What are the Advantages of Using CTE?
- Using CTE improves the readability and enables easy maintenance of complex queries.
- The query can be divided into separate, simple, and logical building blocks, which can be then used to build more complex CTEs until the final result set is generated.
- CTE can be defined in functions, stored procedures, triggers or even views.
- After a CTE is defined, it can be used as a Table or a View and can SELECT, INSERT, UPDATE or DELETE Data.
How can we Rewrite Sub-Queries into Simple Select Statements or with Joins?
Yes. We can rewrite sub-queries using the Common Table Expression (CTE). A Common Table Expression (CTE) is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query.
e.g.
USE AdventureWorks
GO
WITH EmployeeDepartment_CTE AS (
SELECT EmployeeID,DepartmentID,ShiftID
FROM HumanResources.EmployeeDepartmentHistory
)
SELECT ecte.EmployeeId,ed.DepartmentID, ed.Name,ecte.ShiftID
FROM HumanResources.Department ed
INNER JOIN EmployeeDepartment_CTE ecte ON ecte.DepartmentID = ed.DepartmentID
GO
What is CLR?
In SQL Server 2008, SQL Server objects such as user-defined functions can be created using such CLR languages. This CLR language support extends not only to user-defined functions, but also to stored procedures and triggers. You can develop such CLR add-ons to SQL Server using Visual Studio 2008. (Read more here)
What are Synonyms?
Synonyms give you the ability to provide alternate names for database objects. You can alias object names; for example, using the Employee table as Emp. You can also shorten names. This is especially useful when dealing with three and four part names; for example, shortening server.database.owner.object to object. (Read more here)
What is LINQ?
Language Integrated Query (LINQ) adds the ability to query objects using .NET languages. The LINQ to SQL object/relational mapping (O/RM) framework provides the following basic features:
- Tools to create classes (usually called entities) mapped to database tables
- Compatibility with LINQ’s standard query operations
- The DataContext class with features such as entity record monitoring, automatic SQL statement generation, record concurrency detection, and much more
What are Isolation Levels?
Transactions specify an isolation level that defines the degree to which one transaction must be isolated from resource or data modifications made by other transactions. Isolation levels are described in terms of which concurrency side-effects, such as dirty reads or phantom reads, are allowed.
Transaction isolation levels control the following:
- Whether locks are taken when data is read, and what type of locks are requested.
- How long the read locks are held.
- Whether a read operation referencing rows modified by another transaction
- blocks until the exclusive lock on the row is freed,
- retrieves the committed version of the row that existed at the time the statement or transaction started, and
- reads the uncommitted data modification.(Read more here)
What is Use of EXCEPT Clause?
EXCEPT clause is similar to MINUS operation in Oracle. The EXCEPT query and MINUS query return all rows in the first query that are not returned in the second query. Each SQL statement within the EXCEPT query and MINUS query must have the same number of fields in the result sets with similar data types. (Read more here)
What is XPath?
XPath uses a set of expressions to select nodes to be processed. The most common expression that you’ll use is the location path expression, which returns back a set of nodes called a node set. XPath can use both an unabbreviated and abbreviated syntax. The following is the unabbreviated syntax for a location path:
/axisName::nodeTest[predicate]/axisName::nodeTest[predicate]
What is NOLOCK?
Using the NOLOCK query optimizer hint is generally considered a good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken on data when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay (blocking). (Read more here)
What is the Difference between Update Lock and Exclusive Lock?
When Exclusive Lock is on any process, no other lock can be placed on that row or table. Every other process have to wait till Exclusive Lock completes its tasks.
Update Lock is a type of Exclusive Lock, except that it can be placed on the row which already has Shared Lock on it. Update Lock reads the data of the row which has the Shared Lock as soon as the Update Lock is ready to change the data it converts itself to the Exclusive Lock.How will you Handle Error in SQL SERVER 2008?
SQL Server now supports the use of TRY…CATCH constructs for providing rich error handling. TRY…CATCH lets us build error handling at the level we need, in the way we need to by setting a region where if any error occurs, it will break out of the region and head to an error handler. The basic structure is as follows:
BEGIN TRY <code> END TRY BEGIN CATCH <code> END CATCH
So if any error occurs in the TRY block, then execution is diverted to the CATCH block, and the error can be resolved.
What is RAISEERROR?
