The realm of More Significant Changes is not where you often want to be. There’s a lot of scaffolding code you need to write. Basically, suppose you want to make a repair on the 5th story exterior of an 8-story building. You have a couple of options: the YOLO option is to kick everybody out of the building and have people rappel from the top of the building down to the 5th story to make their changes. They need all of the people out of the building because shut up it’s a strained analogy. This approach is kind of inconvenient: people have to stay out of your building until your people are done repairing the exterior. That’s blocking in the database world.
On the other side, you can build a bunch of scaffolding and attach it to the exterior of the building, perform your repairs, and tear down that scaffolding. While the scaffolding is up, people come and go like normal and don’t even think about it. As you tear the scaffolding down, you temporarily block the door for a moment as you’re finishing your work. This is much more convenient for end users and fits the “near-zero downtime” approach we’re taking.
Strained analogies aside, this is a long post on making a series of table-related changes without your end users noticing.