I have a limited exposure to DB and have only used DB as an application programmer. I want to know about Clustered and Non clustered indexes.
I googled and what I found was :
A clustered index is a special type of index that reorders the way
records in the table are physically
stored. Therefore table can have only
one clustered index. The leaf nodes
of a clustered index contain the data
pages. A nonclustered index is a
special type of index in which the
logical order of the index does not
match the physical stored order of
the rows on disk. The leaf node of a
nonclustered index does not consist of
the data pages. Instead, the leaf
nodes contain index rows.
What I found in SO was What are the differences between a clustered and a non-clustered index?.
Can someone explain this in plain English?
With a clustered index the rows are stored physically on the disk in the same order as the index. Therefore, there can be only one clustered index.
With a non clustered index there is a second list that has pointers to the physical rows. You can have many non clustered indices, although each new index will increase the time it takes to write new records.
It is generally faster to read from a clustered index if you want to get back all the columns. You do not have to go first to the index and then to the table.
Writing to a table with a clustered index can be slower, if there is a need to rearrange the data.
A clustered index means you are telling the database to store close values actually close to one another on the disk. This has the benefit of rapid scan / retrieval of records falling into some range of clustered index values.
For example, you have two tables, Customer and Order:
Customer
----------
ID
Name
Address
Order
----------
ID
CustomerID
Price
If you wish to quickly retrieve all orders of one particular customer, you may wish to create a clustered index on the "CustomerID" column of the Order table. This way the records with the same CustomerID will be physically stored close to each other on disk (clustered) which speeds up their retrieval.
P.S. The index on CustomerID will obviously be not unique, so you either need to add a second field to "uniquify" the index or let the database handle that for you but that's another story.
Regarding multiple indexes. You can have only one clustered index per table because this defines how the data is physically arranged. If you wish an analogy, imagine a big room with many tables in it. You can either put these tables to form several rows or pull them all together to form a big conference table, but not both ways at the same time. A table can have other indexes, they will then point to the entries in the clustered index which in its turn will finally say where to find the actual data.
In SQL Server, row-oriented storage both clustered and nonclustered indexes are organized as B trees.
(Image Source)
The key difference between clustered indexes and non clustered indexes is that the leaf level of the clustered index is the table. This has two implications.
The rows on the clustered index leaf pages always contain something for each of the (non-sparse) columns in the table (either the value or a pointer to the actual value).
The clustered index is the primary copy of a table.
Non clustered indexes can also do point 1 by using the INCLUDE clause (Since SQL Server 2005) to explicitly include all non-key columns but they are secondary representations and there is always another copy of the data around (the table itself).
CREATE TABLE T
(
A INT,
B INT,
C INT,
D INT
)
CREATE UNIQUE CLUSTERED INDEX ci ON T(A, B)
CREATE UNIQUE NONCLUSTERED INDEX nci ON T(A, B) INCLUDE (C, D)
The two indexes above will be nearly identical. With the upper-level index pages containing values for the key columns A, B and the leaf level pages containing A, B, C, D
There can be only one clustered index per table, because the data rows
themselves can be sorted in only one order.
The above quote from SQL Server books online causes much confusion
In my opinion, it would be much better phrased as.
There can be only one clustered index per table because the leaf level rows of the clustered index are the table rows.
The book's online quote is not incorrect but you should be clear that the "sorting" of both non clustered and clustered indices is logical, not physical. If you read the pages at leaf level by following the linked list and read the rows on the page in slot array order then you will read the index rows in sorted order but physically the pages may not be sorted. The commonly held belief that with a clustered index the rows are always stored physically on the disk in the same order as the index key is false.
This would be an absurd implementation. For example, if a row is inserted into the middle of a 4GB table SQL Server does not have to copy 2GB of data up in the file to make room for the newly inserted row.
