We have a table with about 100,000 record which is used frequently in our applications. We had an identity (ID) columns and had a clustered index on it and everything worked good. But for some reasons we had to use a Uniqueidentifier column as Primary key. So we add a non clustered index on it and removed the clustered index on ID column. But now, we have lots of performance degradation issuses from our customer in peak times. Is it because the table has no clustered index now?
The fact that you added a primary key by no means implies you had to drop the clustered index. The two concepts are distinct. You can have an uniqueidentifier PK implemented by a non clustered index and a separate clustered index of choice (eg. the old ID column).
But the real question is How did you change your application when you added the uniqueidentifier PK? Did you also modified the application code to retrieve the records by this new PK (by the uniqueidentifier)? Did you update all joins to reference the new PK? Did you modified all foreign key cosntraints that referenced the old ID column? Or does the application continue to retrieve the data using the old identity ID column? My expectation is that you changed both the application and the table, and the access is now prevalent on the form of SELECT ... FROM table WHERE pk=#uniqueidentifier. If only such access occurs, then the table should perform OK even with a non-clustered uniqueidentifier primary key and no clustered index. So there must be something else at play:
your application continues to access the table based on the old identity ID column
there are joins in your query based on the old identity ID column
there are foreign key constraints referencing the table on the old ID column
Ultimately you have a performance troubleshooting issue at hand and approach it as a performance troubleshooting problem. I have two great resources for you:the Waits and Queue methodology and the Performance Troubleshooting Flowchart
Hi I think you can make uniqueidentifier column as clustered index with NEWSEQUENTIALID() instead of NEWID(). As newsequentialid generates the sequential ids and for clustered index its the best.
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I am using SQL Server 2012 and for one of the table I see it has created primary key non-clustered (composite key) and clustered index on different column? Can somebody help me to understand what will happen in this situation?
Does this going to degrade performance for DML operations? If yes how to measure it?
Will this be causing locking/blocking/deadlocks for this table when performing DML operation during concurrency ?
Note: this table has a huge number of records in it ~10 million
One common scenario where you might end up with a primary key which is a non clustered composite key is a junction table. A junction table mainly exists to store a relationship between two primary key values from other tables. A simple example would be storing say relationships between students and the courses they take. As such, the primary (unique) key in such a table would actually be the combination of the two foreign key columns. That being said, there can still be a clustered index on some other column. There is nothing at all out of the ordinary here, assuming such a table falls in line with your design intentions.
I am doing a review of some DB tables that were created in our project and came across this. The table contains an Identity column (ID) which is the primarykey for the table and a clustered index has been defined using this ID column. But when I look at the SPROC that retrieves records from this table, I see that the ID column is never used in the query and they query the records based on a USERID column (this column is not unique) and there can be multiple records for the same USERID.
So my question is there any advantage/purpose in creating a clustered index when we know that the records wont be queried with that column?
If the IDENTITY column is never used in WHERE and JOIN clauses, or referenced by foreign keys, perhaps USERID should be a clustered primary key. I would question the need for the ID column at all in that case.
The best choice for the clustered index depends much on how the table is queried. If the majority of queries are by USERID, then it should probably be a unique clustered index (or clustered unique constraint) and the ID column non-clustered.
Keep in mind that the clustered index key is implicitly included in all non-clustered indexes as the row locator. The implication is that non-clustered indexes may more likely cover queries and non-clustered index leaf node pages wider as a result.
I would say your table is mis-designed. Someone apparently thought every table needs a primary key and the primary key is the clustered index. Adding a system-generated unique number as an identifier just adds noise if that number isn't used anywhere. Noise in the clustered index is unhelpful, to say the least.
They are different concepts, by the way. A primary key is a data modeling concern, a logical concept. An index is a physical design issue. A SQL DBMS must support primary keys, but need not have any indexes, clustered or no.
