What are the differences between a clustered and a non-clustered index

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Clustered indexes physically order the data on the disk. This means no extra data is needed for the index, but there can be only one clustered index (obviously). Accessing data using a clustered index is fastest.

All other indexes must be non-clustered. A non-clustered index has a duplicate of the data from the indexed columns kept ordered together with pointers to the actual data rows (pointers to the clustered index if there is one). This means that accessing data through a non-clustered index has to go through an extra layer of indirection. However if you select only the data that's available in the indexed columns you can get the data back directly from the duplicated index data (that's why it's a good idea to SELECT only the columns that you need and not use *)

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Copied from MSDN  the second point of non-clustered index is not clearly mentioned in the other answers     Clustered   Clustered indexes sort and store the data rows in the table or view based on their key values  These are the columns included in the index definition  There can be only one clustered index per table  because the data rows themselves can be stored in only one order  The only time the data rows in a table are stored in sorted order is when the table contains a clustered index  When a table has a clustered index  the table is called a clustered table  If a table has no clustered index  its data rows are stored in an unordered structure called a heap    Nonclustered   Nonclustered indexes have a structure separate from the data rows  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  The pointer from an index row in a nonclustered index to a data row is called a row locator  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 table  the row locator is the clustered index key

User · Answer

Clustered indexes physically order the data on the disk. This means no extra data is needed for the index, but there can be only one clustered index (obviously). Accessing data using a clustered index is fastest.

All other indexes must be non-clustered. A non-clustered index has a duplicate of the data from the indexed columns kept ordered together with pointers to the actual data rows (pointers to the clustered index if there is one). This means that accessing data through a non-clustered index has to go through an extra layer of indirection. However if you select only the data that's available in the indexed columns you can get the data back directly from the duplicated index data (that's why it's a good idea to SELECT only the columns that you need and not use *)

User · Answer

Clustered indexes are stored physically on the table  This means they are the fastest and you can only have one clustered index per table    Non-clustered indexes are stored separately  and you can have as many as you want   The best option is to set your clustered index on the most used unique column  usually the PK  You should always have a well selected clustered index in your tables  unless a very compelling reason--can t think of a single one  but hey  it may be out there--for not doing so comes up

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You might have gone through theory part from the above posts   -The clustered Index as we can see points directly to record i e  its direct so it takes less time for a search  Additionally it will not take any extra memory space to store the index  -While  in non-clustered Index  it indirectly points to the clustered Index then it will access the actual record  due to its indirect nature it will take some what more time to access Also it needs its own memory space to store the index

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An indexed database has two parts  a set of physical records  which are arranged in some arbitrary order  and a set of indexes which identify the sequence in which records should be read to yield a result sorted by some criterion   If there is no correlation between the physical arrangement and the index  then reading out all the records in order may require making lots of independent single-record read operations   Because a database may be able to read dozens of consecutive records in less time than it would take to read two non-consecutive records  performance may be improved if records which are consecutive in the index are also stored consecutively on disk   Specifying that an index is clustered will cause the database to make some effort  different databases differ as to how much  to arrange things so that groups of records which are consecutive in the index will be consecutive on disk   For example  if one were to start with an empty non-clustered database and add 10 000 records in random sequence  the records would likely be added at the end in the order they were added   Reading out the database in order by the index would require 10 000 one-record reads   If one were to use a clustered database  however  the system might check when adding each record whether the previous record was stored by itself  if it found that to be the case  it might write that record with the new one at the end of the database   It could then look at the physical record before the slots where the moved records used to reside and see if the record that followed that was stored by itself   If it found that to be the case  it could move that record to that spot   Using this sort of approach would cause many records to be grouped together in pairs  thus potentially nearly doubling sequential read speed   In reality  clustered databases use more sophisticated algorithms than this   A key thing to note  though  is that there is a tradeoff between the time required to update the database and the time required to read it sequentially   Maintaining a clustered database will significantly increase the amount of work required to add  remove  or update records in any way that would affect the sorting sequence   If the database will be read sequentially much more often than it will be updated  clustering can be a big win   If it will be updated often but seldom read out in sequence  clustering can be a big performance drain  especially if the sequence in which items are added to the database is independent of their sort order with regard to the clustered index

