Does SQL Server creates Non clustered index by default

Question

Ya, it is a duplicate of this. But I just needs a clarification on this article by Pinal Dave, which says the following:

Scenario 4: Primary Key defaults to Clustered Index with other index defaults to Non-clustered index

In this case we will create two indexes on the both the tables but we will not specify the type of the index on the columns. When we check the results we will notice that Primary Key is automatically defaulted to Clustered Index and another column as a Non-clustered index.
-- Case 4 Primary Key and Defaults
USE TempDB
GO
-- Create table
CREATE TABLE TestTable
(ID INT NOT NULL PRIMARY KEY,
Col1 INT NOT NULL UNIQUE)
GO
-- Check Indexes
SELECT OBJECT_NAME(OBJECT_ID) TableObject,
[name] IndexName,
[Type_Desc]
FROM sys.indexes
WHERE OBJECT_NAME(OBJECT_ID) = 'TestTable'
GO
-- Clean up
DROP TABLE TestTable
GO

score 17 · Accepted Answer · answered May 25 '13 at 06:05

The only indexes that get created automatically:

the clustered index on your primary key (unless you specify otherwise - if you define your primary key to be nonclustered, then a nonclustered index will be created)
a unique nonclustered index when you apply a UNIQUE CONSTRAINT to a column (or set of columns)

score 2 · Answer 2 · edited May 23 '17 at 12:24

Just to spell it out - the Result of Pinal Dave's example are indexes similar to the following:

TestTable   PK__TestTabl__3214EC2703317E3D  CLUSTERED
TestTable   UQ__TestTabl__A259EE55060DEAE8  NONCLUSTERED

Which can be explained as follows:

PK Clustered

If a table is created with a primary key, then it is a Clustered Table, and the Clustered Index is defaulted to the Primary Key unless you specify otherwise. (Tables without a Clustered Index are Heaps)

UQ Nonclustered

SQL does not usually create any non-clustered indexes on a table by default.

However, as Marc has pointed out, because the table has a column with a UNIQUE constraint, (Col1 INT NOT NULL UNIQUE), MS SQL implements the constraint as a unique, non-clustered index on that column.

See also: Is the Sql Server Unique Key also an Index?

Does SQL Server creates Non clustered index by default

2 Answers2