Instead, a page split occurs. Each page at the leaf level of both clustered and non clustered indexes has the address (File: Page) of the next and previous page in logical key order. These pages need not be either contiguous or in key order.
e.g. the linked page chain might be 1:2000 <-> 1:157 <-> 1:7053
When a page split happens a new page is allocated from anywhere in the filegroup (from either a mixed extent, for small tables or a non-empty uniform extent belonging to that object or a newly allocated uniform extent). This might not even be in the same file if the filegroup contains more than one.
The degree to which the logical order and contiguity differ from the idealized physical version is the degree of logical fragmentation.
In a newly created database with a single file, I ran the following.
CREATE TABLE T
(
X TINYINT NOT NULL,
Y CHAR(3000) NULL
);
CREATE CLUSTERED INDEX ix
ON T(X);
GO
--Insert 100 rows with values 1 - 100 in random order
DECLARE #C1 AS CURSOR,
#X AS INT
SET #C1 = CURSOR FAST_FORWARD
FOR SELECT number
FROM master..spt_values
WHERE type = 'P'
AND number BETWEEN 1 AND 100
ORDER BY CRYPT_GEN_RANDOM(4)
OPEN #C1;
FETCH NEXT FROM #C1 INTO #X;
WHILE ##FETCH_STATUS = 0
BEGIN
INSERT INTO T (X)
VALUES (#X);
FETCH NEXT FROM #C1 INTO #X;
END
Then checked the page layout with
SELECT page_id,
X,
geometry::Point(page_id, X, 0).STBuffer(1)
FROM T
CROSS APPLY sys.fn_PhysLocCracker( %% physloc %% )
ORDER BY page_id
The results were all over the place. The first row in key order (with value 1 - highlighted with an arrow below) was on nearly the last physical page.
Fragmentation can be reduced or removed by rebuilding or reorganizing an index to increase the correlation between logical order and physical order.
After running
ALTER INDEX ix ON T REBUILD;
I got the following
If the table has no clustered index it is called a heap.
Non clustered indexes can be built on either a heap or a clustered index. They always contain a row locator back to the base table. In the case of a heap, this is a physical row identifier (rid) and consists of three components (File:Page: Slot). In the case of a Clustered index, the row locator is logical (the clustered index key).
For the latter case if the non clustered index already naturally includes the CI key column(s) either as NCI key columns or INCLUDE-d columns then nothing is added. Otherwise, the missing CI key column(s) silently gets added to the NCI.
SQL Server always ensures that the key columns are unique for both types of indexes. The mechanism in which this is enforced for indexes not declared as unique differs between the two index types, however.
Clustered indexes get a uniquifier added for any rows with key values that duplicate an existing row. This is just an ascending integer.
For non clustered indexes not declared as unique SQL Server silently adds the row locator into the non clustered index key. This applies to all rows, not just those that are actually duplicates.
The clustered vs non clustered nomenclature is also used for column store indexes. The paper Enhancements to SQL Server Column Stores states
Although column store data is not really "clustered" on any key, we
decided to retain the traditional SQL Server convention of referring
to the primary index as a clustered index.
I realize this is a very old question, but I thought I would offer an analogy to help illustrate the fine answers above.
CLUSTERED INDEX
If you walk into a public library, you will find that the books are all arranged in a particular order (most likely the Dewey Decimal System, or DDS). This corresponds to the "clustered index" of the books. If the DDS# for the book you want was 005.7565 F736s, you would start by locating the row of bookshelves that is labeled 001-099 or something like that. (This endcap sign at the end of the stack corresponds to an "intermediate node" in the index.) Eventually you would drill down to the specific shelf labelled 005.7450 - 005.7600, then you would scan until you found the book with the specified DDS#, and at that point you have found your book.
