What is an index in SQL

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What is an index in SQL  Can you explain or reference to understand clearly    Where should I use an index

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An index is used to speed up the performance of queries. It does this by reducing the number of database data pages that have to be visited/scanned.

In SQL Server, a clustered index determines the physical order of data in a table. There can be only one clustered index per table (the clustered index IS the table). All other indexes on a table are termed non-clustered.

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Indexes are all about finding data quickly   Indexes in a database are analogous to indexes that you find in a book  If a book has an index  and I ask you to find a chapter in that book  you can quickly find that with the help of the index  On the other hand  if the book does not have an index  you will have to spend more time looking for the chapter by looking at every page from the start to the end of the book   In a similar fashion  indexes in a database can help queries find data quickly  If you are new to indexes  the following videos  can be very useful  In fact  I have learned a lot from them   Index Basics Clustered and Non-Clustered Indexes  Unique and Non-Unique Indexes  Advantages and disadvantages of indexes

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An index is used for several different reasons  The main reason is to speed up querying so that you can get rows or sort rows faster  Another reason is to define a primary-key or unique index which will guarantee that no other columns have the same values

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First we need to understand how normal  without indexing  query runs  It basically traverse each rows one by one and when it finds the data it returns  Refer the following image   This image has been taken from this video      So suppose query is to find 50   it will have to read  49 records as a linear search    Refer the following image   This image has been taken from this video      When we apply indexing  the query will quickly find out the data without reading each one of them just by eliminating half of the data in each traversal like a binary search  The mysql indexes are stored as B-tree where all the data are in leaf node

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INDEXES - to find data easily   UNIQUE INDEX - duplicate values are not allowed   Syntax for INDEX  CREATE INDEX INDEX NAME ON TABLE NAME COLUMN     Syntax for UNIQUE INDEX  CREATE UNIQUE INDEX INDEX NAME ON TABLE NAME COLUMN

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If you re using SQL Server  one of the best resources is its own Books Online that comes with the install   It s the 1st place I would refer to for ANY SQL Server related topics   If it s practical  how should I do this   kind of questions  then StackOverflow would be a better place to ask   Also  I haven t been back for a while but sqlservercentral com used to be one of the top SQL Server related sites out there

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INDEX is a performance optimization technique that speeds up the data retrieval process  It is a persistent data structure that associated with a Table  or View  in order to increase performance during retrieving the data from that table  or View    Index based search is applied more particularly when your queries include WHERE filter  Otherwise  i e  a query without WHERE-filter selects whole data and process  Searching whole table without INDEX is called Table-scan   You will find exact information for Sql-Indexes in clear and reliable way  follow these links    For cocnept-wise understanding      http   dotnetauthorities blogspot in 2013 12 Microsoft-SQL-Server-Training-Online-Learning-Classes-INDEX-Overview-and-Optimizations html For implementation-wise understanding      http   dotnetauthorities blogspot in 2013 12 Microsoft-SQL-Server-Training-Online-Learning-Classes-INDEX-Creation-Deletetion-Optimizations html

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An index is used to speed up searching in the database  MySQL have some good documentation on the subject  which is relevant for other SQL servers as well   http   dev mysql com doc refman 5 0 en mysql-indexes html  An index can be used to efficiently find all rows matching some column in your query and then walk through only that subset of the table to find exact matches  If you don t have indexes on any column in the WHERE clause  the SQL server has to walk through the whole table and check every row to see if it matches  which may be a slow operation on big tables   The index can also be a UNIQUE index  which means that you cannot have duplicate values in that column  or a PRIMARY KEY which in some storage engines defines where in the database file the value is stored   In MySQL you can use EXPLAIN in front of your SELECT statement to see if your query will make use of any index  This is a good start for troubleshooting performance problems  Read more here  http   dev mysql com doc refman 5 0 en explain html

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An index is an on-disk structure associated with a table or view that speeds retrieval of rows from the table or view  An index contains keys built from one or more columns in the table or view  These keys are stored in a structure  B-tree  that enables SQL Server to find the row or rows associated with the key values quickly and efficiently   Indexes are automatically created when PRIMARY KEY and UNIQUE constraints are defined on table columns  For example  when you create a table with a UNIQUE constraint  Database Engine automatically creates a nonclustered index    If you configure a PRIMARY KEY  Database Engine automatically creates a clustered index  unless a clustered index already exists  When you try to enforce a PRIMARY KEY constraint on an existing table and a clustered index already exists on that table  SQL Server enforces the primary key using a nonclustered index   Please refer to this for more information about indexes  clustered and non clustered   https   docs microsoft com en-us sql relational-databases indexes clustered-and-nonclustered-indexes-described view sql-server-ver15  Hope this helps

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Well in general index is a B-tree  There are two types of indexes  clustered and nonclustered    Clustered index creates a physical order of rows  it can be only one and in most cases it is also a primary key - if you create primary key on table you create clustered index on this table also    Nonclustered index is also a binary tree but it doesn t create a physical order of rows  So the leaf nodes of nonclustered index contain PK  if it exists  or row index    Indexes are used to increase the speed of search  Because the complexity is of O log N   Indexes is very large and interesting topic  I can say that creating indexes on large database is some kind of art sometimes

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A clustered index is like the contents of a phone book  You can open the book at  Hilditch  David  and find all the information for all of the  Hilditch s right next to each other  Here the keys for the clustered index are  lastname  firstname    This makes clustered indexes great for retrieving lots of data based on range based queries since all the data is located next to each other   Since the clustered index is actually related to how the data is stored  there is only one of them possible per table  although you can cheat to simulate multiple clustered indexes    A non-clustered index is different in that you can have many of them and they then point at the data in the clustered index  You could have e g  a non-clustered index at the back of a phone book which is keyed on  town  address   Imagine if you had to search through the phone book for all the people who live in  London  - with only the clustered index you would have to search every single item in the phone book since the key on the clustered index is on  lastname  firstname  and as a result the people living in London are scattered randomly throughout the index   If you have a non-clustered index on  town  then these queries can be performed much more quickly   Hope that helps

[sql] What is an index in SQL?

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