Manually Numbering Rows

Some people wonder why OIDs and sequences are needed at all. Why can't a database user just find the highest number in use, add one, and use the result as the new unique row number? In reality, OIDs and sequences are preferred for several reasons:

• Performance
• Concurrency
• Standardization

First, it is usually a slow process to scan all numbers currently in use to find the next available number. Referring to a counter in a separate location is faster. Second, if one user gets the highest number, and another user is looking for the highest number at the same time, the two users might choose the same next-available highest number. Of course, in this case, the number would not be unique. Such concurrency problems do not occur when using OIDs or sequences. Third, it is more reliable to use database-supplied unique number generation than to generate unique numbers manually.