Month: April 2014
MySQL 5.7 supports the GB18030 Chinese Character Set
My former boss at MySQL sent out a notice that MySQL 5.7.4 now supports the GB18030 character set, thus responding to requests that have been appearing since 2005. This is a big deal because the Chinese government demands GB18030 support, and because the older simplified-Chinese character sets (gbk and gb2312) have a much smaller repertoire (that is, they have too few characters). And this is real GB18030 support — I can define columns and variables with CHARACTER SET GB18030. That’s rare — Oracle 12c and SQL Server 2012 and PostsgreSQL 9.3 can’t do it. (They allow input from GB18030 clients but they convert it immediately to Unicode.) Among big-time DBMSs, until now, only DB2 has treated GB18030 as a first-class character set.
We’re talking about the current version of the standard, GB18030-2005 “IT Chinese coded character set”, especially its description of 70,244 Chinese characters. I couldn’t puzzle out the Chinese wording in the official document, all I could do was use translate.google.com on some excerpts. But I’ve been told that the MySQL person who coded this feature is Chinese, so they’ll have had better luck. What I could understand was what are the difficult characters, what are the requirements for a claim of support, and what the encoding should look like. From the coder’s comments, it’s clear that part was understood. I did not check whether there was adherence for non-mandatory parts, such as Tibetan script.
The repertoire of GB18030 ought to be the same as the Unicode repertoire. So I took a list of every Unicode character, converted to GB18030, and converted back to Unicode. The result in every case was the same Unicode character that I’d started with. That’s called “perfect round tripping”. As I explained in an earlier blog post “The UTF-8 World Is Not Enough”, storing Chinese characters with a Chinese character set has certain advantages. Well, now the biggest disadvantage has disappeared.
Hold on — how is perfect round tripping possible, given that MySQL frequently refers to Unicode 4.0, and some of the characters in GB18030-2005 are only in Unicode 4.1? Certainly that ought to be a problem according to the Unicode FAQ and this extract from Ken Lunde’s book. But it turns out to be okay because MySQL doesn’t actually disallow those characters — it accepts encodings which are not assigned to characters. Of course I believe that MySQL should have upgraded the Unicode support first, and added GB18030 support later. But the best must not be an enemy of the good.
Also the conversions to and from gb2312 work fine, so I expect that eventually gb2312 will become obsolete. It’s time for mainland Chinese users to consider switching over to gb18030 once MySQL 5.7 is GA.
The new character set comes with three collations: one trivial, one tremendous, one tsk, tsk.
The trivial collation is gb18030_bin. As always the bin stands for binary. I expect that as always this will be the most performant collation, and the only one that guarantees that no two characters will ever have the same weight.
The tremendous collation is gb18030_unicode_520_ci. The “unicode_520” part of the name really does mean that the collation table comes from “Unicode 5.2” and this is the first time that MySQL has taken to heart the maxim: what applies to the superset can apply to the subset. In fact all MySQL character sets should have Unicode collations, because all their characters are in Unicode. So to test this, I went through all the Unicode characters and their GB18030 equivalents, and compared their weights with WEIGHT_STRING:
WEIGHT_STRING(utf32_char COLLATE utf32_unicode_520_ci) to
WEIGHT_STRING(gb18030_char COLLATE gb18030_unicode_520_ci).
Every utf32 weight was exactly the same as the gb18030 weight.
The tsk, tsk collation is gb18030_chinese_ci.
The first bad thing is the suffix chinese_ci, which will make some people think that this collation is like gb2312_chinese_ci. (Such confusion has happened before for the general_ci suffix.) In fact there are thousands of differences between gb2312_chinese_ci and gb18030_chinese_ci. Here’s an example.
mysql> CREATE TABLE t5 -> (gb2312 CHAR CHARACTER SET gb2312 COLLATE gb2312_chinese_ci, -> gb18030 CHAR CHARACTER SET gb18030 COLLATE gb18030_chinese_ci); Query OK, 0 rows affected (0.22 sec) mysql> INSERT INTO t5 VALUES ('[','['),(']',']'); Query OK, 2 rows affected (0.01 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> SELECT DISTINCT gb2312 from t5 ORDER BY gb2312; +--------+ | gb2312 | +--------+ | ] | | [ | +--------+ 2 rows in set (0.00 sec) mysql> SELECT DISTINCT gb18030 from t5 ORDER BY gb18030; +---------+ | gb18030 | +---------+ | [ | | ] | +---------+ 2 rows in set (0.00 sec)
See the difference? The gb18030 order is obviously better — ‘]’ should be greater than ‘[‘ — but when two collations are wildly different they shouldn’t both be called “chinese_ci”.
The second bad thing is the algorithm. The new chinese_ci collation is based on pinyin for Chinese characters, and binary comparisons of the UPPER() values for non-Chinese characters. This is pretty well useless for non-Chinese. I can bet that somebody will observe “well, duh, it’s a Chinese character set” — but I can’t see why one would use an algorithm for Latin/Greek/Cyrillic/etc. characters that’s so poor. There’s a Common Locale Data Repository for tailoring for Chinese, there are MySQL worklog tasks that explain the brave new world, there’s no need to invent an idiolect when there’s a received dialect.
The documentation isn’t up to date yet — there’s no attempt to explain what the new character set and its collations are about, and no mention at all in the FAQ.
But the worklog task WL#4024: gb18030 Chinese character set gives a rough idea of what the coder had in mind before starting. It looks as if WL#4024 was partly copied from http://icu-project.org/docs/papers/unicode-gb18030-faq.html so that’s also worth a look.
For developers who just need to know what’s going on now, just re-read this blog post. What I’ve described should be enough for people who care about Chinese.
I didn’t look for bugs with full-text or LIKE searches, and I didn’t look at speed. But I did look hard for problems with the essentials, and found none. Congratulations are due.
Update on July 17 2014: A source at MariaDB tells me that they’ve worked on GB18030 too. And the MySQL folks have closed a GB18030 bug that ‘Y’ = tilde” in the Chinese collation — not realizing that it’s a duplicate of a bug that I reported seven years ago.