[antoncohen]

MySQL has some robust tooling in this space. Some of the tools use triggers to copy to a new table. GitHub's gh-ost[1] is probably the state of the art, and uses the binary log stream to replicate the data.

[1] https://github.com/github/gh-ost

[taspeotis]

Postgres has some less robust tooling, at least both of these carry the "experimental" tag:

https://news.ycombinator.com/item?id=29825520

https://github.com/shayonj/pg-osc

But, still, they do it for you.