Clustered Indexes are really important and special. Special because there can only be 1 Clustered Index in a table and they decide the order in which data is stored in tables.
Clustered Index help the database uniquely identify individual rows in a table. As mentioned in my previous blog if the Clustered Index do not uniquely identify a row, SQL Server automatically adds a 4 byte uniqueidentifier column to make it unique.
However we should be really careful not to add too many columns in a Clustered Index in trying to make it unique, because all Non-Clustered Indexes store the columns from Clustered Index at the end to point to a row in a table.
Therefore, more the no. of columns in a Clustered Index will be, more the size of Non-Clustered Index will be.
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