问题
What is the difference between utf8mb4_0900_ai_ci and utf8_unicode_ci database text coding in mysql (especially in terms of performance) ?
回答1:
- The encoding is the same. That is, the bytes look the same.
- The character set is different. utf8mb4 has more characters.
- The collation (how comparisions are done) is different.
- The perfomance is different, but it rarely matters.
utf8_unicode_ci implies the CHARACTER SET utf8, which includes only the 1-, 2-, and 3-byte UTF-8 characters. Hence it excludes most Emoji and some Chinese characters.
utf8mb4_unicode_ci implies the CHARACTER SET utf8mb4 is the corresponding COLLATION for the 4-byte CHARACTER SET utf8mb4.
The Unicode organization has been evolving the specification over the years. Here are the mappings from its "versions" to MySQL Collations:
4.0 _unicode_
5.20 _unicode_520_
9.0 _0900_
Most of the differences will be in areas that most people never encounter. One example: At some point, a change allowed Emoji to be distinguished and ordered in some manner.
The suffix (MySQL doc):
_bin -- just compare the bits; don't consider case folding, accents, etc
_ci -- explicitly case insensitive (A=a) and implicitly accent insensitive (a=á)
_ai_ci -- explicitly case insensitive and accent insensitive
_as (etc) -- accent-sensitive (etc)
Performance:
_bin -- simple, fast
_general_ci -- fails to compare multiple letters; eg ss=ß, so somewhat fast
... -- slower
_900_ -- (8.0) much faster because of a rewrite
However: The speed of collation is usually the least of the performance issues in queries. INDEXes, JOINs, subqueries, table scans, etc are much more critical to performance.
来源：https://stackoverflow.com/questions/54885178/whats-the-difference-between-utf8-unicode-ci-and-utf8mb4-0900-ai-ci