Exactly. I think the prior HN discussions failed to call out how painful it is to do any sort of schema migration against a big InnoDB table [1][2].
Well known MySQL uses such as Facebook TAO and this Uber Schemaless are typically abstractions built on top of MySQL, which means the schemas are pretty much static, and they don't feel the schema migration pain.
For a typical RoR startup that relies on a RDBMS, please, stay away from MySQL.
[1] Yes, I know about the INSTANT ADD COLUMN patch from Tencent Games that landed in MySQL 8.0, and which has had major bug fixes in at least 8.0.14 and 8.0.20.
[2] A side effect is that MySQL now has a thriving ecosystem of schema migration tools (pt-osc, lhm, gh-ost), while Postgres has none, and there are situations where there is indeed no choice but to rewrite the table, e.g. changing a column type from int to bigint.
Well known MySQL uses such as Facebook TAO and this Uber Schemaless are typically abstractions built on top of MySQL, which means the schemas are pretty much static, and they don't feel the schema migration pain.
For a typical RoR startup that relies on a RDBMS, please, stay away from MySQL.
[1] Yes, I know about the INSTANT ADD COLUMN patch from Tencent Games that landed in MySQL 8.0, and which has had major bug fixes in at least 8.0.14 and 8.0.20.
[2] A side effect is that MySQL now has a thriving ecosystem of schema migration tools (pt-osc, lhm, gh-ost), while Postgres has none, and there are situations where there is indeed no choice but to rewrite the table, e.g. changing a column type from int to bigint.