NON-CLUSTERED INDEX
But if you didn't come into the library with the DDS# of your book memorized, then you would need a second index to assist you. In the olden days you would find at the front of the library a wonderful bureau of drawers known as the "Card Catalog". In it were thousands of 3x5 cards -- one for each book, sorted in alphabetical order (by title, perhaps). This corresponds to the "non-clustered index". These card catalogs were organized in a hierarchical structure, so that each drawer would be labeled with the range of cards it contained (Ka - Kl, for example; i.e., the "intermediate node"). Once again, you would drill in until you found your book, but in this case, once you have found it (i.e, the "leaf node"), you don't have the book itself, but just a card with an index number (the DDS#) with which you could find the actual book in the clustered index.
Of course, nothing would stop the librarian from photocopying all the cards and sorting them in a different order in a separate card catalog. (Typically there were at least two such catalogs: one sorted by author name, and one by title.) In principle, you could have as many of these "non-clustered" indexes as you want.
Find below some characteristics of clustered and non-clustered indexes:
Clustered Indexes
Clustered indexes are indexes that uniquely identify the rows in an SQL table.
Every table can have exactly one clustered index.
You can create a clustered index that covers more than one column. For example: create Index index_name(col1, col2, col.....).
By default, a column with a primary key already has a clustered index.
Non-clustered Indexes
Non-clustered indexes are like simple indexes. They are just used for fast retrieval of data. Not sure to have unique data.
Clustered Index
A clustered index determines the physical order of DATA in a table. For this reason, a table has only one clustered index(Primary key/composite key).
"Dictionary" No need of any other Index, its already Index according to words
Nonclustered Index
A non-clustered index is analogous to an index in a Book. The data is stored in one place. The index is stored in another place and the index has pointers to the storage location. this help in the fast search of data. For this reason, a table has more than 1 Nonclustered index.
"Biology Book" at starting there is a separate index to point Chapter location and At the "END" there is another Index pointing the common WORDS location
A very simple, non-technical rule-of-thumb would be that clustered indexes are usually used for your primary key (or, at least, a unique column) and that non-clustered are used for other situations (maybe a foreign key). Indeed, SQL Server will by default create a clustered index on your primary key column(s). As you will have learnt, the clustered index relates to the way data is physically sorted on disk, which means it's a good all-round choice for most situations.
Clustered Index
A Clustered Index is basically a tree-organized table. Instead of storing the records in an unsorted Heap table space, the clustered index is actually B+Tree index having the Leaf Nodes, which are ordered by the clusters key column value, store the actual table records, as illustrated by the following diagram.
The Clustered Index is the default table structure in SQL Server and MySQL. While MySQL adds a hidden clusters index even if a table doesn't have a Primary Key, SQL Server always builds a Clustered Index if a table has a Primary Key column. Otherwise, the SQL Server is stored as a Heap Table.
The Clustered Index can speed up queries that filter records by the clustered index key, like the usual CRUD statements. Since the records are located in the Leaf Nodes, there's no additional lookup for extra column values when locating records by their Primary Key values.
For example, when executing the following SQL query on SQL Server:
SELECT PostId, Title
FROM Post
WHERE PostId = ?
You can see that the Execution Plan uses a Clustered Index Seek operation to locate the Leaf Node containing the Post record, and there are only two logical reads required to scan the Clustered Index nodes:
|StmtText |
|-------------------------------------------------------------------------------------|
|SELECT PostId, Title FROM Post WHERE PostId = #P0 |
| |--Clustered Index Seek(OBJECT:([high_performance_sql].[dbo].[Post].[PK_Post_Id]), |
| SEEK:([high_performance_sql].[dbo].[Post].[PostID]=[#P0]) ORDERED FORWARD) |
Table 'Post'. Scan count 0, logical reads 2, physical reads 0
Non-Clustered Index
Since the Clustered Index is usually built using the Primary Key column values, if you want to speed up queries that use some other column, then you'll have to add a Secondary Non-Clustered Index.
The Secondary Index is going to store the Primary Key value in its Leaf Nodes, as illustrated by the following diagram:
So, if we create a Secondary Index on the Title column of the Post table:
CREATE INDEX IDX_Post_Title on Post (Title)
And we execute the following SQL query:
SELECT PostId, Title
FROM Post
WHERE Title = ?