RaiseError generates an error message and initiates error processing for the session. RAISERROR can either reference a user-defined message stored in the sys.messages catalog view or build a message dynamically. The message is returned as a server error message to the calling application or to an associated CATCH block of a TRY…CATCH construct. (Read more here)
How to Rebuild the Master Database?
Master database is system database and it contains information about running server’s configuration. When SQL Server 2005 is installed, it usually creates master, model, msdb, tempdb, resourcedb and the distribution system database by default. Only the Master database is the one which is absolutely a must-have database. Without the Master database, the SQL Server cannot be started. This is the reason why it is extremely important to backup the Master database.
To rebuild the Master database, run Setup.exe, verify, and repair a SQL Server instance, and rebuild the system databases. This procedure is most often used to rebuild the master database for a corrupted installation of SQL Server. (Read more here)
What is the XML Datatype?
The xml data type lets you store XML documents and fragments in a SQL Server database. An XML fragment is an XML instance that has a missing single top-level element. You can create columns and variables of the xml type and store XML instances in them. The xml data type and associated methods help integrate XML into the relational framework of SQL Server.
What is Data Compression?
In SQL SERVE 2008, Data Compression comes in two flavors:
- Row Compression
- Page Compression
Row Compression
Row compression changes the format of physical storage of data. It minimize the metadata (column information, length, offsets etc) associated with each record. Numeric data types and fixed-length strings are stored in variable-length storage format, just like Varchar. (Read more here)
Page Compression
Page compression allows common data to be shared between rows for a given page. It uses the following techniques to compress data:
- Row compression.
- Prefix Compression. For every column in a page, duplicate prefixes are identified. These prefixes are saved in compression information headers which resides after the page header. A reference number is assigned to these prefixes and that reference number is replaced where ever those prefixes are being used.
Dictionary Compression
Dictionary compression searches for duplicate values throughout the page and stores them in CI. The main difference between prefix and dictionary compression is that the former is only restricted to one column while the latter is applicable to the complete page.
What is Use of DBCC Commands?
The Transact-SQL programming language provides DBCC statements that act as Database Console Commands for SQL Server. DBCC commands are used to perform the following tasks.
- Maintenance tasks on database, index, or filegroup.
- Tasks that gather and display various types of information.
- Validation operations on a database, table, index, catalog, filegroup, or allocation of database pages.
- Miscellaneous tasks such as enabling trace flags or removing a DLL from memory. (Read more here)
How to Copy the Tables, Schema and Views from one SQL Server to Another?
There are multiple ways to do this.
- “Detach Database” from one server and “Attach Database” to another server.
- Manually script all the objects using SSMS and run the script on a new server.
- Use Wizard of SSMS. (Read more here)
How to Find Tables without Indexes?
Run the following query in the Query Editor.
USE <database_name>;
GO
SELECT SCHEMA_NAME(schema_id) AS schema_name
,name AS table_name
FROM sys.tables
WHERE OBJECTPROPERTY(OBJECT_ID,'IsIndexed') = 0
ORDER BY schema_name, table_name;
GO
How to Copy Data from One Table to Another Table?
There are multiple ways to do this.
1) INSERT INTO SELECT
This method is used when table is already created in the database earlier and data have to be inserted into this table from another table. If columns listed in the INSERT clause and SELECT clause are same, listing them is not required.
2) SELECT INTO
This method is used when table is not created earlier and it needs to be created when data from one table must be inserted into a newly created table from another table. The new table is created using the same data types as those in selected columns. (Read more here)
What is Catalog Views?
Catalog views return information that is used by the SQL Server Database Engine. Catalog Views are the most general interface to the catalog metadata and provide the most efficient way to obtain, transform, and present customized forms of this information. All user-available catalog metadata is exposed through catalog views.
What is PIVOT and UNPIVOT?
A Pivot Table can automatically sort, count, and total the data stored in one table or spreadsheet and create a second table displaying the summarized data. The PIVOT operator turns the values of a specified column into column names, effectively rotating a table.
In simpler word UNPIVOT table is reverse of PIVOT Table, however it is not exactly true. UNPIVOTING is for sure reverse operation to PIVOTING but if during PIVOTING process data aggregated the UNPIVOT table does not return to original table. (Read more here)
What is a Filestream?
