Is there any harm in resetting the auto-increment?

Question

I have a 100 million rows, and it\'s getting too big. I see a lot of gaps. (since I delete, add, delete, add.)

I want to fill these gaps with auto-increment. If I do

Answer 1

Since you can't change the next auto-increment value, you have other options. The datatype switch could be done, but it seems a little unsettling to me since you don't actually have that many rows. You'd have to make sure your code can handle IDs that large, which may or may not be tough for you.

Are you able to do much downtime? If you are, there are two options I can think of:

1. Dump/reload the data. You can do this so it won't keep the ID numbers. For example you could use a SELECT ... INTO to copy the data, sans-IDs, to a new table with identical DDL. Then you drop the old table and rename the new table to the old name. Depending on how much data there is, this could take a noticeable about of time (and temporary disk space).

2. You could make a little program to issue UPDATE statements to change the IDs. If you let that run slowly, it would "defragment" your IDs over time. Then you could temporarily stop the inserts (just a minute or two), update the last IDs, then restart it. After updating the last IDs you can change the AUTO_INCREMENT value to be the next number and your hole will be gone. This shouldn't cause any real downtime (at least on InnoDB), but it could take quite a while depending on how aggressive your program is.

Of course, both of these ignore referential integrity. I'm assuming that's not a problem (log statements that aren't used as foreign keys, or some such).