MySQL/PostgreSQL – How to Alter Table on Live Production Databases

alter-tableMySQLpostgresql

How do most "popular" (MySQL, Postgres…) database system handle altering tables on live production databases (like adding, deleting or changing the type of colums)?

I know the correct way is to backup everything schedule downtime and do then do the changes.

But… does any current database system support doing these things "on-line" without stopping anything? (maybe just delaying the queries that reference a column that is just being changed/deleted)

And what does it happen when I just do an ALTER TABLE... on a live running database?
Does everything stop when this happens?
Can data get corrupted?
etc.

Again, I'm mostly referring to Postgres or MySQL as these are what I encounter.

(And, yes, anytime I had to do this before I did it "the right way", backing things up, scheduling downtine etc. …but I just want to know if it's possible to do this sort and things "quick and dirty" or if there is any DB system that actually has support for "quick, live and dirty" schema changes)

Someone just suggested Online Schema Change for MySQL from Facebook script (with a tutorial here and source here)… seems like a nice way to automate a the set of "hacky" ways to do it… has anyone ever used it in something resemblig production?

Best Answer

When you issue an ALTER TABLE in PostgreSQL it will take an ACCESS EXCLUSIVE lock that blocks everything including SELECT. However, this lock can be quite brief if the table doesn't require re-writing, no new UNIQUE, CHECK or FOREIGN KEY constraints need expensive full-table scans to verify, etc.

If in doubt, you can generally just try it! All DDL in PostgreSQL is transactional, so it's quite fine to cancel an ALTER TABLE if it takes too long and starts holding up other queries. The lock levels required by various commands are documented in the locking page.

Some normally-slow operations can be sped up to be safe to perform without downtime. For example, if you have table t and you want to change column customercode integer NOT NULL to text because the customer has decided all customer codes must now begin with an X, you could write:

ALTER TABLE t ALTER COLUMN customercode TYPE text USING ( 'X'||customercode::text );

... but that would lock the whole table for the re-write. So does adding a column with a DEFAULT. It can be done in a couple of steps to avoid the long lock, but applications must be able to cope with the temporary duplication:

ALTER TABLE t ADD COLUMN customercode_new text;
BEGIN;
LOCK TABLE t IN EXCLUSIVE MODE;
UPDATE t SET customercode_new = 'X'||customercode::text;
ALTER TABLE t DROP COLUMN customercode;
ALTER TABLE t RENAME COLUMN customercode_new TO customercode;
COMMIT;

This will only prevent writes to t during the process; the lock name EXCLUSIVE is somewhat deceptive in that it excludes everything except SELECT; the ACCESS EXCLUSIVE mode is the only one that excludes absolutely everyting. See lock modes. There's a risk that this operation could deadlock-rollback due to the lock upgrade required by the ALTER TABLE, but at worst you'll just have to do it again.

You can even avoid that lock and do the whole thing live by creating a trigger function on t that whenever an INSERT or UPDATE comes in, automatically populates customercode_new from customercode.

There are also built-in tools like CREATE INDEX CONCURRENTLY and ALTER TABLE ... ADD table_constraint_using_index that're designed to allow DBAs to reduce exclusive locking durations by doing work more slowly in a concurrency-friendly way.

The pg_reorg tool or its successor pg_repack can be used for some table restructuring operations as well.

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