标签

PostgreSQL , conversion , pg_conversion , 拼音 , 编码转换 , convert , convert_to , convert_from

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背景

国内的应用，在文本排序上基本都是按照拼音来进行排序的。

在不同的字符集中，汉字的编码可能不一样，比如UTF8和GBK，其中GBK是按拼音的顺序进行编码的，而UTF8则不是。

所以如果你的数据库使用了UTF8编码，对中文字段进行排序时，可能得到的并不是按拼音排序的结果。

在PostgreSQL中，中文按拼音排序的编码包括GB18030, EUC_CN, GBK, BIG5, EUC_TW 等。

为了得到拼音排序，可以使用编码转换后的值来排序，索引也可以使用编码转换的表达式索引。

编码

PostgreSQL支持的编码如下

https://www.postgresql.org/docs/9.6/static/multibyte.html

PostgreSQL Character Sets

Name	Description	Language	Server?	Bytes/Char	Aliases
BIG5	Big Five	Traditional Chinese	No	1-2	WIN950, Windows950
EUC_CN	Extended UNIX Code-CN	Simplified Chinese	Yes	1-3	-
EUC_JP	Extended UNIX Code-JP	Japanese	Yes	1-3	-
EUC_JIS_2004	Extended UNIX Code-JP, JIS X 0213	Japanese	Yes	1-3	-
EUC_KR	Extended UNIX Code-KR	Korean	Yes	1-3	-
EUC_TW	Extended UNIX Code-TW	Traditional Chinese, Taiwanese	Yes	1-3	-
GB18030	National Standard	Chinese	No	1-4	-
GBK	Extended National Standard	Simplified Chinese	No	1-2	WIN936, Windows936
ISO_8859_5	ISO 8859-5, ECMA 113	Latin/Cyrillic	Yes	1	-
ISO_8859_6	ISO 8859-6, ECMA 114	Latin/Arabic	Yes	1	-
ISO_8859_7	ISO 8859-7, ECMA 118	Latin/Greek	Yes	1	-
ISO_8859_8	ISO 8859-8, ECMA 121	Latin/Hebrew	Yes	1	-
JOHAB	JOHAB	Korean (Hangul)	No	1-3	-
KOI8R	KOI8-R	Cyrillic (Russian)	Yes	1	KOI8
KOI8U	KOI8-U	Cyrillic (Ukrainian)	Yes	1	-
LATIN1	ISO 8859-1, ECMA 94	Western European	Yes	1	ISO88591
LATIN2	ISO 8859-2, ECMA 94	Central European	Yes	1	ISO88592
LATIN3	ISO 8859-3, ECMA 94	South European	Yes	1	ISO88593
LATIN4	ISO 8859-4, ECMA 94	North European	Yes	1	ISO88594
LATIN5	ISO 8859-9, ECMA 128	Turkish	Yes	1	ISO88599
LATIN6	ISO 8859-10, ECMA 144	Nordic	Yes	1	ISO885910
LATIN7	ISO 8859-13	Baltic	Yes	1	ISO885913
LATIN8	ISO 8859-14	Celtic	Yes	1	ISO885914
LATIN9	ISO 8859-15	LATIN1 with Euro and accents	Yes	1	ISO885915
LATIN10	ISO 8859-16, ASRO SR 14111	Romanian	Yes	1	ISO885916
MULE_INTERNAL	Mule internal code	Multilingual Emacs	Yes	1-4
SJIS	Shift JIS	Japanese	No	1-2	Mskanji, ShiftJIS, WIN932, Windows932
SHIFT_JIS_2004	Shift JIS, JIS X 0213	Japanese	No	1-2	-
SQL_ASCII	unspecified (see text)	any	Yes	1	-
UHC	Unified Hangul Code	Korean	No	1-2	WIN949, Windows949
UTF8	Unicode, 8-bit	all	Yes	1-4	Unicode
WIN866	Windows CP866	Cyrillic	Yes	1	ALT
WIN874	Windows CP874	Thai	Yes	1	-
WIN1250	Windows CP1250	Central European	Yes	1	-
WIN1251	Windows CP1251	Cyrillic	Yes	1	WIN
WIN1252	Windows CP1252	Western European	Yes	1	-
WIN1253	Windows CP1253	Greek	Yes	1	-
WIN1254	Windows CP1254	Turkish	Yes	1	-
WIN1255	Windows CP1255	Hebrew	Yes	1	-
WIN1256	Windows CP1256	Arabic	Yes	1	-
WIN1257	Windows CP1257	Baltic	Yes	1	-
WIN1258	Windows CP1258	Vietnamese	Yes	1	ABC, TCVN, TCVN5712, VSCII

