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Sunday, August 31, 2014

Temporary Tables in SQL Server

In this article, I am giving a quick overview about temporary tables . Please give your valuable suggestions and feedback to improve this article.

Temporary tables are similar to Permanent tables but Temporary tables are stored in TEMPDB System Database. Click here to know more about SQL Server System Databases. Temporary tables supports all kinds of operation that one normal table supports. Temporary tables can be created at run time in Stored Procedure. Temporary tables can’t be used in User Defined Function. Temporary tables help the developer to improve performance tuning of query.You can’t use stored procedure SP_Rename to rename the Temporary Table name; it can be used to rename columns of Temporary table. Stored procedures can reference temporary tables that are created during the current session. Within a stored procedure, you cannot create a temporary table, drop it, and then create a new temporary table with the same name.

In SQL Server based on scope and behavior there are two types of temp tables.

1.       Local Temp Table
2.       Global Temp Table

Local Temp Table: Local temporary table name is stared with single hash ("#") sign. Local tempoarary Table scope is Local which means Local temp tables are only available to the current connection for the user; and they are automatically deleted when the user disconnects the connection from instance of SQL Server. If User wants to drop the table then using DROP command it can be deleted.Local temporary tables are visible only to their creators during the same connection to an instance of SQL Server. They cannot therefore be used in views and you cannot associate triggers with them.Constraints and Indexes can be created on Local Temporary table, but we can't create Foreign key reference. 

Syntax to create Local Temporary Table

Create Table #Table_Name (Column data_type [Width], ….n)

Let’s create one Local Scope Temporary Table and understands the above explained concept.

USE TEACHMESQLSERVER
GO
CREATE TABLE #LOCALEMPTEMPTABLE (EMPID INT, EMPNAME VARCHAR(100))

The above script will create a temporary local scope table in TEMPDB database. You can find it in Tempdb database using below query

GO
SELECT * FROM TEMPDB.SYS.TABLES

As I have already told, you can perform any operation on Temporary table that a Permanent table supports. Let’s insert few records in recently created temporary table. Insert query will be same to insert data in Temporary tables.

GO
INSERT INTO #LOCALEMPTEMPTABLE VALUES ( 1, 'GHANESH');

Let’s fetch the record from the #LocalEMPTempTable table using below query.

GO
SELECT * FROM #LOCALEMPTEMPTABLE


After execution of all these statements, if you close the query window and again open a new connection and then execute "Insert" or "Select" Command on #LocalEMPTempTable table, it will throw below error.

Msg 208, Level 16, State 0, Line 1
Invalid object name '#LOCALEMPTEMPTABLE'.

This is because the scope of Local Temporary table is only bounded with the current connection of current user. You are using a new connection in which the Local Temporary Table was not created.

Global Temp Table: Global Temporary tables name starts with a double hash ("##"). Scope of Global temporary table is Global which means Once this table has been created by a connection, like a permanent table it is then available to any user by any connection. It will be automatically deleted once all connections have been closed from the instance of SQL Server. User can also drop the table using DROP command. It is suggested to Drop the tables manually using DROP statement if it is not required anymore. Constraints and Indexes can be created on Global Temporary table, but we can't create foreign key references.They cannot be used in views and you cannot associate triggers with them.

Syntax to create Global Temporary Table

Create Table ##Table_Name (Column data_type [Width], ….n)

Let’s create one Global Scope Temporary Table and understands the above explained concept.

USE TEACHMESQLSERVER
GO
CREATE TABLE ##GLOBALEMPTEMPTABLE (EMPID INT, EMPNAME VARCHAR(100))

The above script will create a temporary Global scope table in TEMPDB database. You can find the table in Tempdb database using below query.

GO
SELECT * FROM TEMPDB.SYS.TABLES

As I have already told, you can perform any operation on Temporary table that a Permanent table supports. Let’s insert few records in recently created temporary table. Insert query will be same to insert data in Temporary tables.

GO
INSERT INTO ##GlobalEMPTEMPTABLE VALUES ( 1, 'PRASAD'), ( 2, 'ANSHUL')

Let’s fetch the record from the ##GlobalEMPTempTable table using below query.

