When I use the below query to create index for a column in MS SQL Server
create index IX_indexname on tablename(columnname);
without mentioning it as clustered or non clustered index, what index will be created? Which is the default index?
Default would be non clustered.
You can refer to documentation when in doubt..
Below is the syntax from MSDN for create index statement
CREATE [ UNIQUE ] [ CLUSTERED | NONCLUSTERED ] INDEX index_name
everything in Square brackets is optional,so you would be left with
create index index_name
MSDN again says
If CLUSTERED is not specified, a nonclustered index is created.
As you're referring to clustered indexes, I'm guessing this question refers to MS SQL Server.
Unless you explicitly specify it to be clustered, an index would be non-clustered.
Related
Primary Index in DB2 Vs Clustered index in SQL. While migrating db2 to SQL i came to know that primary key in db2 is creating primary index not clustered index.
I found primary index is different from clustered index in DB2 is it correct? In that case which index we need to use in SQL?
A SQL Server Clustered Index is basically like the following index in DB2
CREATE INDEX ... ON ... (pk-columns) INCLUDE (all-other-columns)
I.e. it is useful if your main use of this index is to fetch by the pk-columns and select many other columns.
As this index contains all columns, there is no separate table structure (no heap table) for SQL Server tables that have a clustered index.
IMHO the clustered index is heavily over used in SQL Server. I wrote this article to explain one problem clustered indexes introduce:
https://use-the-index-luke.com/blog/2014-01/unreasonable-defaults-primary-key-clustering-key
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Just wanted to know what happens internally in SQL Server when we create,
Non Clustered over a Clustered Index.
Clustered Index over Non Clustered Index.
Non Clustered Index Over Non Clustered Index.
Please comment.
For all of the execution plans I have used the same base table filled with meaningless data:
CREATE TABLE dbo.T (ID INT NOT NULL, Filler CHAR(200) NULL);
INSERT dbo.T (ID, Filler)
SELECT ROW_NUMBER() OVER(ORDER BY Object_ID),
CAST(NULLIF(ABS(Object_ID) % 10, 0) AS CHAR(200))
FROM sys.all_objects
1. What happens internally in SQL Server when we create a Non Clustered over a Clustered Index.
This is just a normal create index process, so sorts the data in the manor specified and creates the relevant nodes up from the leaf nodes. The leaf node will store a "pointer" back to the clustered index to allow for key lookups.
So once the clustered index is in place on dbo.T (ID) the execute plan for creating the non-clustered index shows the sort:
And hovering over the sort shows that it is ordering by Filler, then adding it's own sort to ensure the sort is deterministic:
2. What happens internally in SQL Server when we create a Clustered Index over Non Clustered Index.
I think to explain this properly I need to explain how a clustered index on a table with no clustered index would work. A table with no clustered index is called a "Heap" table, this just means that the data is stored in no specific order and is usually just stored in the order it is inserted in. Essentailly SQL Server builds its own clustered index clustered on an internal column RowID, however without the constraints of an explicit clustered index it is free to move the data around if and when it sees fit (More reading on Forwarding records), the non-clustered index will then store a the rowID at the leaf level so it has a way of performing lookups.
When you then create a clustered index on a heap table the table has to be rebuilt, ordering by the columns you have specified, this means that all indexes are also dropped and rebuilt. to show this I first added the non clustered index to dbo.T:
CREATE NONCLUSTERED INDEX IX_T_Filler ON dbo.T (Filler);
Unlike above you can see that a table scan is done as there is no clustered index to use, and the sort done when creating the index does not include the ID column as it did above:
Then afterwards add the clustered index:
CREATE CLUSTERED INDEX IX_T_ID ON dbo.T (ID);
You can see in the execution plan that the nonclustered index is also rebuilt so the leaf will point to the new clustered index rather than the row ID as it did previously. (Note the second query is the same as in the first part when the nonclustered index was built on the clustered index)
3. What happens internally in SQL Server when we create a Non Clustered Index Over Non Clustered Index.
Nonclustered indexes are completely independent of each other, so this is the same as 1 (or the first part of 2 if there is no clustered key), i.e. It does not matter how many non clustered indexes already exist, the method of creating a new one remains the same.
I have following table schema -
CREATE TABLE [dbo].[TEST_TABLE]
(
[TEST_TABLE_ID] [int] IDENTITY(1,1) NOT NULL,
[NAME] [varchar](40) NULL,
CONSTRAINT [PK_TEST_TABLE] PRIMARY KEY CLUSTERED
(
[TEST_TABLE_ID] ASC
)
)
I have inserted huge data in TEST_TABLE.
As I have marked TEST_TABLE_ID column as primary key, clustered index will be created on TEST_TABLE_ID.
When I am running following query, execution plan is showing Clustered Index Scan which is expected.
