SQL Server Clustered Indexes
※ Download: Sql server primary key clustered
Table is still on Heap. I have been already able to use this knowledge to address performance issues that came up this week in capacity testing. Even with a clustered index, you are not guaranteed a sort order UNLESS you use ORDER BY.
What it does not do is attemtpt to maintain the pages in contiguous physical order on disk. I am thus only ever likely to SELECT WHERE or ORDER BY the Country column; a clustered index on the Primary Key doesn't do me any good, I'm not accessing this data by PK, I'm accessing it by this other column.
SQL Server Clustered Indexes - The question is: Can we apply Non-clustered Index on Primary Key Column, Can we apply Clustered Index on Non-Primary key column? The main reason for this confusion is, SQL Server creates default Clustered Index on the Primary Key column.
A primary key is a logical concept - it's the unique identifier for a row in a table. As such, it has a bunch of attributes - it may not be null, and it must be unique. Of course, as you're likely to be searching for records by their unique identifier a lot, it would be good to have an index on the primary key. A clustered index is a physical concept - it's an index that affects the order in which records are stored on disk. This makes it a very fast index when accessing data, though it may slow down writes if your primary key is not a sequential number. Yes, you can have a primary key without a clustered index - and sometimes, you may want to for instance when your primary key is a combination of foreign keys on a joining table, and you don't want to incur the disk shuffle overhead when writing. Yes, you can create a clustered index on columns that aren't a primary key. A table can have a primary key that is not clustered, and a clustered table does not require a primary key. So the answer to both questions is yes. A clustered index stores all columns at the leaf level. That means a clustered index contains all data in the table. A table without a clustered index is called a heap. A primary key is a unique index that is clustered by default. By default means that when you create a primary key, if the table is not clustered yet, the primary key will be created as a clustered unique index. Unless you explicitly specify the nonclustered option. Actually, I recommend reading the whole site from the beginning until you reach the clustering topic to really understand what's going on. Now, to your questions... Can a TABLE have primary key without Clustered Index? Yes, use NONCLUSTERED keyword when declaring your primary key to make a heap-based table. Unlike some other DBMSes, MS SQL Server will let you have a clustering index that is different from primary key, or even without having the primary key at all. Please note that the benefits of clustering are most visible for range scans. Can anybody briefly tell me the relationship of primary key and clustered index? Under MS SQL Server, primary key is also clustered by default. You can change that default, as discussed above. Answers taken from Can a TABLE have primary key without Clustered Index? Can a TABLE have Clustered Index without primary key? A Primary Key is a constraint that ensures uniqueness of the values, such that a row can always be identified specifically by that key. A non-clustered index is a logical ordering of rows, by one or more of its columns. A clustered index is when the actual table is physically ordered by a particular column. A table will not always have a clustered index ie while it'll be physically ordered by something, that thing might be undefined. A table cannot have more than one clustered index, although it can have a single composite clustered index ie the table is physically ordered by eg Surname, Firstname, DOB. The PK is often but not always a clustered index. But if there is not a primary key set yet, and there is a clustered index, then we can't create a primary key with Clustered Index.
TIP : You can create only One Clustered Index per Table. As the name implies it is just a constraint, and with that constraint you tell SQL Server that you want to have unique values in a specific column or in a specific group of columns. Nupur Dave is a social media enthusiast and and an independent consultant. It will fail if there are any other tables with foreign key relationships to dbo. Example query 2 SELECT c. Either that or drop the clustered index, create your new clustered index and then create your old clustered index as a non clustered index. Genders or InvoiceTypes or PaymentType are another example of tables that should be clustered by its PK because you'll usually join them on GenderId, InvoiceTypeId or PaymentTypeId. Next, expand the Indexes folder, and right-click on the key name will open the context menu.