GO
SELECT * FROM ##GLOBALEMPTEMPTABLE


After execution of all these statements, if you open a new query window and then execute "Insert" or "Select" Command on ##GLOBALEMPTempTable table, it will execute successfully.
This is because the scope of Global Temporary table is not bound with the current connection of current user. Global temporary tables are visible to all SQL Server connections. When you create one of these, all the users can see it.

In What scenarios Temp Tables should be used?
  • When a large number of row manipulation is required in stored procedures.
  • When we are having a complex join operation.
  • Temporary tables can replace the cursor because we can store the result set data into a temp table, and then we can manipulate the data from there.

Key points to remember about Temp Tables.
  • Temporary Tables can’t have foreign key constraint.
  • We can’t create Views on Temporary Tables.
  • We can’t create Triggers on Temporary Tables.
  • Temporary Tables will be stored in Tempdb system database.
  • Temporary Tables are automatically deleted based on their scope.
  • Temporary Tables can’t be renamed using SP_Rename sytem stored prodecure.
  • Temporary Tables can’t be used in User Defined Function.
  • The best way to use a temporary table is to create it and then fill it with data. i.e., instead of using Select into temp table, create temp table 1st and populate with data later.
  • Temporary Tables can have Indexes, Constraints (except Foreign key constraint).
  • Temporary Tables Supports Transactions.
  • Temporary Tables Supports Error Handling.
  • Temporary Tables Supports DDL, DML commands.
  • Tables need to be deleted when they are done with their work.
Click here to know about Differences Between Temporary Tables and Table Variable. Don’t forget to share on Facebook if you like the article.

Difference Between Truncate and Delete

This is one of the most important questions generally asked in the interview.


  • Truncate releases allocated memory but Delete operation doesn't release allocated memory.
  • Truncate is faster than Delete because it perform the operation on Table but delete performs operation on each row.

I hope you enjoyed the article don't forget to share on Facebook. Please leave your comments below if there are any questions.

Friday, August 29, 2014

Transaction in SQL Server

In this article, I am giving a quick overview about Transactions in SQL Server. After completing this article you will understand:

What is Transaction in SQL Server?
What are different TCL commands?
What are different modes of Transactions?

Please give your valuable suggestions and feedback to improve this article.

What is Transaction in SQL Server?

Transaction is a single unit of work; if a Transaction is successful then all of the data modifications made during the transaction are committed and become a permanent part of the database. If a Transaction encounters any errors and must be cancelled or rolled back, then all of the data modification is erased. A Transaction always follows ACID rules. Click here to know about ACID Properties of Transaction.

What are different TCL commands?

To manage the Transaction we have TCL (Transaction Control language) with three commands in it COMMIT, ROLLBACK and Save Transaction. Click here to know about types of SQL Commands.

COMMIT- Commit Marks the end of a successful implicit or explicit transaction. COMMIT Transaction makes all data modifications performed since the start of the transaction a permanent part of the database.

ROLLBACK- Roll back an Explicit or Implicit transaction to the beginning of the transaction or to the Save point inside the transaction. Transaction can ROLLBACK Truncated table data.

Save Transaction- A user can set a save point or marker within a transaction. The save point defines a location to which a transaction can return if part of the transaction is conditionally canceled. If a transaction is rolled back to a save point, it must be proceed to completion with more TSQL statement if needed and a COMMIT Transaction statement, or it must be canceled altogether by rolling the transaction back to its beginning. To cancel an entire transaction, use the ROLLBACK Transaction statement.

What are different modes of Transactions?

There are three modes of Transaction in SQL Server

1.      Auto Commit Transaction
2.      Implicit Transaction – A new transaction is implicitly started when the prior transaction completes, but each transaction is explicitly completed with a COMMIT or ROLLBACK keyword.
3.      Explicit Transaction – Each Transaction is explicitly stared with the Begin Transaction keyword and explicitly ended with a COMMIT or ROLLBACK keyword.

