I believe when I create an identity column it gets indexed automatically, but I'm not 100% sure.
Should I create an index for an identity column, or is it created automatically?
create table test (Id int identity)
go
sp_help test
The object 'test' does not have any indexes, or you do not have permissions.
No constraints are defined on object 'test', or you do not have permissions.
As a general practice you would create a unique index on your identity column, this speeds up lookups.
Usually you would like your identity columns to be 'clustered indexes' as well (Id int identity primary key is the shortcut notation), meaning table is layed out on disk in the same order your identity column is. This optimizes for inserts, as the page being inserted into tends to be in memory. In some cases, when you are doing ranged lookups very frequently on other data in the table, you may consider clustering other columns instead, as SQL Server only allows you one clustered index per table.
If you create a column with a PRIMARY KEY constraint, a clustered index will be created by default (assuming this is a new table and no such index is already defined).
Being an IDENTITY field has nothing to do with it.
So, to answer you question - if you are going to query/sort with this field and it is not a primary key, you should define an index on it (rule of thumb, always test for your scenario).
By default, your primary key column(s) will be made into a clustered index; this is a special index that is not separate from the data - but instead, the data is ordered in the table by that indexes values.
I say 'by default', because you can change the clustered index to another field if you wanted. If you've chosen a good identity value for your primary key, you will almost never want to change this, though.
Related
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.
In SQL Server, I have a non nullable column with a unique clustered index on it.
If I make this column a Primary Key the exact same index is created automatically plus
the column is recognized as a Primary Key.
I understand the abstract/semantic difference.
(Primary Key identifies the entity, while any other column with this index may not.
For example, a Person can have Email field which is Unique,Non-nullable... but can be changed)
But what bothers me is the actual difference when it comes to the DB engine itself.
What will happen if I will just create an Id column, make it non-nullable, create a unique clustered index for it, make it Identity Increment, but without the Primary Key constraint?
In what scenarios the Primary Key constraint comes into play?
(I've looked at many related questions before asking this, but all the answers I saw ended up with an abstract/theoretical explanation).
Nothing will be different really. You specify PRIMARY KEY to relay your intentions, not so that the engine does anything differently. When constructing a query plan, the optimizer will still use the uniqueness for all of its properties, and will still use the clustered index for all of its properties, regardless of whether you technically created it as a PRIMARY KEY. When creating a FOREIGN KEY, you can still reference the column(s) specified as unique (clustered or not). The difference is solely in the metadata (sys.indexes.is_primary_key) and in SSMS' representation to you (oh and the fact that you can create a unique clustered index on a NULLable column, but you can't create a PRIMARY KEY on that column).
In fact there are many cases where you want to completely separate the clustered index from the PRIMARY KEY. If you have a table where the PK is a GUID, for example, and you are typically running date range queries against the table, you are probably better off having the PK be non-clustered and have a clustered index on a naturally increasing column (the datetime column) - both to minimize page splits on heavy insert activity and also to best assist date range queries. The non-clustered index will be perfectly fine for looking up individual GUIDs. (I wanted to mention that because a lot of people think the primary key has to be clustered. Not true.)
Also interesting to note that if you create a PRIMARY KEY constraint, then create a unique clustered index with the same name using DROP_EXISTING, the is_primary_key column will still be 1 and Object Explorer will still show the index name under Keys.
Here is one scenario - a lot of code to data mapping frameworks look at the database metadata (what are the primary keys, foreign keys, etc) to determine how code is executed. For example Hibernate requires a primary key.
A typical scenario might be generating a where clause for an update.
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.
In a SQL Server db, what is the difference between a Primary Key and an Identity column? A column can be a primary key without being an indentity. A column cannot, however, be an identity without being a primary key.
In addition to the differences, what does a PK and Identity column offer that just a PK column doesn't?
A column can definitely be an identity without being a PK.
An identity is simply an auto-increasing column.
A primary key is the unique column or columns that define the row.
These two are often used together, but there's no requirement that this be so.
This answer is more of WHY identity and primary key than WHAT they are since Joe has answered WHAT correctly above.
An identity is a value your SQL controls. Identity is a row function. It is sequential, either increasing or decreasing in value, at least in SQL Server. It should never be modified and gaps in the value should be ignored. Identity values are very useful in linking table B to table A since the value is never duplicated. The identity is not the best choice for a clustered index in every case. If a table contains audit data the clustered index may be better being created on the date occurred as it will present the answer to the question " what happened between today and four days ago" with less work because the records for the dates are sequential in the data pages.