We can see that an Index Seek operation is used to locate the Leaf Node in the IDX_Post_Title index that can provide the SQL query projection we are interested in:
|StmtText |
|------------------------------------------------------------------------------|
|SELECT PostId, Title FROM Post WHERE Title = #P0 |
| |--Index Seek(OBJECT:([high_performance_sql].[dbo].[Post].[IDX_Post_Title]),|
| SEEK:([high_performance_sql].[dbo].[Post].[Title]=[#P0]) ORDERED FORWARD)|
Table 'Post'. Scan count 1, logical reads 2, physical reads 0
Since the associated PostId Primary Key column value is stored in the IDX_Post_Title Leaf Node, this query doesn't need an extra lookup to locate the Post row in the Clustered Index.
Clustered Index
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 sorted 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.
You can add nonkey columns to the leaf level of the nonclustered index to by-pass existing index key limits, and execute fully covered, indexed, queries. For more information, see Create Indexes with Included Columns. For details about index key limits see Maximum Capacity Specifications for SQL Server.
Reference: https://learn.microsoft.com/en-us/sql/relational-databases/indexes/clustered-and-nonclustered-indexes-described
Let me offer a textbook definition on "clustering index", which is taken from 15.6.1 from Database Systems: The Complete Book:
We may also speak of clustering indexes, which are indexes on an attribute or attributes such that all of tuples with a fixed value for the search key of this index appear on roughly as few blocks as can hold them.
To understand the definition, let's take a look at Example 15.10 provided by the textbook:
A relation R(a,b) that is sorted on attribute a and stored in that
order, packed into blocks, is surely clusterd. An index on a is a
clustering index, since for a given a-value a1, all the tuples with
that value for a are consecutive. They thus appear packed into
blocks, execept possibly for the first and last blocks that contain
a-value a1, as suggested in Fig.15.14. However, an index on b is
unlikely to be clustering, since the tuples with a fixed b-value
will be spread all over the file unless the values of a and b are
very closely correlated.
Note that the definition does not enforce the data blocks have to be contiguous on the disk; it only says tuples with the search key are packed into as few data blocks as possible.
A related concept is clustered relation. A relation is "clustered" if its tuples are packed into roughly as few blocks as can possibly hold those tuples. In other words, from a disk block perspective, if it contains tuples from different relations, then those relations cannot be clustered (i.e., there is a more packed way to store such relation by swapping the tuples of that relation from other disk blocks with the tuples the doesn't belong to the relation in the current disk block). Clearly, R(a,b) in example above is clustered.
To connect two concepts together, a clustered relation can have a clustering index and nonclustering index. However, for non-clustered relation, clustering index is not possible unless the index is built on top of the primary key of the relation.
"Cluster" as a word is spammed across all abstraction levels of database storage side (three levels of abstraction: tuples, blocks, file). A concept called "clustered file", which describes whether a file (an abstraction for a group of blocks (one or more disk blocks)) contains tuples from one relation or different relations. It doesn't relate to the clustering index concept as it is on file level.
However, some teaching material likes to define clustering index based on the clustered file definition. Those two types of definitions are the same on clustered relation level, no matter whether they define clustered relation in terms of data disk block or file. From the link in this paragraph,
An index on attribute(s) A on a file is a clustering index when: All tuples with attribute value A = a are stored sequentially (= consecutively) in the data file
Storing tuples consecutively is the same as saying "tuples are packed into roughly as few blocks as can possibly hold those tuples" (with minor difference on one talking about file, the other talking about disk). It's because storing tuple consecutively is the way to achieve "packed into roughly as few blocks as can possibly hold those tuples".
Clustered Index:
Primary Key constraint creates clustered Index automatically if no clustered Index already exists on the table. Actual data of clustered index can be stored at leaf level of Index.
