_general_ci Versus _unicode_ci Collations

https://dev.mysql.com/doc/refman/8.0/en/charset-unicode-sets.html

For any Unicode character set, operations performed using the xxx_general_ci collation are faster than those for the xxx_unicode_ci collation. For example, comparisons for the utf8mb4_general_ci collation are faster, but slightly less correct, than comparisons for utf8mb4_unicode_ci. The reason is that utf8mb4_unicode_ci supports mappings such as expansions; that is, when one character compares as equal to combinations of other characters. For xample, 
ß is equal to ss in German and some other languages. utf8mb4_unicode_ci also supports contractions and ignorable characters. utf8mb4_general_ci is a legacy collation that does not support expansions, contractions, or ignorable characters. It can make only one-to-one comparisons between characters.

 

https://rastalion.me/mysql-8-0-1-버전부터-채택된-utf8mb4_0900_ai_ci의-한글-사용에-대한-문제점/

utf8mb4_0900_ai_ci에서 "가"와 "ㄱㅏ"를 같은 문자열로 판정한다는 글.