I can see from an earlier question that log file backup may be performed simultaneously with either full or differential data file backups.
Imagine for a moment that full backups run during a window of time that doesn't overlap with any other backup. However differential and log backups may overlap.
If, for example, this produces a differential backup at 11am on a given day, but also produces a log file backup at 11am on the same day, how do you decide which files to restore?
- 01:00 AM = full backup
- 10:00 AM = differential backup
- 10:00 AM = log backup
- 10:15 AM = log backup
- 10:30 AM = log backup
- 10:45 AM = log backup
- 11:00 AM = differential backup
- 11:00 AM = log backup
- 11:05 AM = disaster
After restoring the full backup, do you restore the second differential backup and the last log backup? Or do you restore the first backup and all the log backups (even though the 10AM log backup really has the same issue as the 11AM backup - coinciding with the differential backup in each case).
I can see another question here for which some answers suggest third-party scripts to restore backup file headers in order to determine sequence numbers - do we really have to resort to header restores to determine which files are able to be restored in sequence?
Edit (19/12/18): I think my original question was worded poorly, so the answers it has received are varied. The key part I wanted to understand was actually mentioned by Jonathan in a comment on Sean's answer, and Jerry mentioned it too - that the restore sequence will handle the scenario if the log file contains information about data changes that have already been accounted for by the differential backup restore. (Sorry I didn't word the question well!) Sean's answer gives lots of additional info that is also helpful to understanding the restore sequence, so I can't pick an individual answer here, but I will upvote everything that helped with clarifications. Thanks.