Non Clustered Index:
Actual data of non clustered index is not directly found at leaf node, instead it has to take an additional step to find because it has only values of row locators pointing towards actual data.
Non clustered Index can't be sorted as clustered index. There can be multiple non clustered indexes per table, actually it depends on the sql server version we are using. Basically Sql server 2005 allows 249 Non Clustered Indexes and for above versions like 2008, 2016 it allows 999 Non Clustered Indexes per table.
Clustered Index - A clustered index defines the order in which data is physically stored in a table. Table data can be sorted in only way, therefore, there can be only one clustered index per table. In SQL Server, the primary key constraint automatically creates a clustered index on that particular column.
Non-Clustered Index - A non-clustered index doesn’t sort the physical data inside the table. In fact, a non-clustered index is stored at one place and table data is stored in another place. This is similar to a textbook where the book content is located in one place and the index is located in another. This allows for more than one non-clustered index per table.It is important to mention here that inside the table the data will be sorted by a clustered index. However, inside the non-clustered index data is stored in the specified order. The index contains column values on which the index is created and the address of the record that the column value belongs to.When a query is issued against a column on which the index is created, the database will first go to the index and look for the address of the corresponding row in the table. It will then go to that row address and fetch other column values. It is due to this additional step that non-clustered indexes are slower than clustered indexes
Differences between clustered and Non-clustered index
There can be only one clustered index per table. However, you can
create multiple non-clustered indexes on a single table.
Clustered indexes only sort tables. Therefore, they do not consume
extra storage. Non-clustered indexes are stored in a separate place
from the actual table claiming more storage space.
Clustered indexes are faster than non-clustered indexes since they
don’t involve any extra lookup step.
For more information refer to this article.
Related
Here is one of the definitions I found for clustered Index:
When is a file is organized so that the ordering of data records is
the same as or close to the ordering of data entries in some index, we
say that the index is clustered.
I'm having trouble understanding the above sentence regarding the clustered Indexes. The things I know about clustered index are:
Clustered indexes reorders the way the records are physically stored in the table, so only one clustered index is possible
Clustered index is created on non key attribute
Well for clustered index we have many view to look into
A clustered index is a type of index where the table records are physically re-ordered to match the index.
Clustered indexes are efficient on columns that are searched for a range of values. After the row with first value is found using a clustered index, rows with subsequent index values are guaranteed to be physically adjacent, thus providing faster access for a user query or an application
You also have to understand the Non-Clustered Index
In other words, a clustered index stores the actual data, where a non-clustered index is a pointer to the data. In most DBMSs, you can only have one clustered index per table, though there are systems that support multiple clusters (DB2 being an example).
Like a regular index that is stored unsorted in a database table, a clustered index can be a composite index, such as a concatenation of first name and last name in a table of personal information.
There are several example and explanations. And this is What do Clustered and Non clustered index actually mean? one of them.
I was seaching how to move a table from one filegroup to the other, and I had some doubts as to why most of the replies I found dealt with clustered indexes, considering that my question had to do with tables.
Then I looked at How I can move table to another filegroup?, and it says that the clustered index is the table data, which explains the reasoning behind recreating a clustered index with CREATE CLUSTERED INDEX.
But in that same question it says that if my clustered index is unique, then do something else.
My question: I assume that when I create tables on a database, a clustered index is created for that table. So how can it not be unique?
Thanks.
If you have an int array and you store the number 1 twice in it - how can that array not be unique?! (Trick question to get you thinking. It clearly can be not unique.) Being unique is a constraint on the data. Fundamentally, there is nothing preventing you from creating multiple rows that have the same values in all columns.
In a heap this is not a problem physically at all. The internal row identifier is it's location on disk.
In a b-tree based index (a "clustered index") the physical data structure indeed requires uniqueness. Note, that the logical structure (the table) does not. This is a physical concern. It's an implementation detail. SQL Server does this by internally appending a key column that contains a sequence number that is counted upwards. This disambiguates the records. You can observe this effect by creating more than 2^32 rows with the same non-unique key. You will receive an error.