Filestream allows you to store large objects in the file system and have these files integrated within the database. It enables SQL Server-based applications to store unstructured data such as documents, images, audios and videos in the file system. FILESTREAM basically integrates the SQL Server Database Engine with New Technology File System (NTFS); it basically stores the data in varbinary (max) data type. Using this data type, the unstructured data is stored in the NTFS file system, and the SQL Server Database Engine manages the link between the Filestream column and the actual file located in the NTFS. Using Transact-SQL statements users can insert, update, delete and select the data stored in FILESTREAM-enabled tables.
What is SQLCMD?
sqlcmd is enhanced version of the isql and osql, and it provides way more functionality than other two options. In other words, sqlcmd is better replacement of isql (which will be deprecated eventually) and osql (not included in SQL Server 2005 RTM). sqlcmd can work in two modes – i) BATCH and ii) interactive modes. (Read more here)
What do you mean by TABLESAMPLE?
TABLESAMPLE allows you to extract a sampling of rows from a table in the FROM clause. The rows retrieved are random and they are not in any order. This sampling can be based on a percentage of number of rows. You can use TABLESAMPLE when only a sampling of rows is necessary for the application instead of a full result set. (Read more here)
What is ROW_NUMBER()?
ROW_NUMBER() returns a column as an expression that contains the row’s number within the result set. This is only a number used in the context of the result set; if the result changes, the ROW_NUMBER() will change.
What are Ranking Functions?
Ranking functions return a ranking value for each row in a partition. All the ranking functions are non-deterministic. The different Ranking functions are as follows:
ROW_NUMBER () OVER ([
Returns the sequential number of a row within a partition of a result set, starting at 1 for the first row in each partition.
RANK () OVER ([
Returns the rank of each row within the partition of a result set.
DENSE_RANK () OVER ([
Returns the rank of rows within the partition of a result set, without any gaps in the ranking. (Read more here )
What is Change Data Capture (CDC) in SQL Server 2008?
Change Data Capture (CDC) records INSERTs, UPDATEs, and DELETEs applied to SQL Server tables and makes a record available of what changed, where, and when, in simple relational ‘change tables’ rather than in an esoteric chopped salad of XML. These change tables contain columns that reflect the column structure of the source table you have chosen to track along with the metadata needed to understand the changes that have been made.3) Questions on SQL SERVER 2008
What are the basic functions for master, msdb, model, tempdb and resource databases?
The master database holds information for all the databases located on the SQL Server instance, and it is the glue that holds the engine together. Because SQL Server cannot start without a functioning master database, you must administer this database with care.
The msdb database stores information regarding database backups, SQL Agent information, DTS packages, SQL Server jobs, and some replication information such as for log shipping.
The tempdb holds temporary objects such as global and local temporary tables and stored procedures.
The model is essentially a template database used in the creation of any new user database created in the instance.
The resource Database is a read-only database that contains all the system objects that are included in the 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.
What is the Maximum Number of Index per Table?
For SQL Server 2005:
1 Clustered Index + 249 Nonclustered Index = 250 Index.
For SQL Server 2008:
1 Clustered Index + 999 Nonclustered Index = 1000 Index. (Read more here)
Explain Few of the New Features of SQL Server 2008 Management Studio
SQL Server 2008 Microsoft has upgraded SSMS with many new features as well as added tons of new functionalities requested by DBAs for long time.
A few of the important new features are as follows:
- IntelliSense for Query Editing
- Multi Server Query
- Query Editor Regions
- Object Explorer Enhancements
- Activity Monitors
Explain IntelliSense for Query Editing:
After implementing IntelliSense, we will not have to remember all the syntax or browse online references. IntelliSense offers a few additional features besides just completing the keyword.
Explain MultiServer Query:
SSMS 2008 has a feature to run a query on different servers from one query editor window. First of all, make sure that you registered all the servers under your registered server. Once they are registered, right click on server group name and click New Query.
e.g. for server version information,
SELECT
SERVERPROPERTY('Edition') AS Edition,
SERVERPROPERTY('ProductLevel') AS ProductLevel,
SERVERPROPERTY('ProductVersion') AS ProductVersion
Explain Query Editor Regions:
When the T-SQL code is more than hundreds of lines, after a while, it becomes more and more confusing.
The regions are defined by the following hierarchy:
From first GO command to the next GO command.
Statements between BEGIN – END, BEGIN TRY – END TRY, BEGIN CATCH – END CATCH
Explain Object Explorer Enhancements:
In Object Explorer Detail, the new feature is Object Search. Enter any object name in the object search box and the searched result will be displayed in the same window as Object Explorer Detail.