Auto Commit

By default SQL Server uses Auto commit transaction mode which means after executing each statement it will automatically commit it

Implicit Transactions

By default SQL Server uses Auto commit transaction mode but you can change it to Implicit Transaction mode using below query

SET IMPLICIT_TRANSACTONS ON

When SET IMPLICIT_TRANSACTIONS is set to ON, then it sets the connection into Implicit Transaction Mode, but when it is set to OFF, then it returns the connection to auto commit transaction mode. It is to be noted that When Transaction mode is changed to Implicit Transaction mode, it will remain ON only for the same connection it was set to ON.

Let’s understand the Transaction mode using simple examples

As I have already told there are three types of TRANSACTION MODE - Auto commit, IMPLICIT and EXPLICIT. By default in SQL Server Transaction mode is auto commit. But you can change to IMPLICIT Transaction mode using SET IMPLICIT_TRANSACTIONS command. Once you change it to IMPLICIT mode, Transaction mode will be changed to IMPLICIT mode but only for that particular connection. When you open another connection for that new connection TRANSACTION mode will be auto commit.

Let’s open two connections and in first connection we will SET IMPLICIT_TRANSACTIONS ON and in second connection we haven’t SET it to ON.


In first connection I am creating EMP table and in Second connection I am creating DEP table. From the below Image you can clearly see. I ran both the connection’s command and it ran successfully.



As I have already told in my beginning that if Transaction mode is set to IMPLICIT then we have to end the transaction by COMMIT or ROLLBACK but in my first connection I have not done that.  Now let’s close the first connection.


As you can see from the above Image when I tried to close the connection I got a pop up message, which is saying I have not committed the transaction. If I will click on Yes then it will commit the Transaction and EMP table will be created in the database for permanently. If I click on NO then Transaction will be ROLLED BACK and EMP table will not be created in TEACHMESQLSERVER database. Just for testing purpose I am clicking on NO.


Now let’s try to close the second connection, you can clearly see from the below Image when I try to close second connection it is not asking whether I want to commit or not.  It is asking if I want to save my SQL command then I can save it. I am not saving the SQL command and clicking on NO.


Now again I create a new connection, let’s see how many table our TEACHMESQLSERVER database contains. As you can see from the below result, it say only DEP table is present in our database. This DEP table was created by the second connection. EMP table is not present in our database which was created by first connection. Even after successful execution EMP table is not present in the database it is because when we closed the First connection we clicked on NO. It didn't commit but it rolled back the transaction. 



Explicit Transactions

Explicit Transaction start with BEGIN TRAN or BEGIN TRANSACTION keyword then it contains TSQL statements and ends with COMMIT or ROLLBACK keywords.

Syntax

BEGIN TRANSACTION
SQL Statement
COMMIT | ROLLBACK

Let’s have one-one example for both COMMIT and ROLLBACK.
We are inserting a record in DEP table and committing our transaction

USE TEACHMESQLSERVER
GO
BEGIN TRANSACTION
INSERT INTO DEP VALUES(1)
COMMIT
GO

SELECT * FROM DEP

As you can see this records becomes permanent and stored in DEP table because we COMMITTED our transaction.

In this below query we are inserting another record in DEP table but we are rolling back our transaction.

USE TEACHMESQLSERVER
GO
BEGIN TRANSACTION
INSERT INTO DEP VALUES(2)
ROLLBACK
GO
SELECT * FROM DEP

When we fetch all the records from DEP table, we don’t find second record. It is because we ROLLED BACK the transaction. When we ROLL BACK our transaction it restore the table data to last COMMITTED stage. As you know last COMMIT state was when we inserted record one. So it is showing 1 in the result set.



As you know TRUNCATE command can’t be rolled back until it is used within transaction. If you don’t know about what are the difference between Truncate and Delete command then Click here
If you will run the below command then you will find, Truncate can also be rolled back if it is used in Transaction.
  
USE TEACHMESQLSERVER
GO
BEGIN TRANSACTION
TRUNCATE TABLE DEP
ROLLBACK
GO
SELECT * FROM DEP

We can also save transaction point in our Transaction; those save point can be used for roll back. Below example shows a simple example of SAVE TRANSACTION. We are inserting three records but we have created on transaction point after inserting first record. After two more insert operation we are rolling back to Saved Transaction point named as one and then committing the transaction.