So there's a hidden column in the table that you cannot access. It's officially called "uniqueifier". Internally, it's used to complete the CI key to make it unique. It's stored and used everywhere where normally the unique CI key would be used: In the CI, in non-unique NCIs, in the lock hash and in query plans.
If Clustered Index is not unique then SQL Server internally creates Uniquifier to make uniqueness on that record. I will try to explain with an example:
CREATE TABLE Test2 (Col1 INT, Col2 INT)
CREATE CLUSTERED INDEX idxClustered ON Test2 (Col1)
CREATE NONCLUSTERED INDEX idxNonClustered ON test2 (Col2)
Here cluserered index is not unique
INSERT INTO Test2 VALUES (1,1), (2,2)
INSERT INTO Test2 VALUES (3,3)
INSERT INTO Test2 VALUES (3,3)
--Get the Page Number of the Non Clustered Index
DBCC IND (Test, Test2, -1)
--Examine the Results of the Page
--Not to run in production
DBCC TRACEON (3604);
DBCC PAGE(Test, 1, 3376, 3);
You will see Uniquifier key with corresponding uniqueness value... If your clustered index is Unique Clustered Index then It will not have that Uniquifier attribute.
**usr* has a good post worth reading. I will add here from Microsofts Documentation.
First, you are not alone with Clustered-Indexes. Honestly, the name itself is somewhat confusing (Structured-Indexes or Disk-Indexes would probably be better in SQL).
Refer back to the official documentation from MSDN. Any alterations by me are in italics:
A Clustered Index is an on-disk structure of the table. This means the values are pointing to a physical location. This is why when you move the table you need to recreate the Index because the physical location has been altered.
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 sorted 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 (like pointers, this is a logical ordering of the data that consumes a fraction of the physical disk space).
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 (think ordered).
For a heap, a row locator is a pointer to the row.
For a clustered table, the row locator is the clustered index key.
ABSTRACT VIEW:
A table created is not necessarily a clustered (ordered) table.
An index does not necessarily have to be unique. It is an abstract view of the table.
Unique means that a value or set of values will not repeat themselves. If you wish to enforce this, you can add a constraint by the index (i.e. UNIQUE CLUSTERED INDEX) or a CONSTRAINT such as PRIMARY KEY if you wish this to be managed in the table structure itself.
You may have multiple unique indexes since as long as the values are represented logically, they will not share the same value as another row pointer.
Consider you have Columns A, B, and C in a given table.
Column A was created with a UNIQUE CLUSTERED INDEX. This means that either A already had an enforceable UNIQUE constraint (like PK, UNIQUE CONSTRAINT) or was DECLARED EXPLICITLY.
A Column Group {B,C} could be a unique index so long as B and C never repeat itself together. In the same way, you could theoretically have indexes with the groups {A}, {B,C}, {A,C}, and every one of them be unique. Recall that an index is a logical ordering of the data so they likely will not have the same logical value (and thus are unique).
HOWEVER: unless the datatype, constraint (including the INDEX constraint), or table structure enforces a unique constraint on a COLUMN, you should not assume the index is unique. Furthermore, you cannot create a UNIQUE index if there are more than one rows containing the same combination of NULL values since SQL Server will treat them as the same value (NULL being unknown).
Will SQL Server use your indexes, unique or not? Well that is another story and depends on a number of things. But hopefully you find this post helpful.
Sources:
MSDN - Clustered and Nonclustered Indexes Described
A clustered index doesn't have to be unique. But, there can be only one clustered index on a table, because a clustered index actually determines the physical order of the table rows on disk (but I find it confusing to say that the clustered index is the table data, per se, even though they are strongly tied to each other).
HERE is a good post all about non-unique clustered indexes. Even if the index was the entire row of data, you can certainly have duplicate rows (no PK), which would equate to duplicate clustered index nodes.
