Eight physically contiguous pages in SQL Server database are called the extent. One page is 8 KB, therefore one extent is 64 KB.
There are two types of extents in SQL Sever:
- All eight physically contiguous pages belong and can be used only by a single object. This is a uniform extent.
- Each of the eight physically contiguous pages belongs and can be used by eight different objects. This type of extent is known as mixed.
Usually all new tables or indexes are allocated pages from mixed extents. SQL Server extent switches from mixed to uniform type only when a table or index has at least eight or more pages.