If USERID is what is usually used to search the table, it should be in your clustered index. The clustered index need not be unique and need not be the primary key. I would look at the data carefully to see if some combination of USERID and another column (or two, or more) form a unique identifier for the row. If so, I'd make that the primary key (and clustered index), with USERID as the first column. If query analysis showed that many queries use only USERID and nothing else (for existence testing) I might create a separate index just of USERID.
If no combination of columns constitutes a unique identifier, you have logical problem, to wit: what does the row mean? What aspect of the real world does it represent?
A basic tenet of the Relational Model is that elements in a relation (rows in a table) are unique, that each one identifies something. If two rows are identical, they identify the same thing. What does it mean to delete one of them? Is the thing that they both identify still there, or not? If it is, what purpose did the 2nd row serve?
I hope that gives you another way to think about clustered indexes and keys. I wouldn't be surprised if you find other tables that could be improved, too.
By default, a SQLServer table clustered index is the PK. If I define that:
such key is a GUID and
I will generate at random a.k.a Guid.NewGuid() and
forget to set the clustered index to a more meaningful column
will SQLServer reorder the pages as a new record enters the table or will it just "ignore" the clustered part of the index?
I think you got it reversed: by default, the PK is also the clustered index. You must choose the data type and what columns to include in the PK. SQL Server won't set a default PK for you. Without a PK, a table is a heap.
Using GUID as a PK is bad practice. You will cause unnecessary page splits upon INSERT. If the data does not have a natural key, use an IDENTITY column instead.
So you create PK and it defaults to a clustered. You change it to a non clustered index. What makes you think there is a clustered index hanging around? There is no clustered part to the index if you changed it to a non clustered.
I've been using SQL Server for quite a while, I always create database with design view.
The steps I took to create the table is:
Right Click Table -> New Table
I always have the first column as SOMETHING_ID (int) -> I make SOMETHING_ID as Identity with auto increment of 1
-> Then I add other columns
-> Save and use
As you can see, I didn't define SOMETHING_ID by right clicking it and SET AS PRIMARY.
Will there be any performance impact in this case?
Yes, it can impact performance because creating the primary key essentially makes an index for it. So when you join tables on that key it will improve performance greatly if there are indexes.... particularly if you have lots of data.
What you really need to do is to create a clustered index. A primary index, by default is a clustered index (but you can create a primary index that is not a clustered index). A table without a clustered index is called a heap and except for very special occasions you should have a clustered index on every table. A primary index is a index that has only unique values and does not have any (not even one) null index value.
A query that uses a clustered index is usually a very effective one but if there is not clustered index (even if the table has indexes) it can end up with forwarding pointers all over the place and searching for all the rows for a given customer can require SQL Server to read many, many pages.
To create a clustered index on a table you can use syntax such as
create clustered index ix1_table1 on table1(id)
The column(s) used in a index of any kind can occur anywhere in a table and does not necessarily have to be identity columns.
By not creating Primary key you're breaking the rule of First Normal Form in Normalization.
Disadvantages of not having Primary Key
Chances of Duplicates
Your table won't be clustered with clustered index
You won't be able to do Primary Key-Foreign Key relationship with other table.
I am just wondering can we create a Primary key on a table in sql server without any type of index on it?
No. As an implementation detail, SQL Server maintains the primary key using an index. You cannot prevent it from doing so. The primary key:
Ensures that no duplicate key values exist
Allows individual rows to be identified/accessed
SQL Server already has mechanisms that offer these features - unique indexes - so it uses those to enforce the constraint.
You can create a table with a primary key that is not a clustered index by adding the keyword NONCLUSTERED after the primary key word.
Actually indexing functions in the same way a book is traversed. One cannot go to a particular page or topic unless there is page number and their relation to the topics. This "Paging" (ordering) of rows is done physically by Clustered Index in SQL Server. If there is no PK in a table one can add clustered index on any unique-key qualifying column(s). As there cannot be more than one clustered index on a table, and all other non-clustered indices depend on clustered index for search/traversal, you cannot make a column PK without clustered index.