Additionally, there are new wizards which help you perform several tasks, from policy management to disk monitoring. One cool thing is that everything displayed in the object explorer details screen can be right away copied and pasted to Excel without any formatting issue.
Explain Activity Monitors:
There are four graphs
- percent; Processor Time,
- Waiting Tasks,
- Database I/O,
- Batch Requests/Sec
What is Service Broker?
Service Broker is a message-queuing technology in SQL Server that allows developers to integrate SQL Server fully into distributed applications. Service Broker is a feature which provides facility to SQL Server to send an asynchronous, transactional message. It allows a database to send a message to another database without waiting for the response; so the application will continue to function if the remote database is temporarily unavailable. (Read more here)
Where are SQL server Usernames and Passwords Stored in the SQL server?
They get stored in System Catalog Views, sys.server_principals and sys.sql_logins. However, you will not find password stored in plain text.
What is Policy Management?
Policy Management in SQL SERVER 2008 allows you to define and enforce policies for configuring and managing SQL Server across the enterprise. Policy-Based Management is configured in SQL Server Management Studio (SSMS). Navigate to the Object Explorer and expand the Management node and the Policy Management node; you will see the Policies, Conditions, and Facets nodes. (Read More Here)
What is Database Mirroring?
Database mirroring can be used with replication to provide availability for the publication database. Database mirroring involves two copies of a single database that typically reside on different computers. At any given time, only one copy of the database is currently available to clients, which is known as the principal database. Updates made by the clients to the principal database are applied to the other copy of the database, known as the mirror database. Mirroring involves applying the transaction log from every insertion, update, or deletion made on the principal database onto the mirror database.
What are Sparse Columns?
A sparse column is another tool used to reduce the amount of physical storage used in a database. They are the ordinary columns that have an optimized storage for null values. Sparse columns reduce the space requirements for null values at the cost of more overhead to retrieve non-null values. (Read more here)
What does TOP Operator Do?
The TOP operator is used to specify the number of rows to be returned by a query. The TOP operator has new addition in SQL SERVER 2008 that it accepts variables as well as literal values and can be used with INSERT, UPDATE, and DELETE statements.
What is CTE?
CTE is the abbreviation for Common Table Expression. A CTE is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query. (Read more here)
What is MERGE Statement?
MERGE is a new feature that provides an efficient way to perform multiple DML operations. In previous versions of SQL Server, we had to write separate statements to INSERT, UPDATE, or DELETE data based on certain conditions, but now, using MERGE statement, we can include the logic of such data modifications in one statement that even checks when the data is matched, then just update it, and when unmatched, insert it. One of the most important advantages of MERGE statement is all the data is read and processed only once. (Read more here)
What is Filtered Index?
Filtered Index is used to index a portion of rows in a table that means it applies filter on INDEX which improves query performance, reduces index maintenance costs, and reduces index storage costs when compared with full-table indexes. When we see an Index created with a WHERE clause, then that is actually a FILTERED INDEX.
Which are the New Data Types Introduced in SQL SERVER 2008?
The GEOMETRY Type: The GEOMETRY datatype is a system .NET common language runtime (CLR) datatype in SQL Server. This type represents data in a two-dimensional Euclidean coordinate system.
The GEOGRAPHY Type: The GEOGRAPHY datatype’s functions are the same as with GEOMETRY. The difference between the two is that when you specify GEOGRAPHY, you are usually specifying points in terms of latitude and longitude.
New Date and Time Data types: SQL Server 2008 introduces four new data types related to date and time: DATE, TIME, DATETIMEOFFSET, and DATETIME2.
- DATE: The new DATE data type just stores the date itself. It is based on the Gregorian calendar and handles years from 1 to 9999.
- TIME: The new TIME (n) type stores time with a range of 00:00:00.0000000 through 23:59:59.9999999. The precision is allowed with this type. TIME supports seconds down to 100 nanoseconds. The n in TIME(n) defines this level of fractional second precision from 0 to 7 digits of precision.
- The DATETIMEOFFSET Type: DATETIMEOFFSET (n) is the time-zone-aware version of a datetime datatype. The name will appear less odd when you consider what it really is: a date + time + time-zone offset. The offset is based on how far behind or ahead you are from Coordinated Universal Time (UTC) time.
