Log chain theory question

Question

Hello!
As per BOL:
http://msdn.microsoft.com/en-us/library/ms186299(v=sql.105).aspx
begins_log_chain bit 1 - This is the first in a continuous chain of log backups.
A log chain begins with the first
log backup taken after the database is created or when it is switched from the simple to the full or bulk-logged recovery model.
As per http://msdn.microsoft.com/en-us/library/ms190440%28v=sql.105%29.aspx
The Log Chain
A continuous sequence of log backups is called a log chain.
A log chain starts with a full backup of the database. Usually, a new log chain is only started when the database
is backed up for the first time or after the recovery model is switched from simple recovery to full or bulk-logged recovery.
So what really starts a log chain - a DB or a LOG
backup???
Thank you in advance,
Michael

Answer 1

Michael,
     The confusion falls in the fact that once a database has been changed from Simple to full or bulk logged recovery model then the transaction log will continue to truncate itself upon a checkpoint.  The reason is that SQL is
smart enough to know that a database backup has not yet been taken so there is no possibility to recover the database to a point in time.  This is sometimes referred to as pseudo simple recovery.  After a database backup is taken then the transaction
log will only be truncated after a transaction log backup.
Hope this helps
David Dye My Blog