SELECT * FROM TEST_TABLE WHERE TEST_TABLE_ID = 34
But, when I am running following query I was expecting Table Scan as NAME column does not have any index:
SELECT * FROM TEST_TABLE WHERE NAME LIKE 'a%'
But in execution plan it is showing Clustered Index Scan.
As NAME column does not have any index why it is accessing the clustered index?
I believe, this is happening as clustered index resides on data pages.
Can anyone tell me if my assumption is correct? Or is there any other reason?
A clustered index is the index that stores all the table data. So a table scan is the same as a clustered index scan.
In a table without a clustered index (a "heap"), a table scan requires crawling through all data pages. That is what the query optimizer calls a "table scan".
As others explained already, for a table that has a clustered index, a Clustered Index Scan means a Table Scan.
In other words, the table is the clustered index.
What you have wrong is your first query execution plan:
SELECT *
FROM TEST_TABLE
WHERE TEST_TABLE_ID = 34 ;
It does a Clustered Index Seek and not a Scan. It doesn't have to search (scan) the whole table (clustered index), it goes directly to the point (seeks) and checks if a row with id=34 exists.
You can see a simple test in SQL-Fiddle, and how the two execution plans differ.
The table is stored as a clustered index. The only way to scan the table is to scan the clustered index. Only tables with no clustered index can have a "table scan" per se.
It is because this table has a clustered index and it will scan the entire clustered index to return all the rows base on the where clause. How ever you should be seeing a missing index message.
When you build a Clustered Index on a table, then SQL Server logically orders the rows of that table based on the Clustered Index Key, which in your case is Test_Table_ID.
However, when you see the Clustered Index Scan operator, this COULD be a little misleading. If certain conditions are met, (which equate to SQL Server not caring about the order of the data) then SQL Server is still able to perform an unordered allocation scan, which is more similar to a table scan than an clustered index scan, as it actually reads the leaf level of the CI (the tables data pages) in allocation order, based on the IAM chain, as opposed to following the pointers in the index. This can potentially give you a performance improvement, as fragmentation (pages being out of physical order) does not decrease performance
To see if this is happening, look at the Ordered property in the execution plan. If this is set to False, then you have an unordered allocation scan.
Is it possible to convert a clustered index to non clustered index or
non clustered index to clustered index in sql server 2005.
Please convert this query into clustered index:
create index index1 on mytable(firstcolumn)
Please convert this query into non clustered index:
create clustered index clusindex1 on mytable(cluscolumn)
There is more to it than meets the eye
to create a clustered index
drop index mytable.clusindex1
go
create clustered index clusindex1 on mytable(cluscolumn)
to create a non clustered index
drop index mytable.clusindex1
go
create index clusindex1 on mytable(cluscolumn) --non clustered is default
having said that, you can only have one clustered index per table, so if you try to drop an index and recreate it as a clustered index it will fail if you already have a clustered index. Whenever you drop a clustered index all non clustered indexes will also be dropped and recreated pointing to the heap, and then again dropped and recreated when you create the clustered index, now pointing to the clustered index (look up the WITH DROP_EXISTING clause)
I would say lookup how indexing works in Books On Line before you start dropping and recreating indexes
Those aren't queries; they are DDL commands.
Drop and recreate the indexes as desired, like so:
drop index mytable.index1
go
create nonclustered index index1 on mytable (firstcolumn asc)
go
I also wanted to know whether a clustered index could be converted (altered) to be a non-clustered index. I don't believe this can be done. The existing clustered index has to first be dropped and then the new non-clustered index (possibly with the same name as the clustered index) has to be created. The same is true for converting a non-clustered index to a clustered index.
I have no idea why you're asking for the 'queries' to be converted, but #Tahbaza is correct in that the code you included in your question aren't really queries. They are T-SQL statements for making changes to 'data definitions' (i.e. the schema [structure] of your database).
I'm executing the following statement:
UPDATE TOP(1) dbo.userAccountInfo
SET Flags = Flags | #AddValue
WHERE ID = #ID;
The column 'ID' is an INT PRIMARY KEY with IDENTITY constraints.
Flags is a BIGINT NOT NULL.
The execution path indicates that a Clustered Index Update is occurring. A very expensive operation.
There's no indexes covering Flags or ID, except for the primary key.
I feel like the actual execution path should be:
Clustered Index Seek => Update
Tables come in two flavors: clustered indexes and heaps. You have a PRIMARY KEY constraint so you have created implicitly a clustered index. You'd have to go to extra length during the table create for this not to happen. Any update of the 'table' is an update of the clustered index, since the clustered index is the table.
As for the clustered index update being a 'very expensive operation', now that is an urban legend surrounding basic misinformation about how a database works. The correct statement is 'a clustered index update that affects the clustered key has to update the all non-clustered indexes'.
The clustered index is the physical table, so whenever you update any row, you're updating the clustered index.
See this MSDN article