USE TEACHMESQLSERVER
GO
BEGIN TRANSACTION
INSERT INTO DEP VALUES(2)
SAVE TRANSACTION ONE
INSERT INTO DEP VALUES(3)
INSERT INTO DEP VALUES(4)
ROLLBACK TRANSACTION ONE
COMMIT
GO
SELECT * FROM DEP

As you can see form the above result set record 3 and 4 was not inserted in to the table, it is because before committing the transaction we rolled back to the saved transaction point which comes after the first insertion which is 2.

@@TRANCOUNT System Function

Returns the number of BEGIN TRANSACTION statements that have occurred on the current connection.

Showing the effects of the BEGIN and COMMIT statements

PRINT @@TRANCOUNT
--  The BEGIN TRAN statement will increment the
--  transaction count by 1.
BEGIN TRAN
    PRINT @@TRANCOUNT
    BEGIN TRAN
        PRINT @@TRANCOUNT
--  The COMMIT statement will decrement the transaction count by 1.
    COMMIT
    PRINT @@TRANCOUNT
COMMIT
PRINT @@TRANCOUNT


Click here to know how to Manage Transaction in Stored Procedure.

I hope you enjoyed the article don't forget to share on Facebook. Please leave your comments below if there are any questions.

Wednesday, August 27, 2014

Indexes in SQL Server

In this article, I am giving a quick overview about INDEX in SQL Server. After completing this article you will understand:

What is meant by INDEX?
What are different types of INDEX in SQL Server?
How to create INDEX?
How to DROP INDEX?
How to Enable-Disable INDEX?
What is difference between clustered and non-clustered indexes?
What are the disadvantages of indexes?

Please give your valuable suggestions and feedback to improve this article.

What is meant by INDEX?

Index is a database object which is used to improve query performance for data retrieval but it also adds some burden on data modification queries such as INSERT, UPDATE & DELETE. So it is advised to use correct Index type. If your table gets many data modification request then it is advised not to have Clustered index on such table, because it will slow down you DML operations. Index can be created on one or more column (max 16 columns) on existing table or views. When we create the index, it reads the column’s data and forms a relevant data structure to minimize the number of data comparison. Indexes are automatically maintained for a table or view whenever the table data is modified.

Let’s understand Index with some real world examples.

Example 1

Suppose A table CUSTOMERS has around 20 Million records. When we try to retrieve records for two or three customers based on their customer id, all 20 Million records are taken and comparison is made to get a match on the supplied customer id. Just think about now how much time that will take if it is a web based application and there are 30-50 customers that want to access their data through internet.
Does the data server do 20 Million X 30 searches?  The answer is NO because all modern database use the concept of Index.

Example 2

Suppose you have a book which has thousands of pages, Thousands of pages are divided into many chapters. Each chapter has many pages and each page has related topic written, if you want to read a particular topic then how do you find it in your book? Answer is you just look into the Table of Content which is always present in the beginning from Chapter 1.This Table of contents helps you search your topics in all topics and then you jump to a particular page out of those thousands pages. So this Table of Content is nothing but Index for your book. You better know how helpful are those table of content 10-20 pages, if it was not there then it would have been very difficult to search needed topic in those thousands of pages. Table of content will always be created after completing the book.

Let’s relate this understanding with Database, now you assume your Book as a Database Table and Table of Content as Index. After creating and populating a Table or view, you create index on it. Once you create an index on a field, automatically another data structure will be created which holds the field value and pointer to the record it relates to. This index structure is then sort and stores the data rows in the database which allows Binary search on it, which means these Indexes require additional space on the disk, and get stored with a table using MYISAM engine.

Summary

1. Like an index in a book, an index in a database lets you quickly find specific information in a table or indexed view.
2. An Index contains keys built from one or more columns in a Table, or view and pointers that map to the storage location of the specified data.
3. These keys are stored in structure (B-Tree) that enables SQL Server to find the rows associated with the key values quickly and efficiently.
4. We can significantly improve the performance of database queries and applications by creating well designed indexes to support your queries.
5. Indexes can reduce the amount of data that must be read to return the query result set.
6. Indexes can also enforce uniqueness on the rows in a table, ensuring the data integrity of the table data.