- The DATETIME2 Type: It is an extension of the datetime type in earlier versions of SQL Server. This new datatype has a date range covering dates from January 1 of year 1 through December 31 of year 9999. This is a definite improvement over the lower boundary of 1753 of the datetime datatype. DATETIME2 not only includes the larger date range, but also has a timestamp and the same fractional precision that TIME type provides.
How can I Track the Changes or Identify the Latest Insert-Update-Delete from a Table?
In SQL Server 2005 and earlier versions, there is no inbuilt functionality to know which row was recently changed and what the changes were. However, in SQL Server 2008, a new feature known as Change Data Capture (CDC) has been introduced to capture the changed data. (Read more here)
What is the CPU Pressure?
CPU pressure is a state wherein the CPU is fully occupied with currently assigned tasks and there are more tasks in the queue that have not yet started. (Read more here)
How can I Get Data from a Database on Another Server?
If you want to import data only through T-SQL query, then use OPENDATASOURCE function. To repeatedly get data from another server, create a linked server and then use the OPENQUERY function or use 4-part naming. If you are not adhered with T-SQL, then it is better to use import/export wizard, and you can save it as a SSIS package for future use. (Read more here)
What is the Bookmark Lookup and RID Lookup?
When a small number of rows are requested by a query, the SQL Server optimizer will try to use a non-clustered index on the column or columns contained in the WHERE clause to retrieve the data requested by the query. If the query requests data from columns not present in the non-clustered index, then the SQL Server must go back to the data pages to get the data in those columns. Even if the table has a clustered index or not, the query will still have to return to the table or clustered index to retrieve the data.
In the above scenario, if table has clustered index, it is called bookmark lookup (or key lookup); if the table does not have clustered index, but a non-clustered index, it is called RID lookup. (Read more here)
What is Difference between ROLLBACK IMMEDIATE and WITH NO_WAIT during ALTER DATABASE?
ROLLBACK AFTER integer [SECONDS] | ROLLBACK IMMEDIATE:
Specifies whether to roll back after a specified number of seconds or immediately if transaction is not complete.
NO_WAIT:
Specifies that if the requested database state or option change cannot complete immediately without waiting for transactions to commit or roll back on their own, then the request will fail.(Read more here)
What is Difference between GETDATE and SYSDATETIME in SQL Server 2008?
In case of GETDATE, the precision is till milliseconds, and in case of SYSDATETIME, the precision is till nanoseconds.(Read More Here)
How can I Check that whether Automatic Statistic Update is Enabled or not?
The following query can be used to know if Automatic Statistic Update:
SELECT is_auto_create_stats_on,is_auto_update_stats_on
FROM sys.databases
WHERE name =‘YOUR DATABASE NAME
How to Find Index Size for Each Index on Table?
We can use the following query to find the size of index.
SELECT *
FROM sys.indexes
WHERE OBJECT_ID=OBJECT_ID('HumanResources.Shift')
What is the Difference between Seek Predicate and Predicate?
Seek Predicate is the operation that describes the b-tree portion of the Seek. Predicate is the operation that describes the additional filter using non-key columns. Based on the description, it is very clear that Seek Predicate is better than Predicate as it searches indexes, whereas in Predicate, the search is on non-key a column – which implies that the search is on the data in page, files itself.
What are Basics of Policy Management?
SQL server 2008 has introduced a policy management framework, which is the latest technique for SQL server database engine. SQL policy administrator uses SQL Server Management Studio to create policies that can handle entities on the server side like the SQL Server objects and the instance of SQL Server databases. It consists of three components: policy administrators (who create policies), policy management, and explicit administration. Policy-based management in SQL Server assists the database administrators in defining and enforcing policies that tie to database objects and instances. These policies allow the administrator to configure and manage SQL server across the enterprise. (Read more here)
What are the Advantages of Policy Management?
The following advantages can be achieved by appropriate administration of policy management system.
- It interacts with various policies for successful system configuration.
- It handles the changes in the systems that are the result of configuration against authoring policies.
- It reduces the cost of ownership with simple elaboration of administration tasks.
- It detects various compliance issues in SQL Server Management Studio.
What are Policy Management Terms?
To have a better grip on the concept of Policy-based management, there are some key terms you need to understand.
Target – A type of entity that is appropriately managed by Policy-based management. For example, a table, database and index, to name a few.
Facet -A property that can be managed in policy-based management. A clear example of facet is the name of Trigger or the Auto Shrink Property of database.
