It seems that MariaDB 10.3+ added a new optimization (and "select_type" in terms of EXPLAIN), LATERAL DERIVED. See MDEV-17381 for one of examples of when it is used. It also seems that this optimization is controlled by the split_materialized optimizer switch.

All these, though, are NOT properly documented in our KB. Check these pages:

https://mariadb.com/kb/en/optimizer-switch/ - nothing documented for split_materialized, besides a link to MDEV-13369

https://mariadb.com/kb/en/explain/ - "LATERAL DERIVED" is not even listed among "select_type" values and not at all documented

https://mariadb.com/kb/en/optimizations-for-derived-tables/ - does not mention this optimization at all.

So, at least all these pages must be changed to have this optimization listed and properly documented, and probably a separate page for it 9and related bugs) should be added. It seems to cause query performance regressions while upgrading to 10.3 from older versions, so having zero information about it is a problem for community users and customers. Having is enabled by default is also a problem, I;'d say.