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What columns should be included in a non clustered index?
Included columns in a non-clustered index can only be the non-key columns. In the included columns the storage only happens on the leaf level of the index, contrary to the key column of an index. The key column of an index is stored at all levels.
What does include do in non clustered index?
The Include Clause. The include clause allows us to make a distinction between columns we would like to have in the entire index (key columns) and columns we only need in the leaf nodes ( include columns). That means it allows us to remove columns from the non-leaf nodes if we don’t need them there.
When to use clustered or non clustered indexes?
The clustered index controls the sort of the data pages in the disk, including all the columns in the table, although the index is created by one column only. The non-clustered index does not specify the real data order.
How to create a clustered index in SQL Server?
A lookup to the clustered index needs to be performed by SQL Server to get the 3 columns. To create a covering index, we need to add these 3 columns to the index. In this case it would make sense to use included columns. We don’t add the columns as key columns, because it would make the index wider than needed.
Do you need to add 3 columns to clustered index?
This index has the columns that are needed for the WHERE clause, but it doesn’t cover the 3 columns from the SELECT. A lookup to the clustered index needs to be performed by SQL Server to get the 3 columns. To create a covering index, we need to add these 3 columns to the index.
What are the different types of indexes in SQL Server?
SQL Server Indexes can be categorized also to other types, such as the composite index; which is an index that contains more than one column. Unique index; which ensures the uniqueness of each value in the indexed column or columns as a whole. The last type is the covering index which contains all columns needed for a specific query.