User · Answer

Clustered basically means that the data is in that physical order in the table  This is why you can have only one per table   Unclustered means it s  only  a logical order

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Clustered Index   Only one clustered index can be there in a table Sort the records and store them physically according to the order Data retrieval is faster than non-clustered indexes Do not need extra space to store logical structure   Non Clustered Index   There can be any number of non-clustered indexes in a table Do not affect the physical order  Create a logical order for data rows and use pointers to physical data files Data insertion update is faster than clustered index Use extra space to store logical structure     Apart from these differences you have to know that when table is non-clustered  when the table doesn t have a clustered index  data files are unordered and it uses Heap data structure as the data structure

User · Answer

Clustered basically means that the data is in that physical order in the table  This is why you can have only one per table   Unclustered means it s  only  a logical order

User · Answer

Pros   Clustered indexes work great for ranges  e g  select   from my table where my key between  min and  max   In some conditions  the DBMS will not have to do work to sort if you use an orderby statement   Cons   Clustered indexes are can slow down inserts because the physical layouts of the records have to be modified as records are put in if the new keys are not in sequential order

User · Answer

Pros   Clustered indexes work great for ranges  e g  select   from my table where my key between  min and  max   In some conditions  the DBMS will not have to do work to sort if you use an orderby statement   Cons   Clustered indexes are can slow down inserts because the physical layouts of the records have to be modified as records are put in if the new keys are not in sequential order

User · Answer

Clustered Index   There can be only one clustered index for a table  Usually made on the primary key  The leaf nodes of a clustered index contain the data pages    Non-Clustered Index   There can be only 249 non-clustered indexes for a table till sql version 2005 later versions support upto 999 non-clustered indexes   Usually made on the any key  The leaf node of a nonclustered index does not consist of the data pages   Instead  the leaf nodes contain index rows

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A clustered index is essentially a sorted copy of the data in the indexed columns   The main advantage of a clustered index is that when your query  seek  locates the data in the index then no additional IO is needed to retrieve that data   The overhead of maintaining a clustered index  especially in a frequently updated table  can lead to poor performance and for that reason it may be preferable to create a non-clustered index

User · Answer

A clustered index is essentially a sorted copy of the data in the indexed columns   The main advantage of a clustered index is that when your query  seek  locates the data in the index then no additional IO is needed to retrieve that data   The overhead of maintaining a clustered index  especially in a frequently updated table  can lead to poor performance and for that reason it may be preferable to create a non-clustered index

User · Answer

You might have gone through theory part from the above posts   -The clustered Index as we can see points directly to record i e  its direct so it takes less time for a search  Additionally it will not take any extra memory space to store the index  -While  in non-clustered Index  it indirectly points to the clustered Index then it will access the actual record  due to its indirect nature it will take some what more time to access Also it needs its own memory space to store the index

User · Answer

A clustered index is essentially a sorted copy of the data in the indexed columns   The main advantage of a clustered index is that when your query  seek  locates the data in the index then no additional IO is needed to retrieve that data   The overhead of maintaining a clustered index  especially in a frequently updated table  can lead to poor performance and for that reason it may be preferable to create a non-clustered index

User · Answer

Clustered Index   Only one per table Faster to read than non clustered as data is physically stored in index order   Non Clustered Index   Can be used many times per table Quicker for insert and update operations than a clustered index   Both types of index will improve performance when select data with fields that use the index but will slow down update and insert operations   Because of the slower insert and update clustered indexes should be set on a field that is normally incremental ie Id or Timestamp   SQL Server will normally only use an index if its selectivity is above 95

User · Answer

Pros   Clustered indexes work great for ranges  e g  select   from my table where my key between  min and  max   In some conditions  the DBMS will not have to do work to sort if you use an orderby statement   Cons   Clustered indexes are can slow down inserts because the physical layouts of the records have to be modified as records are put in if the new keys are not in sequential order

User · Answer

Clustered indexes are stored physically on the table  This means they are the fastest and you can only have one clustered index per table    Non-clustered indexes are stored separately  and you can have as many as you want   The best option is to set your clustered index on the most used unique column  usually the PK  You should always have a well selected clustered index in your tables  unless a very compelling reason--can t think of a single one  but hey  it may be out there--for not doing so comes up

User · Answer

Clustered Index   Only one clustered index can be there in a table Sort the records and store them physically according to the order Data retrieval is faster than non-clustered indexes Do not need extra space to store logical structure   Non Clustered Index   There can be any number of non-clustered indexes in a table Do not affect the physical order  Create a logical order for data rows and use pointers to physical data files Data insertion update is faster than clustered index Use extra space to store logical structure     Apart from these differences you have to know that when table is non-clustered  when the table doesn t have a clustered index  data files are unordered and it uses Heap data structure as the data structure

User · Answer

Clustered basically means that the data is in that physical order in the table  This is why you can have only one per table   Unclustered means it s  only  a logical order

User · Answer

Clustered basically means that the data is in that physical order in the table  This is why you can have only one per table   Unclustered means it s  only  a logical order

User · Answer

A clustered index actually describes the order in which records are physically stored on the disk  hence the reason you can only have one   A Non-Clustered Index defines a logical order that does not match the physical order on disk

User · Answer

A clustered index is essentially a sorted copy of the data in the indexed columns   The main advantage of a clustered index is that when your query  seek  locates the data in the index then no additional IO is needed to retrieve that data   The overhead of maintaining a clustered index  especially in a frequently updated table  can lead to poor performance and for that reason it may be preferable to create a non-clustered index

User · Answer

Clustered Index   Only one per table Faster to read than non clustered as data is physically stored in index order   Non Clustered Index   Can be used many times per table Quicker for insert and update operations than a clustered index   Both types of index will improve performance when select data with fields that use the index but will slow down update and insert operations   Because of the slower insert and update clustered indexes should be set on a field that is normally incremental ie Id or Timestamp   SQL Server will normally only use an index if its selectivity is above 95

User · Answer

Clustered Index   There can be only one clustered index for a table  Usually made on the primary key  The leaf nodes of a clustered index contain the data pages    Non-Clustered Index   There can be only 249 non-clustered indexes for a table till sql version 2005 later versions support upto 999 non-clustered indexes   Usually made on the any key  The leaf node of a nonclustered index does not consist of the data pages   Instead  the leaf nodes contain index rows

User · Answer

Clustered indexes physically order the data on the disk. This means no extra data is needed for the index, but there can be only one clustered index (obviously). Accessing data using a clustered index is fastest.

All other indexes must be non-clustered. A non-clustered index has a duplicate of the data from the indexed columns kept ordered together with pointers to the actual data rows (pointers to the clustered index if there is one). This means that accessing data through a non-clustered index has to go through an extra layer of indirection. However if you select only the data that's available in the indexed columns you can get the data back directly from the duplicated index data (that's why it's a good idea to SELECT only the columns that you need and not use *)

User · Answer

Clustered Index   Only one per table Faster to read than non clustered as data is physically stored in index order   Non Clustered Index   Can be used many times per table Quicker for insert and update operations than a clustered index   Both types of index will improve performance when select data with fields that use the index but will slow down update and insert operations   Because of the slower insert and update clustered indexes should be set on a field that is normally incremental ie Id or Timestamp   SQL Server will normally only use an index if its selectivity is above 95

User · Answer

A clustered index actually describes the order in which records are physically stored on the disk  hence the reason you can only have one   A Non-Clustered Index defines a logical order that does not match the physical order on disk

User · Answer

Copied from MSDN  the second point of non-clustered index is not clearly mentioned in the other answers     Clustered   Clustered indexes sort and store the data rows in the table or view based on their key values  These are the columns included in the index definition  There can be only one clustered index per table  because the data rows themselves can be stored in only one order  The only time the data rows in a table are stored in sorted order is when the table contains a clustered index  When a table has a clustered index  the table is called a clustered table  If a table has no clustered index  its data rows are stored in an unordered structure called a heap    Nonclustered   Nonclustered indexes have a structure separate from the data rows  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  The pointer from an index row in a nonclustered index to a data row is called a row locator  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 table  the row locator is the clustered index key

