[alexsolo]

Or just because MySQL is really to get started with when using Rails and other frameworks, and that when you're starting a business, you think "if I actually ever run into MySQL scaling issues, that's a good problem to have".

[blueberry]

Name one database that won't have scaling issues for sure. Scaling DB-wise is not only a good problem, it is inevitable if you constantly grow.

[mmt]

Name one database that won't have scaling issues for sure

You are, in effect, asking someone to prove a negative.

I don't think there's any contention that scaling will always be an issue, but, rather, where along the scale curve debilitating issues occur and how painful the resolutions to those issues are.

For example, with MySQL, without sacrificing ACID properties[1], one cannot even separate a table's indexes onto a separate spindle from its data. For rotating storage, this make creating a new index astonishingly painful. Even with SSDs, more sequential/bulk I/O combined with eliminating contention for the paths to the drive, there can be a performance improvement.

Then there's query optimization. Is there even anything remotely close in MySQL to the EXPLAIN ANALYZE functionality in PostgreSQL?

I posit that scaling issues with MySQL come early and, once they do, require sweeping and expensive changes to address. One can't merely exercise a little cleverness and cheap hardware to tide one over for another factor of 2-4 growth.

[1] i.e. InnoDB, not MyISAM