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Comment by jeltz | Hacker News Reader

PostgreSQL may be behind in some other areas but not here. PostgreSQL has some of the best support for online schema changes (transactional DDL for almost everything, lots of work put into reducing lock level of schema changes) and was the first to implement support for JSON.

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When doing the rewrite, I was extremely surprised by how good PostgreSQL's JSON support is; there are a large number of functions for querying nested structures, and the indexing is very powerful. I mapped my data from RethinkDB documents to mostly relational data, but kept a couple of columns as JSONB for now, which saved time and massively simplified the rewrite. I remember trying to fully model similar data back in 2007, and it got overly complicated for no real benefit...

(I'm the author of the blog post.)

Please try to alter a schema of a live multibillion rows table before pretending that it works. Unless they changed how is implemented recently then such an operation can take hours. I love postgres for many reasons so i would like to be proven wrong.

Depends on the schema change. If you have a multibillion row table there are plenty of other things you need to pay close attention to as well, as you're probably all-too aware.

Some schema changes can take prohibitively long on large tables, so you do have to be careful. But there are ways of achieving the same results that are safe for production database. Braintree did a decent writeup on it. https://www.braintreepayments.com/blog/safe-operations-for-h...

Hours is okay, if it works.

Now, if you could just tell this to the people who depend on the software and tell me that hours of down time is not a possibility.

Hours of work <> Hours of downtime

The bigger the database the more complex it will be to implement schema change while keeping track of transactions that happen while your are transitioning. However I see no reason why it shouldn't be feasible given proper amount of preparation. And if short downtime is a goal I don't see why a proper amount of time developing a solution wouldn't make it either.

And on top of that PostgreSQL community is implementing new way to solve problem with each release. I'm not in a huge live-data use case so I might be wrong, but upcoming logical replication look promising for such use cases https://www.depesz.com/2017/02/07/waiting-for-postgresql-10-....

I have never worked with a database of that size but I have done major refactoring of tables with about half a billion rows. And while it can take hours for the schema changes to finish it is hours without any downtime (or just a couple of seconds of downtime during when the exclusive locks need to be acquired). It is tedious to have to wait that long (especially if one has 10 times the rows than what we did), but PostgreSQL can do almost any change online just fine.

What kind of schema changes do you think of that would require that much downtime? Even in complex cases that can be avoided with triggers or even rules.

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