User · Answer

Pros   Clustered indexes work great for ranges  e g  select   from my table where my key between  min and  max   In some conditions  the DBMS will not have to do work to sort if you use an orderby statement   Cons   Clustered indexes are can slow down inserts because the physical layouts of the records have to be modified as records are put in if the new keys are not in sequential order

User · Answer

A clustered index actually describes the order in which records are physically stored on the disk  hence the reason you can only have one   A Non-Clustered Index defines a logical order that does not match the physical order on disk

User · Answer

Clustered Index   Only one per table Faster to read than non clustered as data is physically stored in index order   Non Clustered Index   Can be used many times per table Quicker for insert and update operations than a clustered index   Both types of index will improve performance when select data with fields that use the index but will slow down update and insert operations   Because of the slower insert and update clustered indexes should be set on a field that is normally incremental ie Id or Timestamp   SQL Server will normally only use an index if its selectivity is above 95

User · Answer

Clustered indexes are stored physically on the table  This means they are the fastest and you can only have one clustered index per table    Non-clustered indexes are stored separately  and you can have as many as you want   The best option is to set your clustered index on the most used unique column  usually the PK  You should always have a well selected clustered index in your tables  unless a very compelling reason--can t think of a single one  but hey  it may be out there--for not doing so comes up

User · Answer

Clustered indexes are stored physically on the table  This means they are the fastest and you can only have one clustered index per table    Non-clustered indexes are stored separately  and you can have as many as you want   The best option is to set your clustered index on the most used unique column  usually the PK  You should always have a well selected clustered index in your tables  unless a very compelling reason--can t think of a single one  but hey  it may be out there--for not doing so comes up

User · Answer

A clustered index actually describes the order in which records are physically stored on the disk  hence the reason you can only have one   A Non-Clustered Index defines a logical order that does not match the physical order on disk

User · Answer

Clustered indexes physically order the data on the disk. This means no extra data is needed for the index, but there can be only one clustered index (obviously). Accessing data using a clustered index is fastest.

All other indexes must be non-clustered. A non-clustered index has a duplicate of the data from the indexed columns kept ordered together with pointers to the actual data rows (pointers to the clustered index if there is one). This means that accessing data through a non-clustered index has to go through an extra layer of indirection. However if you select only the data that's available in the indexed columns you can get the data back directly from the duplicated index data (that's why it's a good idea to SELECT only the columns that you need and not use *)

User · Answer

An indexed database has two parts  a set of physical records  which are arranged in some arbitrary order  and a set of indexes which identify the sequence in which records should be read to yield a result sorted by some criterion   If there is no correlation between the physical arrangement and the index  then reading out all the records in order may require making lots of independent single-record read operations   Because a database may be able to read dozens of consecutive records in less time than it would take to read two non-consecutive records  performance may be improved if records which are consecutive in the index are also stored consecutively on disk   Specifying that an index is clustered will cause the database to make some effort  different databases differ as to how much  to arrange things so that groups of records which are consecutive in the index will be consecutive on disk   For example  if one were to start with an empty non-clustered database and add 10 000 records in random sequence  the records would likely be added at the end in the order they were added   Reading out the database in order by the index would require 10 000 one-record reads   If one were to use a clustered database  however  the system might check when adding each record whether the previous record was stored by itself  if it found that to be the case  it might write that record with the new one at the end of the database   It could then look at the physical record before the slots where the moved records used to reside and see if the record that followed that was stored by itself   If it found that to be the case  it could move that record to that spot   Using this sort of approach would cause many records to be grouped together in pairs  thus potentially nearly doubling sequential read speed   In reality  clustered databases use more sophisticated algorithms than this   A key thing to note  though  is that there is a tradeoff between the time required to update the database and the time required to read it sequentially   Maintaining a clustered database will significantly increase the amount of work required to add  remove  or update records in any way that would affect the sorting sequence   If the database will be read sequentially much more often than it will be updated  clustering can be a big win   If it will be updated often but seldom read out in sequence  clustering can be a big performance drain  especially if the sequence in which items are added to the database is independent of their sort order with regard to the clustered index

[sql-server] What are the differences between a clustered and a non-clustered index?

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