[2017] Update: MySQL 5.6 has support for online index updates
In MySQL 5.6 and higher, the table remains available for read and write operations while the index is being created or dropped. The CREATE INDEX or DROP INDEX statement only finishes after all transactions that are accessing the table are completed, so that the initial state of the index reflects the most recent contents of the table. Previously, modifying the table while an index is being created or dropped typically resulted in a deadlock that cancelled the INSERT, UPDATE, or DELETE statement on the table.
[2015] Updating table indicies blocks writes in MySQL 5.5
From the answer above:
“If your using a version greater than 5.1 indices are created while the database is online. So not to worry you won’t interrupt production system use.”
This is ****FALSE**** (at least for MyISAM / InnoDB tables, which is what 99.999% of people out there use. Clustered Edition is different.)
Doing UPDATE operations on a table will BLOCK while the index is being created. MySQL is really, really stupid about this (and a few other things).
Test Script:
(
for n in {1..50}; do
#(time mysql -uroot -e 'select * from website_development.users where id = 41225\G'>/dev/null) 2>&1 | grep real;
(time mysql -uroot -e 'update website_development.users set bio="" where id = 41225\G'>/dev/null) 2>&1 | grep real;
done
) | cat -n &
PID=$!
sleep 0.05
echo "Index Update - START"
mysql -uroot website_development -e 'alter table users add index ddopsonfu (last_name, email, first_name, confirmation_token, current_sign_in_ip);'
echo "Index Update - FINISH"
sleep 0.05
kill $PID
time mysql -uroot website_development -e 'drop index ddopsonfu on users;'
My Server (InnoDB):
Server version: 5.5.25a Source distribution
Output (notice how the 6th operation blocks for the ~400ms it takes to finish the index update):
1 real 0m0.009s
2 real 0m0.009s
3 real 0m0.009s
4 real 0m0.012s
5 real 0m0.009s
Index Update - START
Index Update - FINISH
6 real 0m0.388s
7 real 0m0.009s
8 real 0m0.009s
9 real 0m0.009s
10 real 0m0.009s
11 real 0m0.009s
Vs read operations which don’t block (swap the line comment in the script):
1 real 0m0.010s
2 real 0m0.009s
3 real 0m0.009s
4 real 0m0.010s
5 real 0m0.009s
Index Update - START
6 real 0m0.010s
7 real 0m0.010s
8 real 0m0.011s
9 real 0m0.010s
...
41 real 0m0.009s
42 real 0m0.010s
43 real 0m0.009s
Index Update - FINISH
44 real 0m0.012s
45 real 0m0.009s
46 real 0m0.009s
47 real 0m0.010s
48 real 0m0.009s
Updating MySQL’s Schema without downtime
Thusfar, there’s only one method I know of to update a MySql schema and not suffer an availability outage. Circular masters:
- Master A has your MySQL database running on it
- Bring Master B into service and have it replicate writes from Master A ( B is a slave of A)
- Perform the schema update on Master B. It will fall behind during the upgrade
- Let Master B catch up. Invariant: Your schema change MUST be capable of processing commands replicated from a downversion schema. Indexing changes qualify. Simple column additions usually qualify. Removing a column? probably not.
- ATOMICALLY swap all clients from Master A to Master B. If you want to be safe (trust me, you do), you should ensure that the last write to A is replicated to B BEFORE B takes its first write. If you allow concurrent writes to 2+ masters, … you better understand MySQL replication at a DEEP level or you are headed for a world of pain. Extreme pain. Like, do you have a column that is AUTOINCREMENT??? you are screwed (unless you use even numbers on one master and odds on the other). Do NOT trust MySQL replication to “do the right thing”. It is NOT smart and will not save you. It’s just slightly less safe than copying binary transaction logs from the command-line and replaying them by hand. Still, disconnecting all clients from the old master and flipping them to the new master can be done in a matter of seconds, vastly faster than waiting for a multi-hour schema upgrade.
- Now Master B is your new master. You have the new schema. Life is good. Have a beer; the worst is over.
- Repeat the process with Master A, upgrading his schema so that he becomes your new secondary master, ready to take over in the event that your primary master (master B now) loses power or just up and dies on you.
An easy way to update schema this isn’t. Workable in a serious production environment; yes, it is. Please, please, please, if there is an easier way to add an index to a MySQL table without blocking writes, let me know.
Googling lead me to this article which describes a similar technique. Even better, they advise drinking at the same point in the proceedure (Note that I wrote my answer before reading the article)!
Percona’s pt-online-schema-change
The article I linked above talks about a tool, pt-online-schema-change, that works as follows:
- Create new table with same structure as original.
- Update schema on new table.
- Add a trigger on the original table so that changes are kept in-sync with the copy
- Copy rows in batches from original table.
- Move original table out of the way and replace with new table.
- Drop old table.
I’ve never tried the tool myself. YMMV
RDS
I’m currently using MySQL through Amazon’s RDS. It’s a really nifty service that wraps up and manages MySQL, letting you add new read replicas with a single button and transparently upgrade the database across hardware SKU’s. It’s really convenient. You don’t get SUPER access to the database, so you can’t screw with replication directly (is this a blessing or curse?). However, you can use Read Replica Promotion to make your schema changes on a read-only slave, then promote that slave to become your new master. Exactly the same trick as I described above, just vastly easier to execute. They still don’t do much to help you with the cut-over. You have to reconfigure and restart your app.