What's the difference between utf8_general_ci and utf8_unicode_ci

问题

Between utf8_general_ci and utf8_unicode_ci, are there any differences in terms of performance?

回答1:

These two collations are both for the UTF-8 character encoding. The differences are in how text is sorted and compared.

Note: You should use utf8mb4 rather than utf8. They both refer to the UTF-8 encoding, but the older utf8 had a MySQL-specific limitation preventing use of characters numbered above 0xFFFD.

Note: Newer versions of MySQL have updated Unicode sorting rules, available under names such as utf8mb4_0900_ci for rules based on Unicode 9.0 - and with no equivalent general variant.

Key differences

utf8mb4_unicode_ci is based on the official Unicode rules for universal sorting and comparison, which sorts accurately in a wide range of languages.
utf8mb4_general_ci is a simplified set of sorting rules which aims to do as well as it can while taking many short-cuts designed to improve speed. It does not follow the Unicode rules and will result in undesirable sorting or comparison in some situations, such as when using particular languages or characters.

On modern servers, this performance boost will be all but negligible. It was devised in a time when servers had a tiny fraction of the CPU performance of today's computers.

Note: there exists now an updated version of utf8mb4_unicode_ci called utf8mb4_0900_ai_ci - this is based on changes in Unicode version 9.0, and is also apparently faster. It adopts a new naming scheme whereby 0900 is the Unicode version and ai means accent-insensitive - like the previous utf8mb4_unicode_ci, accents in letters are not considered significant.

Benefits of utf8mb4_unicode_ci over utf8mb4_general_ci

utf8mb4_unicode_ci, which uses the Unicode rules for sorting and comparison, employs a fairly complex algorithm for correct sorting in a wide range of languages and when using a wide range of special characters. These rules need to take into account language-specific conventions; not everybody sorts their characters in what we would call 'alphabetical order'.

As far as Latin (ie "European") languages go, there is not much difference between the Unicode sorting and the simplified utf8mb4_general_ci sorting in MySQL, but there are still a few differences:

For examples, the Unicode collation sorts "ß" like "ss", and "Œ" like "OE" as people using those characters would normally want, whereas utf8mb4_general_ci sorts them as single characters (presumably like "s" and "e" respectively).
Some Unicode characters are defined as ignorable, which means they shouldn't count toward the sort order and the comparison should move on to the next character instead. utf8mb4_unicode_ci handles these properly.

In non-latin languages, such as Asian languages or languages with different alphabets, there may be a lot more differences between Unicode sorting and the simplified utf8mb4_general_ci sorting. The suitability of utf8mb4_general_ci will depend heavily on the language used. For some languages, it'll be quite inadequate.

What should you use?

There is almost certainly no reason to use utf8mb4_general_ci anymore, as we have left behind the point where CPU speed is low enough that the performance difference would be important. Your database will almost certainly be limited by other bottlenecks than this.

In the past, some people recommended to use utf8mb4_general_ci except when accurate sorting was going to be important enough to justify the performance cost. Today, that performance cost has all but disappeared, and developers are treating internationalization more seriously.

There's an argument to be made that if speed is more important to you than accuracy, you may as well not do any sorting at all. It's trivial to make an algorithm faster if you do not need it to be accurate. So, utf8mb4_general_ci is a compromise that's probably not needed for speed reasons and probably also not suitable for accuracy reasons.

One other thing I'll add is that even if you know your application only supports the English language, it may still need to deal with people's names, which can often contain characters used in other languages in which it is just as important to sort correctly. Using the Unicode rules for everything helps add peace of mind that the very smart Unicode people have worked very hard to make sorting work properly.

What the parts mean

Firstly, ci is for case-insensitive sorting and comparison. This means it's suitable for textual data, and case is not important. The other types of collation are cs (case-sensitive) for textual data where case is important, and bin, for where the encoding needs to match, bit for bit, which is suitable for fields which are really encoded binary data (including, for example, Base64). Case-sensitive sorting leads to some weird results and case-sensitive comparison can result in duplicate values differing only in letter case, so case-sensitive collations are falling out of favor for textual data - if case is significant to you, then otherwise ignorable punctuation and so on is probably also significant, and a binary collation might be more appropriate.

Next, unicode or general refers to the specific sorting and comparison rules - in particular, the way text is normalized or compared. There are many different sets of rules for the utf8mb4 character encoding, with unicode and general being two that attempt to work well in all possible languages rather than one specific one. The differences between these two sets of rules are the subject of this answer. Note that unicode uses rules from Unicode 4.0. Recent versions of MySQL add the rulesets unicode_520 using rules from Unicode 5.2, and 0900 (dropping the "unicode_" part) using rules from Unicode 9.0.

And lastly, utf8mb4 is of course the character encoding used internally. In this answer I'm talking only about Unicode based encodings.

回答2:

I wanted to know what is the performance difference between using utf8_general_ci and utf8_unicode_ci, but I did not find any benchmarks listed on the internet, so I decided to create benchmarks myself.

