I thought we could only place one clustered index on one table, and put multiple non-clustered indexes on a table, but using the code below I can easily add more than one clustered index to my table.
CREATE CLUSTERED INDEX TBL_MULTI_LC_HIST ON dbo.TBL_MULTI_LC_HIST (ID,AsOfDate)
Is this completely wrong?
It isn't possible to create multiple clustered indexes for a single table. From the docs (emphasis mine):
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.
For example this will fail:
CREATE TABLE Thing
(
Column1 INT NOT NULL,
Column2 INT NOT NULL
)
CREATE CLUSTERED INDEX IX1 ON dbo.Thing(Column1)
CREATE CLUSTERED INDEX IX2 ON dbo.Thing(Column2)
Error:
Cannot create more than one clustered index on table 'dbo.Thing'. Drop the existing clustered index 'IX1' before creating another.
Example: http://www.sqlfiddle.com/#!18/53a63/1
You can however have a single index with multiple columns in it which is perhaps where you are getting confused:
CREATE CLUSTERED INDEX IX3 ON dbo.Thing(Column1, Column2)
You can only have one clustered index. A "Clustered" index IS the row... it contains all the columns. Every other index would just contain a pointer to the clustered row. The key of the clustered index enforces an 'ordering' on the rows by default.
If there is no clustered index, then the rows are basically stored in a heap, with no order or structure.
Related
I have a Table to make a Clustered Primary Key.
CREATE TABLE dbo.SampleTable
(
C1 INT NOT NULL,
C2 INT NOT NULL )
First Way is making Primary Key index with Clustered index.
ALTER TABLE dbo.SampleTable ADD CONSTRAINT IDX_SampleTable PRIMARY KEY CLUSTERED (C1, C2)
Second Way is CREATE CLUSTERED INDEX after ADD CONSTRAINT PRIMARY KEY NONCLUSTERED about same columns.
ALTER TABLE dbo.SampleTable ADD CONSTRAINT IDX_SampleTable PRIMARY KEY NONCLUSTERED (C1, C2)
CREATE CLUSTERED INDEX IDX_SampleTable2 ON dbo.SampleTable (C1 ,C2) -- Can not create Same Name With above Constraint Name
Is there a difference in performance from the above two methods?
Is there a way do not recommend using it?
Yes, there is a difference. By specifying CLUSTERED, you instruct the database to store the data in a certain way. Basically, it enforces that subsequent indexes are stored on subsequent data blocks on the hard drive.
By creating a clustered primary key as in your first statement, all the data in the table will always have unique values in C1, C2 and the data is always stored in subsequent data blocks.
In the second example, you do NOT enforce this CLUSTERED behaviour through the primary key, but through a separate index. Though the effects are the same now, you might choose to remove (or temporarily disable) the index and then the data would no longer be guaranteed to get stored in a CLUSTERED fashion.
Bottom line: In practice these two statements are the same now, but might make a difference in the future because the CLUSTERED property is not integrated in the PK, but in a separate index.
Creating a Nonclustered Primary Key and then creating a Clustered index on the columns within the Primary key is not a good idea. Effectively you'll create 2 indexes on the columns (C1 and C2 in this case), however, it's very unlikely the nonclustered index will ever be used. This is because the Clustered Index is very likely going to be the first choice for the RDBMS, as the pages will be in the order of the Clustered Index. Also, when using a non-clustered index the data engine will still need to refer to the Clustered Index afterwards, to find out the exact location of the row (in the pages).
If you do want a clustered index on your Primary Key(s) then create the key as a Clustered Primary Key. This is not to say that your Primary Key should always be Clustered, but that is a very different subject.
This depends from your datas:
https://learn.microsoft.com/en-gb/sql/relational-databases/indexes/clustered-and-nonclustered-indexes-described?view=sql-server-2017
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.
So the clustered key influence the format of your physical data structure.
Could someone simplify the explanation of adding a UNIQUE CONSTRAINT to a column please.
When creating the key index does SQL SERVER copy ALL of the information in the row and add it to the index or just the data in the column with the applied UNIQUE CONSTRAINT?
I hope I explained that properly.
Any help will be greatly appreciated.
Lee.
EDIT**
Ok i think i get it?
CREATE TABLE dbo.test
(
Id int NOT NULL,
Name char(10) NOT NULL UNIQUE
);
INSERT INTO dbo.test (id, name) VALUES (1, 'Lee')
INSERT INTO dbo.test (id, name) VALUES (2, 'Paul')
INSERT INTO dbo.test (id, name) VALUES (3, 'Adam')
INSERT INTO dbo.test (id, name) VALUES (4, 'Henry')
In a clustered index the whole table would be sorted like
3, Adam
4, Henry
1, Lee
2, Paul
So with each additional INSERT the server would have to re-sort the entire table based on the name column?
In a nonclustered index there is another "table" that stores the sort?
When creating the key index does SQL SERVER copy ALL of the
information in the row and add it to the index or just the data in the
column with the applied UNIQUE CONSTRAINT?
There is no such a term as "key index".
Indexes can be clustered or non-clustered.
When you declare UNIQUE CONSTRAINT it's logical entity, but it's physically supported by unique index creation
When you create your unique constraint declaring it as clustered, clustered index will be created. If you don't mention clustered in your constraint definition or use explicite nonclustered, non-clustered index will be created.
Non-clustered index is a separate data structure where every row contains key columns.
