doc/design: add pinyin order collation for utf8mb4 charset

docs/design/2020-09-12-utf8mb4-pinyin-order.md

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+# Proposal: support `pinyin` order for `utf8mb4` charset
+
+- Author(s):     [xiongjiwei](https://github.com/xiongjiwei)
+- Last updated:  2020-11-06
+- Discussion at: https://github.com/pingcap/tidb/issues/19747
+
+## Abstract
+This proposal proposes a new feature that supports `pinyin` order for chinese character.
+
+## Background
+It's unable now to order by a column based on its pinyin order. For example:
+
+```sql
+create table t(
+	a varchar(100)
+)
+charset = 'utf8mb4' collate = 'utf8mb4_zh_0900_as_cs';
+
+# insert some data:
+insert into t values ("中文"), ("啊中文");
+
+# a query requires to order by column a in its pinyin order:
+select * from t order by a;
++-----------+
+| a         |
++-----------+
+| 啊中文    |
+| 中文      |
++-----------+
+2 rows in set (0.00 sec)
+```
+
+## Proposal
+
+`pinyin` order for Chinese character supported by this proposal will add a new collation named `utf8mb4_zh_pinyin_tidb_as_cs` which is support all Unicode and sort Chinese characters correctly according to the PINYIN collation in zh.xml file of [CLDR24](http://unicode.org/Public/cldr/24/core.zip), and only support those Chinese characters with `pinyin` in zh.xml currently, we support neither those CJK characters whose category defined in Unicode are Symbol with the same shape as Chinese characters nor the PINYIN characters. In `utf8mb4_zh_pinyin_tidb_as_cs`, `utf8mb4` means charset utf8mb4, `zh` means Chinese language, `pinyin` means it has pinyin order, `tidb` means a special(tidb) version, and `as_cs` means it is accent-sensitive and case-sensitive.
+
+### Advantages
+
+It's a lot of work if we implement `utf8mb4_zh_0900_as_cs`. The implementation of MySQL looks complicated with weight reorders, magic numbers, and some tricks. Implementing `utf8mb4_zh_pinyin_tidb_as_cs` is much easier. It supports all Chinese characters and sorts Chinese characters in pinyin order. It is good enough.
+
+### Disadvantages
+
+It is not compatible with MySQL. MySQL does not have a collation named `utf8mb4_zh_pinyin_tidb_as_cs`.
+
+## Rationale
+
+### How to implement
+
+#### Compare and Key
+
+- For any Chinese character, which has non-zero seq NO. defined in zh.xml according to its gb18030 code, the final weight shall be 0xFFA00000+(seq No.)
+- For any non-Chinese gb18030 character 2 bytes C, the final weight shall be C itself.
+- For any non-Chinese gb18030 character 4 bytes C, the final weight shall be 0xFF000000+diff(C)(we get diff by Algorithm).
+
+### Parser
+
+Choose collation ID `2048` for `utf8mb4_zh_pinyin_tidb_as_cs` and add it into parser.
+
+> MySQL supports two-byte collation IDs. The range of IDs from 1024 to 2047 is reserved for user-defined collations. [see also](https://dev.mysql.com/doc/refman/8.0/en/adding-collation-choosing-id.html)
+
+### Compatibility with current collations
+
+`utf8mb4_zh_pinyin_tidb_as_cs` has same priority with `utf8mb4_unicode_ci` and `utf8mb4_general_ci`, which means these three collations incompatible with each other.
+
+### Alternative
+MySQL has a lot of language specific collations, for `pinyin` order, MySQL uses collation `utf8mb4_zh_0900_as_cs`.
+
+## Compatibility and Migration Plan
+
+### Compatibility issues with MySQL
+
+There is no `utf8mb4_zh_pinyin_tidb_as_cs` collation in MySQL. We can comment `utf8mb4_zh_pinyin_tidb_as_cs` when users need to replicate their data from TiDB to MySQL.
+
+## Open issues (if applicable)
+
+https://github.com/pingcap/tidb/issues/19747
+
+https://github.com/pingcap/tidb/issues/10192