Index') WITH DENSITY_VECTOR

--

DensityVector

Below information is self-explanatory.

--

DBCC SHOW_STATISTICS('TestRIDInxs','Ix_Index') WITH HISTOGRAM

--

Histogram

_____________________________________________________________________

Question 13. Why we can’t put Order by inside the view?

Answer –

There is no need of using ORDER BY in the View. Order by in a View is pointless.
If you try to include ORDER BY in View, it will throw the following error


Msg 1033, Level 15, State 1, Procedure vw_ViewLimit1, Line 5
The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP or FOR XML is also specified.

You can do below like-

--

SELECT * FROM [View_Name] ORDER By [Column_Name]

--

______________________________________________________________________

Question 14. What is a cross apply and how to use this?

Answer –

The APPLY operator comes in two variants, CROSS APPLY and OUTER APPLY. It is useful for joining two SQL tables or XML expressions. CROSS APPLY is equivalent to an INNER JOIN expression and OUTER APPLY is equivalent to a LEFT OUTER JOIN expression. E.g. below-

--

SELECT f.*,
ROW_NUMBER() OVER(ORDER BY file_id, page_id, slot_id) AS Row
FROM flag f 
CROSS APPLY sys.fn_PhysLocCracker(%%physloc%%)
ORDER BY Row

--

_____________________________________________________________________

Question 15. Can we use more than one CTE in a single select query?

Answer – Yes we can. Please check out the example below.

--

;WITH CTE1 AS 
(
	SELECT 10 Col1
),
CTE2 AS 
(
	SELECT 20 Col2
)
SELECT a.Col1,b.Col2
FROM CTE1 a
FULL OUTER JOIN CTE2 b ON a.Col1 = b.Col2

--

_____________________________________________________________________

Question 16. How do you know the total transaction count?

Answer – Please check out the query below-

--

SELECT * FROM sys.dm_os_performance_counters
WHERE 
1=1
AND counter_name = 'Transactions/sec'
AND instance_name = '_Total';

--

Transaction

_____________________________________________________________________

Question 17. I have created a table variable can we use it in a nested stored procedure? If not what is the scope of a table variable?

Answer-

Scope of the Table variable is the Batch or Stored Procedure in which it is declared. And they can’t be dropped explicitly, they are dropped automatically when batch execution completes or the Stored Procedure execution completes.

--

DECLARE @temp TABLE
(
	col1 INT
)

INSERT @temp VALUES(1)
SELECT * FROM @TEMP

GO

SELECT * FROM @temp

--

After executing above script you will get below error.

(1 row(s) affected)

(1 row(s) affected)


Msg 1087, Level 15, State 2, Line 11
Must declare the table variable “@temp”.

_____________________________________________________________________

Question 18. Top performance tuning tools

Answer-

Query Analyzer, SQL Profiler, Index Wizard, Performance Monitor are some Microsoft tools. Some other tools are given below-

• System Stored Procedures (Transact-SQL)
• Red-Gate SQL Monitor
• Trace Flags (Transact-SQL)
• Database Engine Tuning Advisor
• SQL Sentry Performance Advisor

_______________________________________________________________

Question 19. What is a bitmap index?

Answer-

A bitmap index is a special type of structure used by most high-end database management systems to optimize search and retrieval for low-variability data.

_______________________________________________________________

Question 20. Can we fire a trigger manually?

Answer-

No you can’t. If you want a procedure that can be executed manually, then you should just create a stored procedure.

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