Description:
Some character sets are designated as MY_CS_STRNXFRM, meaning that sorting needs to go through my_strnxfrm() (implemented by the charset), and some are not, meaning that a client can do the strnxfrm itself based on cs->sort_order. However, most of the logic related to the latter has been removed already (e.g. filesort always uses my_strnxfrm() since 2003), and now it's mostly in the way. The three main uses left are:
1. A microoptimization for constructing sort keys in filesort.
2. A home-grown implementation of Boyer-Moore for accelerating certain LIKE patterns that should probably be handled through FTS.
3. Some optimizations to MyISAM prefix keys.
Given that our default collation (utf8mb4_0900_ai_ci) now is a strnxfrm-based collation, the benefits of keeping these around for a narrow range of single-byte locales (like latin1_swedish_ci, cp850 and a bunch of more obscure locales) seems dubious. We seemingly can't remove the flag entirely due to #3 seemingly affecting the on-disk MyISAM structure, but we can remove the code for #1 and #2.
How to repeat:
N/A
Suggested fix:
N/A
