Take the 2-minute tour ×
Server Fault is a question and answer site for professional system and network administrators. It's 100% free, no registration required.

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

share|improve this question
add comment

4 Answers

up vote 5 down vote accepted

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.
share|improve this answer
    
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 '10 at 2:21
1  
"Is it really uncommon to modify tables?" - Yes. –  symcbean Aug 27 '10 at 9:37
    
So if I have a products table and want to add a description column, this should be done in another table?? –  Lasse Bunk Aug 3 '13 at 14:17
    
No, but one can argue that if that happens REGULARLY - not as part of a major software upgrade - then someone needs to be fired for not realizing that all the tables should be there in the first place. The problem / trick here is the "regularly", not "Once every couple of months". –  TomTom Jun 23 at 16:19
add comment

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.

share|improve this answer
    
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 '10 at 2:23
add comment

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.

share|improve this answer
    
Thank you Warner, this is helpful. –  apptree Aug 27 '10 at 2:23
add comment

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)
share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.