On the other hand, clustered index (or better call it clustered table) is data itself + searching B-tree above it. In this case no separate structure is created, it's table itself that now is organized not as a heap but as ordered index.
UNIQUE CONSTRAINT will work just as UNIQUE INDEX. There are 2 ways:
With a clustered index the rows are stored physically on the disk in the same order as the index. (hence, only one clustered index is possible)
With a non clustered index there is a second list that has pointers to the physical rows. You can have many non clustered indexes, although each new index will increase the time it takes to write new records.
If you have both clustered and non clustered index, then non clustered index will point to the clustered index column.
THIS 'SO' answer will help you understand it a bit clear.
By default the unique constraint and Unique index will create a non
clustered index if you don't specify any different (and the PK will by
default be created as CLUSTERED if no conflicting clustered index
exists) but you can explicitly specify CLUSTERED/NONCLUSTERED for any
of them.
If I have a query like this:
SELECT * FROM tTable
where tTable does not contain any indexes a table-scan happens, as expected. If I add a clustered index on some column then QO decides to use clustered index scan on this query. Why? Why is clustered-index-scan preferred instead of table-scan in this case?
If I add a clustered index on some column then QO decides to use clustered index scan on this query
because when you create a clustered index on a table,data in table is rearranged in index order..so table it self is clustered index.This is also the reason why you can't have two clustered indexes on same table
To summarize,when you create a clustered index,there is only one structure ,not two(clustered index and table)
The query is "give me all rows and all columns" which means "read every row" which is a scan
There is nothing to do an index seek on, because there is no WHERE clause.
Unlike this:
SELECT * FROM tTable WHERE PrimaryClusteredKeyValue = 45
Then this may use a nonclustered seek followed by a clustered key lookup or it may still scan the clustered index because you ask for all columns. It depends on how many rows gbn will match
SELECT * FROM tTable WHERE NonClusteredOtherColumnValue = 'gbn'
For a table with PK on an identity column, it will be clustered by default. Could it better be non-clustered? The PK is a surrogate key which may never be used for querying directly, it may be used to join another table.
The reason is other indexes will be created for queries. A query which uses a non-clustered index and returned columns are not covered by the index will use less LIO because there is no extra clustered index seek steps?
create table T (
Id int identity(1,1) primary key, -- clustered or non-clustered?
A ....
B ....
C ....
....)
create index ix_A on T (A)
create index ix_..... -- Many indexes can be created for different queries
select A, B
from T
where A between #a and #a+5 -- This query will have less LIO if the PK is non-clustered (seek)
It's perfectly fine to set your surrogate PK to be non-clustered if there is a better candidate in the table for the clustered index.
Good candidates for a clustered index are columns that you will frequently do either range searches ([ColumnName] BETWEEN This AND That) on, or ORDER BY clauses on.
What is Clustered and non clustered indexing? How to index a table using sql server 2000 Enterprise manager?
In a clustered index on ID, the table rows are ordered by ID.
In a non-clustered index on ID, the references to table rows are ordered by ID.
We can compare a database to a CSV file:
ID,Value
-------
1,ReallyReallyLongValue1
3,ReallyReallyLongValue2
In a clustered table, when we insert a new row, we need to squeeze it between the existing rows:
ID,Value
-------
1,ReallyReallyLongValue1
2,ReallyReallyLongValue2
3,ReallyReallyLongValue3
, which is slow on insert but fast on retrieve.
In a non-clustered table, we keep a separate file index file which orders our rows:
Id,RowNumber
------------
1, 1
3, 2
When we insert the new row, we just append it to our main file and update the short index file:
ID,Value
-------
1,ReallyReallyLongValue1
3,ReallyReallyLongValue3
2,ReallyReallyLongValue2
Id,RowNumber
------------
1, 1
2, 3
3, 2
, which is fast on insert but less efficient on retrieve.
In real databases indexes use more efficient binary trees, but the principle remains the same.
Clustered indexes are faster on SELECT, non-clustered indexes are faster on INSERT / UPDATE / DELETE
A clustered index means that the rows are physically ordered by the values in that index. A non-clustered index means that an index table is kept up to date that allows for quick seeking and sorting based upon value, but does not physically order the rows.
Only one clustered index can exist for a table, and if a primary key exists then that is the clustered index (in SQL Server).
A clustered index defines how the actual table is stored. The rows are stored in a way to make searches on the fields in the clustered index fast. (They're not physically stored in the sort order of the index fields, but in a binary tree or something similiar.)
You can have only one clustered index per table. The clustered index contains all fields in the table, for example:
indexfield1 - indexfield2 - field2 - field3 - ....
A non-clustered index is like a separate table. It contains the fields in the index, and a reference to the fields in the table. For example:
secondindexfield1 - secondindexfield2 - reference to table row
When searching a non-clustered index, SQL server will find the value in the index, do a "bookmark lookup" to the table, and retrieve the other row fields from there. This is why non-clustered indexes perform slightly less wel then clustered indexes.
To add an index in SQL Server Management Studio, expand the table node in object view. Right click on "Indexes" and select "New Index".
Clustered Index: Only one clustered index per table is allowed. If an index is clustered, it means that the table on which the clustered index is based is physically sorted according to that index. Think of the page numbers in an encyclopedia.
Non-clustered Index: Can have many non-clustered indexes per table. Think of the keyword index at the back of the book.