What are different types of INDEX in SQL Server?

1. Clustered Index
2. Non Clustered Index
3. Unique
4. Filtered
5. Xml
6. Full Text
7. Spatial
8. Columnstore
9. Index with Included Columns
10. Index on Computed Columns

In this article we will focus only on Clustered and Non Clustered Index.

How to create INDEX?

We can create Index on an empty table or one that contains data, creating an index on an empty table has no performance implications at the time of Index is creation, however performance will be affected when data is added to the table.

Creating Index on large table should be planned carefully so that database performance is not hindered. The best way to create Index on large table is to first create Clustered Index and then create any Non Clustered Index.

When a table has no Clustered Index then its data will be stored in unsorted way and it will be called as Heap. When you insert a new record in Heap, it will be inserted in any random position in the table and position depends on the page free space. If A Clustered index is created on a heap with several existing nonclustered indexes, all the nonclustered indexes must be rebuilt so that they contain the clustering key value instead of the row identifier (RID).

Syntax to Create Index

CREATE [UNIQUE] CLUSTERED |NONCLUSTERED INDEX index_name ON table_name|view_name (Column ASC|DESC [,..n])

Uniqueness can be a property of both clustered and nonclustered indexes. If UNIQUE and CLUSTERED keywords are missing in CREATE INDEX command then it will create Non-Unique Non Clustered Index.

Clustered Index

1. When Clustered Index is created it first sorts the data rows and then stores the data rows in the table or view based on their key values.
2. Whenever new records come into the table, it gets fit to its position based on the key values that is the reason INSERT, UPDATE and DELETE are slow on INDEXED table because it will take some time to rearrange the data Pages.
3. When we create Primary Key constraint in a Table and if Clustered Index is not present on the table then it will automatically create Clustered Index on primary key column with the same name as the Primary Key constraint has.
4. We can have only 1 Clustered index on a Table.
5. Reason for having only 1 Clustered Index on a table is the data rows themselves can be sorted in only one order either ASC or DESC.
6. When a table has Clustered Index then it will be called as Clustered Table.
7. When creating the Clustered Index, SQL Server reads the Primary Key column values and forms a Binary Tree on it. This Binary Tree information is then stored separately in the disc, you can find Index created on Primary Key in database Index Folder.
8. Clustered Index can be created on Non - Primary Key columns. When we create Primary Key later in that case it will not create Clustered index because it is already available, it will create Unique Non Cluster Index.
9. Uniqueness is the property of Clustered Index and Non Clustered Index.
10. When we drop the Index, It leaves the data in sorted order and if new records come then it will be inserted at the random position in the table.
11. Clustered Index leaf nodes contains the Data Pages means row’s data.

Let’s go through with some example to understand Clustered Index

Let’s create a table name as EMP in our tutorial database TEACHMESQLSERVER

USE TEACHMESQLSERVER
IF OBJECT_ID('EMP','U') IS NOT NULL
DROP TABLE EMP
CREATE TABLE EMP(EMPID INT NOT NULL)

As you can see from the below image right now EMP table doesn’t has Index. It is because we have not created the INDEX or we have not defined any Unique or Primary Key constraint on EMPID column at the time of table creation.
Let’s insert few records in the EMP table.

INSERT INTO EMP VALUES(1),(3),(2),(0)
GO
SELECT * FROM EMP


As you can see EMPID was not sorted and stored because it doesn’t have Clustered Index on EMP Table. I have already discussed that we can also create Clustered Index on Non Primary Key columns.

Let’s Create Clustered Index on EMPID column in EMP table.


CREATE CLUSTERED INDEX EMPIDINDEX ON EMP (EMPID ASC)


As you can see now EMP table has one Clustered Index name as EMPIDINDEX.

Now let’s retrieve all the records we had inserted last time from EMP table.

GO
SELECT * FROM EMP


As you can see once after creating Clustered Index we retrieved the data, it is coming in sorted form. It is because at the time of Clustered Index creation we had given sorting Order as ASC. Data can be sort either ASC or DESC. This is the main reason we can have only one Clustered Index on a Table.
Now let’s try to create one more Clustered Index. We already know the answer that SQL Server will not allow to create another Clustered Index on EMP table.