I created a very simple table with 500,000 rows:

CREATE TABLE test(
  ID INT(11) DEFAULT NULL,
  Description VARCHAR(20) DEFAULT NULL
)
ENGINE = INNODB
CHARACTER SET utf8
COLLATE utf8_general_ci;

Then I filled it with random data by running this stored procedure:

CREATE PROCEDURE randomizer()
BEGIN
  DECLARE i INT DEFAULT 0;
  DECLARE random CHAR(20) ;
  theloop: loop
    SET random = CONV(FLOOR(RAND() * 99999999999999), 20, 36);
    INSERT INTO test VALUES (i+1, random);
    SET i=i+1;
    IF i = 500000 THEN
      LEAVE theloop;
    END IF;
  END LOOP theloop;
END

Then I created the following stored procedures to benchmark simple SELECT, SELECT with LIKE, and sorting (SELECT with ORDER BY):

CREATE PROCEDURE benchmark_simple_select()
BEGIN
  DECLARE i INT DEFAULT 0;
  theloop: loop
    SELECT *
    FROM test
    WHERE Description = 'test' COLLATE utf8_general_ci;
    SET i = i + 1;
    IF i = 30 THEN
      LEAVE theloop;
    END IF;
  END LOOP theloop;
END;

CREATE PROCEDURE benchmark_select_like()
BEGIN
  DECLARE i INT DEFAULT 0;
  theloop: loop
    SELECT *
    FROM test
    WHERE Description LIKE '%test' COLLATE utf8_general_ci;
    SET i = i + 1;
    IF i = 30 THEN
      LEAVE theloop;
    END IF;
  END LOOP theloop;
END;

CREATE PROCEDURE benchmark_order_by()
BEGIN
  DECLARE i INT DEFAULT 0;
  theloop: loop
    SELECT *
    FROM test
    WHERE ID > FLOOR(1 + RAND() * (400000 - 1))
    ORDER BY Description COLLATE utf8_general_ci LIMIT 1000;
    SET i = i + 1;
    IF i = 10 THEN
      LEAVE theloop;
    END IF;
  END LOOP theloop;
END;

In the stored procedures above utf8_general_ci collation is used, but of course during the tests I used both utf8_general_ci and utf8_unicode_ci.

I called each stored procedure 5 times for each collation (5 times for utf8_general_ci and 5 times for utf8_unicode_ci) and then calculated the average values.

My results are:

benchmark_simple_select()

with utf8_general_ci: 9,957 ms
with utf8_unicode_ci: 10,271 ms

In this benchmark using utf8_unicode_ci is slower than utf8_general_ci by 3.2%.

benchmark_select_like()

with utf8_general_ci: 11,441 ms
with utf8_unicode_ci: 12,811 ms

In this benchmark using utf8_unicode_ci is slower than utf8_general_ci by 12%.

benchmark_order_by()

with utf8_general_ci: 11,944 ms
with utf8_unicode_ci: 12,887 ms

In this benchmark using utf8_unicode_ci is slower than utf8_general_ci by 7.9%.

回答3:

This post describes it very nicely.

In short: utf8_unicode_ci uses the Unicode Collation Algorithm as defined in the Unicode standards, whereas utf8_general_ci is a more simple sort order which results in "less accurate" sorting results.

回答4:

See the mysql manual, Unicode Character Sets section:

For any Unicode character set, operations performed using the _general_ci collation are faster than those for the _unicode_ci collation. For example, comparisons for the utf8_general_ci collation are faster, but slightly less correct, than comparisons for utf8_unicode_ci. The reason for this is that utf8_unicode_ci supports mappings such as expansions; that is, when one character compares as equal to combinations of other characters. For example, in German and some other languages “ß” is equal to “ss”. utf8_unicode_ci also supports contractions and ignorable characters. utf8_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.

So to summarize, utf_general_ci uses a smaller and less correct (according to the standard) set of comparisons than utf_unicode_ci which should implement the entire standard. The general_ci set will be faster because there is less computation to do.

回答5:

In brief words:

If you need better sorting order - use utf8_unicode_ci (this is the preferred method),

but if you utterly interested in performance - use utf8_general_ci, but know that it is a little outdated.

The differences in terms of performance are very slight.

回答6:

Some details (PL)

As we can read here (Peter Gulutzan) there is difference on sorting/comparing polish letter "Ł" (L with stroke - html esc: Ł) (lower case: "ł" - html esc: ł) - we have following assumption:

utf8_polish_ci      Ł greater than L and less than M
utf8_unicode_ci     Ł greater than L and less than M
utf8_unicode_520_ci Ł equal to L
utf8_general_ci     Ł greater than Z

In polish language letter Ł is after letter L and before M. No one of this coding is better or worse - it depends of your needs.

回答7:

According to this post, there is a considerably large performance benefit on MySQL 5.7 when using utf8mb4_general_ci in stead of utf8mb4_unicode_ci: https://www.percona.com/blog/2019/02/27/charset-and-collation-settings-impact-on-mysql-performance/

来源：https://stackoverflow.com/questions/766809/whats-the-difference-between-utf8-general-ci-and-utf8-unicode-ci

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