MySQL AUTO_INCREMENT does not ROLLBACK

Question

I'm using MySQL's AUTO_INCREMENT field and InnoDB to support transactions. I noticed when I rollback the transaction, the AUTO_INCREMENT field is not rollbacked? I found out that it was designed this way but are there any workarounds to this?

Answer 1

Why do you care if it is rolled back? AUTO_INCREMENT key fields are not supposed to have any meaning so you really shouldn't care what value is used.

If you have information you're trying to preserve, perhaps another non-key column is needed.

Answer 2

I do not know of any way to do that. According to the MySQL Documentation, this is expected behavior and will happen with all *innodb_autoinc_lock_mode* lock modes. The specific text is:

In all lock modes (0, 1, and 2), if a transaction that generated auto-increment values rolls back, those auto-increment values are “lost.” Once a value is generated for an auto-increment column, it cannot be rolled back, whether or not the “INSERT-like” statement is completed, and whether or not the containing transaction is rolled back. Such lost values are not reused. Thus, there may be gaps in the values stored in an AUTO_INCREMENT column of a table.

Answer 3

If you need to have the ids assigned in numerical order with no gaps, then you can't use an autoincrement column. You'll need to define a standard integer column and use a stored procedure that calculates the next number in the insert sequence and inserts the record within a transaction. If the insert fails, then the next time the procedure is called it will recalculate the next id.

Having said that, it is a bad idea to rely on ids being in some particular order with no gaps. If you need to preserve ordering, you should probably timestamp the row on insert (and potentially on update).

Answer 4

It can't work that way. Consider:

• program one, you open a transaction and insert into a table FOO which has an autoinc primary key (arbitrarily, we say it gets 557 for its key value).
• Program two starts, it opens a transaction and inserts into table FOO getting 558.
• Program two inserts into table BAR which has a column which is a foreign key to FOO. So now the 558 is located in both FOO and BAR.
• Program two now commits.
• Program three starts and generates a report from table FOO. The 558 record is printed.
• After that, program one rolls back.

How does the database reclaim the 557 value? Does it go into FOO and decrement all the other primary keys greater than 557? How does it fix BAR? How does it erase the 558 printed on the report program three output?

Oracle's sequence numbers are also independent of transactions for the same reason.

If you can solve this problem in constant time, I'm sure you can make a lot of money in the database field.

Now, if you have a requirement that your auto increment field never have gaps (for auditing purposes, say). Then you cannot rollback your transactions. Instead you need to have a status flag on your records. On first insert, the record's status is "Incomplete" then you start the transaction, do your work and update the status to "compete" (or whatever you need). Then when you commit, the record is live. If the transaction rollsback, the incomplete record is still there for auditing. This will cause you many other headaches but is one way to deal with audit trails.

Answer 5

Let me point out something very important:

You should never depend on the numeric features of autogenerated keys.

That is, other than comparing them for equality (=) or unequality (<>), you should not do anything else. No relational operators (<, >), no sorting by indexes, etc. If you need to sort by "date added", have a "date added" column.

Treat them as apples and oranges: Does it make sense to ask if an apple is the same as an orange? Yes. Does it make sense to ask if an apple is larger than an orange? No. (Actually, it does, but you get my point.)

If you stick to this rule, gaps in the continuity of autogenerated indexes will not cause problems.