Conditions – Criteria that specifies the state of facet to true or false. For example, you can adjust the state of a facet that gives you clear specifications of all stored procedures in the Schema ‘Banking’.
Policy – A set of rules specified for the server objects or the properties of database.
What is the ‘FILLFACTOR’?
A “FILLFACTOR” is one of the important arguments that can be used while creating an index.
According to MSDN, FILLFACTOR specifies a percentage that indicates how much the Database Engine should fill each index page during index creation or rebuild. Fill-factor is always an integer valued from 1 to 100. The fill-factor option is designed for improving index performance and data storage. By setting the fill-factor value, you specify the percentage of space on each page to be filled with data, reserving free space on each page for future table growth.
Specifying a fill-factor value of 70 would imply that 30 percent of each page will be left empty, providing space for index expansion as data is added to the underlying table. The fill-factor setting applies only when the index is created or rebuilt. (Read more here)
Where in MS SQL Server is ’100’ equal to ‘0’?
Fill-factor settings of 0 and 100 are equal! (Read more here)
What are Points to Remember while Using the FILLFACTOR Argument?
- If fill-factor is set to 100 or 0, the Database Engine fills pages to their capacity while creating indexes.
- The server-wide default FILLFACTOR is set to 0.
- To modify the server-wide default value, use the sp_configure system stored procedure.
- To view the fill-factor value of one or more indexes, use sys.indexes.
- To modify or set the fill-factor value for individual indexes, use CREATE INDEX or ALTER INDEX statements.
- Creating a clustered index with a FILLFACTOR < 100 may significantly increase the amount of space the data occupies because the Database Engine physically reallocates the data while building the clustered index. (Read more here)
What is a ROLLUP Clause?
ROLLUP clause is used to do aggregate operation on multiple levels in hierarchy. If we want sum on different levels without adding any new column, then we can do it easily using ROLLUP. We have to just add the WITH ROLLUP Clause in group by clause. (Read more here)
What are Various Limitations of the Views?
- ORDER BY clause does not work in View.
- Regular queries or Stored Procedures give us flexibility when we need another column; we can add a column to regular queries right away. If we want to do the same with Views, then we will have to modify them first.
- Index created on view not used often.
- Once the view is created and if the basic table has any column added or removed, it is not usually reflected in the view till it is refreshed.
- One of the most prominent limitations of the View it is that it does not support COUNT (*); however, it can support COUNT_BIG (*).
- UNION Operation is now allowed in Indexed View.
- We cannot create an Index on a nested View situation means we cannot create index on a view which is built from another view.
- SELF JOIN Not Allowed in Indexed View.
- Outer Join Not Allowed in Indexed Views.
- Cross Database Queries Not Allowed in Indexed View.
What is a Covered index?
It is an index that can satisfy a query just by its index keys without having needed to touch the data pages.
It means that when a query is fired, SQL Server doesn’t need to go to the table to retrieve the rows, but can produce the results directly from the index as the index covers all the columns used in query. (Read more here)
When I Delete any Data from a Table, does the SQL Server reduce the size of that table?
When data are deleted from any table, the SQL Server does not reduce the size of the table right away; however, it marks those pages as free pages, showing that they belong to the table. When new data are inserted, they are put into those pages first. Once those pages are filled up, SQL Server will allocate new pages. If you wait for sometime, the background process de-allocates the pages, finally reducing the page size. (Read more here)
What are Wait Types?
There are three types of wait types, namely,
Resource Waits. Resource waits occur when a worker requests access to a resource that is not available because that resource is either currently used by another worker or it’s not yet available.
Queue Waits. Queue waits occur when a worker is idle, waiting for work to be assigned.
External Waits. External waits occur when an SQL Server worker is waiting for an external event. (Read more here)
How to Stop Log File Growing too Big?
If your Transaction Log file was growing too big and you wanted to manage its size, then instead of truncating transaction log file, you should choose one of the options mentioned below.
1) Convert the Recovery Model to Simple Recovery
If you change your recovery model to Simple Recovery Model, then you will not encounter the extraordinary growth of your log file. However, please note if you have one long running transaction it will for sure grow your log file till the transaction is complete.
2) Start Taking Transaction Log Backup
In this Full Recovery Model, your transaction log will grow until you take a backup of it. You need to take the T-Log Backup at a regular interval. This way, your log would not grow beyond some limits.
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