In the documentation for SQL server 2008 R2 is stated:
Wide keys are a composite of several columns or several large-size columns. The key values from the clustered index are used by all nonclustered indexes as lookup keys. Any nonclustered indexes defined on the same table will be significantly larger because the nonclustered index entries contain the clustering key and also the key columns defined for that nonclustered index.
Does this mean, that when there is a search using non-clustered index, than the clustered indes is search also? I originally thought that the non-clustered index contains ditrectly the address of the page (block) with the row it references. From the text above it seems that it contains just the key from the non-clustered index instead of the address.
Could somebody explain please?
Yes, that's exactly what happens:
SQL Server searches for your search value in the non-clustered index
if a match is found, in that index entry, there's also the clustering key (the column or columns that make up the clustered index)
with that clustered key, a key lookup (often also called bookmark lookup) is now performed - the clustered index is searched for that value given
when the item is found, the entire data record at the leaf level of the clustered index navigation structure is present and can be returned
SQL Server does this, because using a physical address would be really really bad:
if a page split occurs, all the entries that are moved to a new page would be updated
for all those entries, all nonclustered indices would also have to be updated
and this is really really bad for performance.
This is one of the reasons why it is beneficial to use limited column lists in SELECT (instead of always SELECT *) and possibly even include a few extra columns in the nonclustered index (to make it a covering index). That way, you can avoid unnecessary and expensive bookmark lookups.
And because the clustering key is included in each and every nonclustered index, it's highly important that this be a small and narrow key - optimally an INT IDENTITY or something like that - and not a huge structure; the clustering key is the most replicated data structure in SQL Server and should be a small as possible.
The fact that these bookmark lookups are relatively expensive is also one of the reasons why the query optimizer might opt for an index scan as soon as you select a larger number of rows - at at time, just scanning the clustered index might be cheaper than doing a lot of key lookups.
I have few questions about clustered index and column store index.We know in clustered index the physical order of data for particular column is changing and storing in the leaf node of the binary tree.So,my question are:
1)If we create clustered index on columnA,the data of that columnA will be removed from the actual strage place and added to a binary trees leaf? or it is just rearranging the data in the original storage place?
2)What about column store index,here also the data from specific column will be removed from actual storage location and loaded in to another separate file segment?
3)From the above 2 question,If the data is moving from original location,for eg: we have a tableA having coulmns colA,colB and
if we create any of the above 2 indexes on colA, then original location contains only colB data? and colA is in some other location?
1) When you create a clustered index the table is rebuild as a clustered index. There is no 'original storage', the clustered index is the table and the table is the clustered index. A clustered index is organized as a B-tree structure (not to be confused with a binary tree).
2) A columnstore non-clustered index is a secondary index on a table. As with any secondary (non-clustered) indexes every row in the non-clustered index is a copy of the data from the table base heap or clustered index. A columnstore index is not a B-tree, nor a heap, but a new type of data organization optimized for column oriented storage. To understand how column oriented storage works in general, read the C-Store paper.
3) No. Adding non-clustered indexes of any nature, including special indexes like XML, spatial and columnstore, never remove data from the table base heap or clustered index. Non-clustered indexes always contain a copy of the data. The addition of a non-clustered columnstore index does not change in any way rows and columns in the table base heap or clustered index (I suspect what you call the 'actual storage').
What are the differences between a clustered and a non-clustered index?
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%.
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 *)
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.
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.
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.
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.
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.
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.
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.
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.
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
// 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.
Clustered Indexes
Clustered Indexes are faster for retrieval and slower for insertion
and update.
A table can have only one clustered index.
Don't require extra space to store logical structure.
Determines the order of storing the data on the disk.
Non-Clustered Indexes
Non-clustered indexes are slower in retrieving data and faster in
insertion and update.
A table can have multiple non-clustered indexes.
Require extra space to store logical structure.
Has no effect of order of storing data on the disk.