Reorganize and Rebuild Indexes with active connections I want to rebuild or reorganize indexes in a table. What happens if there are active connections to that table at the moment?
I want to rebuild or reorganize indexes in a table


These are two different modes to remove index fragmentation. 


Rebuilding an index means that a whole new set of pages is
allocated for it. 

It cames in two flavours: online and offline. When rebuilding
offline your index is completely unavailable, but the operation is
faster than that online. It can be minimally logged (in simple
or bulk logged recovery models), and even when it's fully logged
it's logged very efficiently (the whole index pages when built are
written to log). 

Online rebuild is available only in SQL Server Enterprise,
Developer, and Evaluation editions. The operation is always
fully logged and produce a notable amount of log as new index is
logged row by row. Besides, more exact term for it is "almost
online operation". Yes there is no lock held for the entire duration
of the index rebuild operation, but the locks are still held.

Here is a great explanation of Paul Randal:

Misconceptions around index rebuilds (allocation, BULK_LOGGED mode, locking)


  Myth 5: online index rebuild doesn't take any locks
  
  This myth is untrue. The 'online' in 'online index operations' is a
  bit of a misnomer.  Online index operations need to take two very
  short-term table locks. An S (Shared) table lock at the start of the
  operation to force all write plans that could touch the index to
  recompile, and a SCH-M (Schema-Modification – think of it as an
  Exclusive) table lock at the end of operation to force all read and
  write plans that could touch the index to recompile.
  
  The most recent time this came up on the forums was someone noticing
  insert queries timing out after an online index rebuild operation had
  just started. The problem is that the  table lock that online index
  rebuild needs has to be entered into the grant queue in the lock
  manager until it can be acquired – and it will stay there until
  existing transactions that are holding conflicting locks either commit
  or roll-back. Any transaction that requires a conflicting lock AFTER
  the index rebuild lock has been queued but not acquired (and then
  released) will wait behind it in the lock grant queue. If the query
  timeout is reached before the transaction can get it's lock, it will
  timeout.

Reorganizing is designed to remove logical fragmentation from the leaf level of an index while 
keeping the index online and as available as possible. When reorganizing an index, SQL Server acquires an Intent-Exclusive lock on the index B-tree. Exclusive page locks are taken on individual pages only while those pages are being manipulated. So you can consider reorganizing an online operation.