I periodically need to make changes to tables in mysql 5.1, mostly adding columns. Very simple with the alter table command. But my tables have up to 40 million rows now and they are growing fast... So those alter table commands take several hours. In a couple months they'll take days I'm guessing.

Since I'm using amazon RDS, I can't have slave servers to play with and then promote to master. So my question is if there's a way to do this with minimal downtime? I don't mind an operation taking hours or even days if users can still use the db of course... Can they at least read while columns are being added? What happens if my app tries to write? Insert or update? If it fails immediately that's actually not so bad, if it just hangs and causes problems for the db server that's a big problem..

This must be a fairly common scaling issue, everyone needs to add columns.. What's typically done to a production db? Slave -> master migration?

Update - I forgot to mention I'm using the innodb storage engine


5 Answers 5


I periodically need to make changes to tables in mysql 5.1, mostly adding columns.

Don't. No really. Just don't. It should be a very rare occasion when this is ever necessary.

Assuming your data really is normalized to start with, the right way to solve the problem is to add a new table with a 1:1 relationship to the base table (non-obligatory on the new table).

Having to add columns regularly is usually an indicator of a database which is not normalized - if your schema is not normalized then that's the problem you need to fix.

Finally, if your schema really, really is normalized and you really, really must keep adding columns then:

  1. Ensure you've got a timestamp column on the database or that it is generating replication logs
  2. Create a copy (B) of the table (A)
  3. add the new columns to B (this will still block with myisam)
  4. disable transactions
  5. rename the original table (A) as something else (backup)
  6. rename the new table (B) with the name of the original table (A)
  7. replay the transactions from the start of the operation from the replication log or from the backup table
  8. enable transactions.
  • 2
    Thank you for your step by step approach. Is it really uncommon to modify tables? I understand that I can instead add another table with the new column (in the case of needing to add a column) and have it reference the original large table in a 1:1 relationship. But it doesn't seem right to have 15 very large 1:1 tables when they should all be in 1 table... The querying performance of course then suffers as well, not to mention the indexing issues. I'm not an expert, but my database is fairly well normalized and it seems natural that I need to periodically modify..
    – apptree
    Aug 27, 2010 at 2:21
  • 2
    "Is it really uncommon to modify tables?" - Yes.
    – symcbean
    Aug 27, 2010 at 9:37
  • 30
    As a dev, especially one that works in start-ups and young companies, I could not agree less with symcbean and @TomTom. Things change, products change, business goals change and the database structure needs to change with them. Providing good DBA service means saying "yes" to those changes, then figuring out how to implement them efficiently. Heavily-normalized databases are a concept that died a long time ago. They result in bad performance and slow dev cycles.
    – pents90
    Apr 15, 2015 at 21:54
  • 5
    Uncommon to change tables??? Maybe in large companies, but in agile team that happens quite often, requirements change...
    – tibo
    Aug 12, 2015 at 0:42
  • 4
    some 8 years later =) I just felt I wanted to say im with pents90 and tibo. What symcbean said 8 years ago is just plain wrong, and probably was then too.
    – Ted
    Aug 4, 2018 at 13:44

I just had to do this recently. What Amazon recommended was using the Percona Toolkit. I downloaded it and was able to run something like:

./pt-online-schema-change h=databasenameHostName,D=databasename,t=tablename --recursion-method=none --execute --user username --password password --alter "MODIFY someColumn newDataType"

and it works great. It tells you how much time remaining in the process.

It actually creates a new table with the new column and then copies the existing data over. Further, it creates a trigger so that new data is also pushed over to the new table. It then renames the tables automagically, drops the old table and you're up and running with the new column and no downtime while you waited for the updates.

  • The Percona team has a short write-up on enabling the log_bin_trust_function_creators feature, through RDS parameter groups (as SET GLOBAL log_bin_trust_function_creators = 1 doesn't work on RDS), required by the pt-online-schema-change tool. More details: percona.com/blog/2016/07/01/pt-online-schema-change-amazon-rds Oct 25, 2016 at 3:34
  • its worked for me
    – Adiii
    Dec 14, 2018 at 14:48

symcbean provides some solid recommendations.

To answer your question, the easiest and best way to mitigate impact is by having multiple databases replicating. Dual master with an appropriate failover procedure stopping replication on the active, which allows an alteration on the inactive without impacting active.

You could potentially do this on a single live database and minimize impact by using a procedure similar to the one I detailed in this answer. Admittedly, this is similar to what symcbean described but includes technical details. You could use an auto_increment field as well and not just timestamp.

Ultimately, if your data set is growing so large, you need to also consider archival between OLTP and OLAP databases. Your transaction dataset should not need to be so large, if you design appropriately.


From the manual: http://dev.mysql.com/doc/refman/5.1/en/alter-table.html

In most cases, ALTER TABLE makes a temporary copy of the original table. MySQL incorporates the alteration into the copy, then deletes the original table and renames the new one. While ALTER TABLE is executing, the original table is readable by other sessions. Updates and writes to the table are stalled until the new table is ready, and then are automatically redirected to the new table without any failed updates.

So, reading will work fine. Writes will be stalled, but executed afterwards. If you want to prevent this, you'll have to modify your software.

  • So I've done this and disabled the parts of my site that write to the table that I'm modifying right now. So far I have received several "Lock wait timeout exceeded; try restarting transaction" exceptions, that's not too bad. However, they were on PURELY read operations...
    – apptree
    Aug 27, 2010 at 2:23

I am in similar situation where I have to alter 1 of my transaction table which is almost 65GB. I hear 2 solutions

  1. Use straight forward ALTER and let it Run (X numbers of hours or day)
  2. Ensure you've got a timestamp column on the database or that it is generating replication logs
    • Create a copy (B) of the table (A)
    • add the new columns to B (this will still block with myisam)
    • disable transactions
    • rename the original table (A) as something else (backup)
    • rename the new table (B) with the name of the original table (A)

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