Let's suppose you have this table: We have made last name the primary key in this table Now lets import some rows into the table and check the fragmentation Notice the fragmentation at 96.48% and all we did was one import of 16426 records. So now we have several questions: How bad is this? Fragmentation causes SQL Server to skip around to read data from your tables. For a one time read on a small table this is virtually meaningless. In a job that may need to read millions of records, this could tear apart your performance. Ok this is bad. How did it happen? The primary key on a table is clustered. This means the data will be stored according to the primary key. In our case, last name. When data doesn't come in the same way you store it (i.e. Customer names are not coming in alphabetically, but rather randomly) SQL Server must constantly split pages to store the data correctly. This causes data to be "fragmented" into multiple areas of the disk instead of one continuou...