In SQL Server's full recovery model, a full database backup does not truncate the transaction log, contrary to what some might expect. This is a fundamental behavior designed to maintain the complete backup chain required for point-in-time recovery.

Microsoft's documentation clearly states this behavior in Recovery Models (SQL Server):

"Under the full recovery model, after you create a full database backup, you must back up the transaction log. Not doing so breaks the chain of transaction log backups, making it impossible to restore log backups taken after the most recent full or differential database backup."

The transaction log maintains all changes since the last log backup because:

• Log backups depend on previous backups in the chain
• Full backups alone don't capture all committed transactions at the exact moment of backup completion
• The log preserves the sequence of operations needed for recovery scenarios

Here's how you can verify this behavior:

-- Create a test database
CREATE DATABASE RecoveryModelTest;
GO

-- Set to full recovery model
ALTER DATABASE RecoveryModelTest SET RECOVERY FULL;
GO

-- Take initial full backup
BACKUP DATABASE RecoveryModelTest TO DISK = 'C:\Backups\RecoveryModelTest_Full.bak';
GO

-- Check log space usage (will show log not truncated)
DBCC SQLPERF(LOGSPACE);
GO

-- Create some transactions
USE RecoveryModelTest;
GO
CREATE TABLE TestTable (ID INT);
GO
INSERT INTO TestTable VALUES (1);
GO 1000

-- Check log space again (will show increased usage)
DBCC SQLPERF(LOGSPACE);
GO

-- Take another full backup
BACKUP DATABASE RecoveryModelTest TO DISK = 'C:\Backups\RecoveryModelTest_Full2.bak';
GO

-- Log space remains allocated until log backup
DBCC SQLPERF(LOGSPACE);
GO

-- Now perform log backup (this WILL truncate)
BACKUP LOG RecoveryModelTest TO DISK = 'C:\Backups\RecoveryModelTest_Log.trn';
GO

-- Verify log truncation occurred
DBCC SQLPERF(LOGSPACE);
GO

Many DBAs assume full backups should truncate logs because:

• Simple recovery model DOES behave this way
• Full backups appear to be "complete" snapshots
• The concept seems logically consistent at first glance

To properly manage transaction logs in full recovery:

1. Schedule regular log backups (not just full backups)
2. Monitor log file growth using DBCC SQLPERF(LOGSPACE)
3. Consider using log shipping or mirroring for critical systems
4. Implement proper retention policies for log backups

Failure to perform log backups can lead to:

• Excessive log file growth
• Potential out-of-space errors
• Longer recovery times
• Increased storage requirements

In SQL Server's full recovery model, the transaction log continues growing until explicitly backed up. Many DBAs assume a full database backup would automatically truncate inactive portions of the log, but this isn't the case. The log remains intact until a BACKUP LOG operation occurs.

Microsoft's documentation clearly states this behavior in Recovery Models (SQL Server):

"Under the full recovery model, you must back up the transaction log. Until you back up the log, the database uses the transaction log to prevent truncation."

The full backup contains all data as of that point in time, but the transaction log contains the complete history needed for:

• Point-in-time recovery
• Log shipping configurations
• Database mirroring synchronization

Here's how to verify this behavior:

-- Create a test database
CREATE DATABASE LogTruncationTest;
GO

-- Set to full recovery
ALTER DATABASE LogTruncationTest SET RECOVERY FULL;
GO

-- Perform initial full backup
BACKUP DATABASE LogTruncationTest 
TO DISK = 'C:\Backups\LogTruncationTest_Full.bak';
GO

-- Check log space usage (will show log not truncated)
DBCC SQLPERF(LOGSPACE);
GO

To properly manage transaction logs in full recovery mode:

-- Regular log backup schedule
BACKUP LOG LogTruncationTest 
TO DISK = 'C:\Backups\LogTruncationTest_Log.trn';
GO

-- Verify log truncation occurred
DBCC SQLPERF(LOGSPACE);
GO

Some administrators believe these operations truncate logs (they don't):

• Full database backups
• Checkpoints (in full recovery mode)
• Database shrink operations