CREATE CLUSTERED INDEX EMPIDINDEX1 ON EMP (EMPID ASC)

Here you go – got error message

Msg 1902, Level 16, State 3, Line 1
Cannot create more than one clustered index on table 'EMP'. Drop the existing clustered index 'EMPIDINDEX' before creating another.

Let’s try to insert new records

INSERT INTO EMP VALUES (8)
INSERT INTO EMP VALUES(6)
GO
SELECT * FROM EMP

As you can see from the above result output, records are coming in ASC order even after we inserted 8 before 6.

As I have already discussed if our table already has Clustered Index and Primary key constraint was not defined at the time of table creation, if we create Primary Key constraints using Alter command on EMP table then it will never create a Clustered Index, It will create a Unique Non Clustered Index.

Let’s create Primary Key constraint on EMPID column.

ALTER TABLE EMP ADD CONSTRAINT PRIMARYKEY PRIMARY KEY (EMPID)


As you can see PRIMARYKEY INDEX is a Unique Non Clustered Index.

You can find all Indexes list in SQL Server from SYS.INDEXES table.

SELECT * FROM SYS.INDEXES

Dropping index will not affect the sorted stored records. Let’s Drop the INDEX. I will discuss how to DROP Index in this article, if DROP command looks strange here then don’t worry.

DROP INDEX EMPIDINDEX ON EMP
ALTER TABLE EMP DROP CONSTRAINT PRIMARYKEY
GO
SELECT * FROM EMP

As you can see Result set is same after dropping the Index. Now let’s insert new records in EMP table.

INSERT INTO EMP VALUES (7)
GO
SELECT * FROM EMP

As you can see from the result set new records were inserted at the end not its key value position.

NonClustered Index
1.  We can have many Nonclustered (max 999) index on a Table.
2.  Uniqueness is the property of Clustered Index and Non Clustered Index.
3.  By default a Nonclustered Index is created if Clustered or Unique options are not specified in the Create Index command.
4.  A Nonclustered index never store data in sorted order if your table has more than 1 column, ASC and DESC keywords will be ignored if used at the time of Nonclustered index creation. If your table has only column then only it will store data in sorted order.
5.  A Nonclustered Index is stored separately from the table, so additional space is required. The amount of space required will depend on the size of table and the number and types of columns used in index.
6.  A Nonclustered Index contains the nonclustered Index key values and each key value entry has a pointer to the data row that contains the key value.
7.  The pointer from an index row in a nonclustered index to a data row is called a row locator.
8.  The Structure of the row locator depends on whether the data pages are stored in a heap or a clustered table. For a heap, a Row locator is a pointer to the row. For a clustered Index, the row locator is the clustered index key.
9. Whenever you create constraint, nonclustered index will be created.

Let’s go through with some example to understand Nonclustered Index

Let’s create a table name as EMP in our tutorial database TEACHMESQLSERVER

USE TEACHMESQLSERVER
IF OBJECT_ID('EMP','U') IS NOT NULL
DROP TABLE EMP
CREATE TABLE EMP(EMPID INT NOT NULL, DEPID INT NOT NULL)


 As you can see from the above image right now EMP table doesn’t has any Index. It is because we have not created any INDEX or we have not defined any constraint at the time of table creation.

Let’s insert few records in the EMP table.

INSERT INTO EMP VALUES(1,11),(3,5),(2,4),(0,7),(1,1)
GO
SELECT * FROM EMP


As you can see from the above result set records are not stored in sorted order. It was stored in the sequence it was written in query.

Let’s Create Nonclustered Index on EMPID column in EMP table.

CREATE NONCLUSTERED INDEX EMPIDINDEX ON EMP (EMPID ASC)
GO
SELECT * FROM EMP



As you can see from the above image Non-Uique, Non-Clustered index has been created.

Now let’s retrieve all the records we had inserted last time from EMP table.

GO
SELECT * FROM EMP


As you can see from the above result set, records were not stored in sorted ordered even after defining the ordering type (ASC) at the time of EMPIDINDEX Index creation.