A primary key makes the column or columns in a row unique. Primary key is a column function. Only one primary key may be defined on any table, but multiple unique indexes may be created which simulates the primary key. Clustering the primary key is not always the correct choice. Consider a phone book. If the phone book is clustered by the primary key(phone number) the query to return the phone numbers on "First Street" will be very costly.
The general rules I follow for identity and primary key are:
Always use an identity column
Create the clustered index on the column or columns which are used in range lookups
Keep the clustered index narrow since the clustered index is added to the end of every other index
Create primary key and unique indexes to reject duplicate values
Narrow keys are better
Create an index for every column or columns used in joins
These are my GENERAL rules.
A primary key (also known as a candidate key) is any set of attributes that have the properties of uniqueness and minimality. That means the key column or columns are constrained to be unique. In other words the DBMS won't permit any two rows to have the same set of values for those attributes.
The IDENTITY property effectively creates an auto-incrementing default value for a column. That column does not have to be unique though, so an IDENTITY column isn't necessarily a key.
However, an IDENTITY column is typically intended to be used as a key and therefore it usually has a uniqueness constraint on it to ensure that duplicates are not permitted.
Major Difference between Primary and Identity Column
Primary Column:
Primary Key cannot have duplicate values.
It creates a clustered index for the Table.
It can be set for any column type.
We need to provide the primary column value while inserting in the table.
Identity Column:
Identity Column can have duplicate value.
It can only be set for Integer related columns like int, bigint, smallint, tinyint or decimal
No need to insert values in the identity column. It is inserted automatically based on the seed.
EDITS MADE BASED ON FEEDBACK
A key is unique to a row. It's a way of identifying a row. Rows may have none, one, or several keys. These keys may consist of one or more columns.
Keys are indexes with a unique constraint. This differentiates them from non-key indexes.
Any index with multi-columns is called a "composite index".
Traditionally, a primary key is viewed as the main key that uniquely identifies a row. There may only be one of these.
Depending on the table's design, one may have no primary key.
A primary key is just that - a "prime key". It's the main one that specifies the unique identity of a row. Depending on a table's design, this can be a misnomer and multiple keys express the uniqueness.
In SQL Server, a primary key may be clustered. This means the remaining columns are attached to this key at the leaf level of the index. In other words, once SQL Server has found the key, it has also found the row (to be clear, this is because of the clustered aspect).
An identity column is simply a method of generating a unique ID for a row.
These two are often used together, but this is not a requirement.
You can use IDENTITY not only with integers, but also with any numeric data type that has a scale of 0
primary key could have scale but its not required.
IDENTITY, combined with a PRIMARY KEY or UNIQUE constraint, lets you provide a simple unique row identifier
Primary key emphasizing on uniqueness and avoid duplication value for all records on the same column, while identity provides increasing numbers in a column without inserting data.
Both features could be on a single column or on difference one.
I believe when I create an identity column it gets indexed automatically, but I'm not 100% sure.
Should I create an index for an identity column, or is it created automatically?
create table test (Id int identity)
go
sp_help test
The object 'test' does not have any indexes, or you do not have permissions.
No constraints are defined on object 'test', or you do not have permissions.
As a general practice you would create a unique index on your identity column, this speeds up lookups.
Usually you would like your identity columns to be 'clustered indexes' as well (Id int identity primary key is the shortcut notation), meaning table is layed out on disk in the same order your identity column is. This optimizes for inserts, as the page being inserted into tends to be in memory. In some cases, when you are doing ranged lookups very frequently on other data in the table, you may consider clustering other columns instead, as SQL Server only allows you one clustered index per table.
If you create a column with a PRIMARY KEY constraint, a clustered index will be created by default (assuming this is a new table and no such index is already defined).
Being an IDENTITY field has nothing to do with it.
So, to answer you question - if you are going to query/sort with this field and it is not a primary key, you should define an index on it (rule of thumb, always test for your scenario).
By default, your primary key column(s) will be made into a clustered index; this is a special index that is not separate from the data - but instead, the data is ordered in the table by that indexes values.
I say 'by default', because you can change the clustered index to another field if you wanted. If you've chosen a good identity value for your primary key, you will almost never want to change this, though.