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Does creating a non-clustered index lock the table?
Yes it will lock the table. For a nonclustered index, should be a shared lock, so just blocks writers. Unless you have Enterprise edition and can build the index online.
Does a non-clustered index have to be unique?
Both clustered and nonclustered indexes can be unique. This means no two rows can have the same value for the index key. Otherwise, the index is not unique and multiple rows can share the same key value.
Why do we have multiple non clustered indexes in SQL Server?
We can have multiple non-clustered indexes in SQL tables because it is a logical index and does not sort data physically as compared to the clustered index. Let’s understand the non-clustered index in SQL Server using an example. In this execution plan, we can notice that the table scan changes to a clustered index seek:
When do you create a unique nonclustered index?
UNIQUE constraints. When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. You can specify a unique clustered index if a clustered index on the table does not already exist. For more information, see Unique Constraints and Check Constraints.
Which is a good column for a non clustered index?
The semi-unique columns that have a large number of distinct values are good candidates also as Non-clustered index key columns. For the column that has few numbers of distinct values, such as the Gender column, you can take benefits from creating a filtered index, as we will see in the next article.
How to create a non clustered index in SSMS?
Non-clustered indexes can be created using SSMS by expanding the Tables folder under your database. To accomplish this, expand the table on which you plan to create the Non-clustered index on, then right-click on the Indexes node under your table and choose to create the Non-Clustered Index type from the New Index option, as shown below: