On the one hand, MySQL has offered this for two decades, by virtue of pluggable storage engines being core to its design. Some storage engines use clustered indexes and some do not. The user can decide which one matches their use-case; very large companies can design their own custom special-purpose storage engines; etc.

On the other hand, mixing storage engines in a single db instance has operational downsides (especially re: crash-safe replication). And InnoDB is by far the dominant storage engine, and is probably unlikely to offer nonclustered indexing, so from that perspective I agree with your point.

I still remember the "good" ol days when the default MySQL engine was MyISAM. Even after InnoDB became the default, a lot of people were still configuring it for MyISAM for their 30 user webapp because they'd heard it was faster, and besides, you can ensure data integrity in code, right?

I made a bit of money freelancing on "my database for my LAMP stack app is corrupt!" issues by a) demonstrating that InnoDB wouldn't slow down their webapp in any measurable form and then b) trying to save and normalise as much data as possible.

It'll be interesting to see how things shake out when some of the other implementations using postgres's pluggable storage API start maturing. I wonder if it'll have some of the same operational downsides that mixing storage in MySQL has.

Good question. I assume it depends on how Postgres handles multi-engine transactions, and how it stores replication state metadata. A good discussion of the issue in MySQL/MariaDB is here: https://kristiannielsen.livejournal.com/19223.html

Apparently MariaDB 10.3+ has this solution implemented, which is cool, never knew that before. I don't think there's anything equivalent in MySQL.