Now let’s create one more Nonclustered Index on DEPID column in EMP table. We have already discussed that we can create many nonclustered index on a table.

CREATE NONCLUSTERED INDEX DEPIDINDEX ON EMP (DEPID DESC)
GO
SELECT * FROM EMP

Now once again let’s retrieve all the records we had inserted last time from EMP table.

GO
SELECT * FROM EMP

As you can see from the above result set, records were not stored in sorted ordered even after defining the ordering type (DESC) at the time of DEPIDINDEX Index creation. This clearly shows that Nonclustered Index never stores the data in any order.

Let’s insert a new record in EMP table

INSERT INTO EMP VALUES (-2,6)
GO
SELECT * FROM EMP

As you can see from the above result set, new record was inserted at the end of all records.

What is difference between clustered and non-clustered indexes?

1.   A table can have multiple non-clustered indexes. But, a table can have only one clustered index.
2.  A clustered index can be a disadvantage because any time a change is made to a value of an indexed column, the subsequent possibility of re-sorting rows to maintain order is a definite performance hit.
3.   A Clustered Index is faster than a Nonclustered Index but in some condition Non-Clustered index would perform better. You can refer to this article.
4.  If Clustered Index is disabled then you can’t perform DML operation, but if Nonclustered index is disabled then also you can perform DML operation.
5.   Clustered Index determines the Storage Order of rows in the table, hence doesn’t require additional disk space but where as a Nonclustered Index is stored separately from the table, so additional space is required.
6.   A clustered index determines the order in which the rows of the table will be stored on disk (asc or desc) – and it actually stores row level data in the leaf nodes of the index itself. A non-clustered index has no effect on which the order of the rows will be stored.
7.   Using a clustered index is an advantage when groups of data that can be clustered are frequently accessed by some queries. This speeds up retrieval because the data lives close to each other on disk. Also, if data is accessed in the same order as the clustered index, the retrieval will be much faster because the physical data stored on disk is sorted in the same order as the index.
8.   Non clustered indexes store both a value and a pointer to the actual row that holds that value. Clustered indexes don’t need to store a pointer to the actual row because of the fact that the rows in the table are stored on disk in the same exact order as the clustered index – and the clustered index actually stores the row-level data in it’s leaf nodes.

How to DROP INDEX?
Implicitly created index can’t be dropped using DROP INDEX command which means you can’t drop the Indexes if Indexes were created automatically by any of Primary Key or Unique constraint. To drop the index you have to drop the constraint it will automatically delete the Index.
If a clustered index is dropped on a table that has several nonclustered indexes, the nonclustered indexes are all rebuilt as part of the DROP operation. This may take significant time on large tables.

How to Enable-Disable INDEX?

DROP INDEX index_name ON table_name|view_name

Syntax to Disable Index

You can disable a particular Index using below query syntax
ALTER INDEX Index_name ON table_name|view_name  DISABLE

You can disable all available Index using below query syntax
ALTER INDEX ALL ON table_name|view_name  DISABLE

When you disable the clustered index then you can’t perform DML (INSERT, UPDATE & DELETE) operation. If you try to do then you will get below error message

Msg 8655, Level 16, State 1, Line 1
The query processor is unable to produce a plan because the index ‘Index_name’on table or view 'table_name or View_name' is disabled.

You can perform DML operation if your nonclustered index is disabled.

Syntax to Enable Index

You can enable a particular Index using below query syntax
ALTER INDEX Index_name ON table_name|view_name  REBUILD

You can enable all available Index using below query syntax
ALTER INDEX ALL ON table_name|view_name  REBUILD

What are the disadvantages of indexes?

Additional Disk Space – Clustered Index does not require any additional storage. Every Non-Clustered index requires additional space as it is stored separately from the table. The amount of space required will depend on the size of the table, and the number and types of columns used in index.

Insert, Update and Delete Operation can become Slow  When DML statements modify data in a table, the data in all the indexes also needs to be updated. Indexes can help, to search and locate the rows, that we want to delete, but too many indexes to update can actually hurt the performance of data modifications.

Click here to know how to create index on View.

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