与中文编码排序相关的包括 GB18030, EUC_CN, GBK, BIG5, EUC_TW

简体常用的包括GBK, EUC_CN。

编码转换

在PostgreSQL中，如果要将字符从一个编码转换为另一个编码，需要告诉数据库（create conversion）怎么转换（使用什么C函数），PG内置了一些转换的C函数和转换方法。

https://www.postgresql.org/docs/9.6/static/catalog-pg-conversion.html

pg_conversion

Name	Type	References	Description
oid	oid	-	Row identifier (hidden attribute; must be explicitly selected)
conname	name	-	Conversion name (unique within a namespace)
connamespace	oid	pg_namespace.oid	The OID of the namespace that contains this conversion
conowner	oid	pg_authid.oid	Owner of the conversion
conforencoding	int4	-	Source encoding ID
contoencoding	int4	-	Destination encoding ID
conproc	regproc	pg_proc.oid	Conversion procedure
condefault	bool	-	True if this is the default conversion

查看内置的转换方法

可以看到utf8转换为中文编码的都支持了

postgres=> select * from pg_conversion where conname ~* 'gbk|gb18|euc_cn|euc_tw|big5' order by 1;
     conname     | connamespace | conowner | conforencoding | contoencoding |     conproc     | condefault 
-----------------+--------------+----------+----------------+---------------+-----------------+------------
 big5_to_euc_tw  |           11 |       10 |             36 |             4 | big5_to_euc_tw  | t
 big5_to_mic     |           11 |       10 |             36 |             7 | big5_to_mic     | t
 big5_to_utf8    |           11 |       10 |             36 |             6 | big5_to_utf8    | t
 euc_cn_to_mic   |           11 |       10 |              2 |             7 | euc_cn_to_mic   | t
 euc_cn_to_utf8  |           11 |       10 |              2 |             6 | euc_cn_to_utf8  | t
 euc_tw_to_big5  |           11 |       10 |              4 |            36 | euc_tw_to_big5  | t
 euc_tw_to_mic   |           11 |       10 |              4 |             7 | euc_tw_to_mic   | t
 euc_tw_to_utf8  |           11 |       10 |              4 |             6 | euc_tw_to_utf8  | t
 gb18030_to_utf8 |           11 |       10 |             39 |             6 | gb18030_to_utf8 | t
 gbk_to_utf8     |           11 |       10 |             37 |             6 | gbk_to_utf8     | t
 mic_to_big5     |           11 |       10 |              7 |            36 | mic_to_big5     | t
 mic_to_euc_cn   |           11 |       10 |              7 |             2 | mic_to_euc_cn   | t
 mic_to_euc_tw   |           11 |       10 |              7 |             4 | mic_to_euc_tw   | t
 utf8_to_big5    |           11 |       10 |              6 |            36 | utf8_to_big5    | t
 utf8_to_euc_cn  |           11 |       10 |              6 |             2 | utf8_to_euc_cn  | t
 utf8_to_euc_tw  |           11 |       10 |              6 |             4 | utf8_to_euc_tw  | t
 utf8_to_gb18030 |           11 |       10 |              6 |            39 | utf8_to_gb18030 | t
 utf8_to_gbk     |           11 |       10 |              6 |            37 | utf8_to_gbk     | t
(18 rows)

编码转换函数

注意数据库版本

PostgreSQL 8.x(如Greenplum), 将字符串从原编码转换为指定编码的字符串返回。

可能存在显示的问题。

                              List of functions
   Schema   |     Name      | Result data type | Argument data types |  Type  
------------+---------------+------------------+---------------------+--------
 pg_catalog | convert       | text             | text, name          | normal
 pg_catalog | convert       | text             | text, name, name    | normal

PostgreSQL 9.x, 将源编码字符串的字节流转换为指定编码的字符串的字节流返回。

避免了显示的问题。

                              List of functions
   Schema   |     Name     | Result data type | Argument data types |  Type  
------------+--------------+------------------+---------------------+--------
 pg_catalog | convert      | bytea            | bytea, name, name   | normal
 pg_catalog | convert_from | text             | bytea, name         | normal
 pg_catalog | convert_to   | bytea            | text, name          | normal

如果8.x需要避免显示问题，返回字节流，可以这样使用，推荐使用。

byteain(textout(convert(字符,'源编码','目标编码')))  

例子

当前数据库编码为UTF-8，中文排序未按拼音排序。

postgres=# \l
                             List of databases
   Name    |  Owner   | Encoding | Collate | Ctype |   Access privileges   
-----------+----------+----------+---------+-------+-----------------------
 db0       | postgres | UTF8     | C       | C     | 
 postgres  | postgres | UTF8     | C       | C     | 
 template0 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
 template1 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
(4 rows)

postgres=# select * from (values ('刘德华'), ('刘少奇')) t(id) order by id;
   id   
--------
 刘少奇
 刘德华
(2 rows)

按拼音排序方法(目标可以改成EUC_CN)

8.x

postgres=> select * from (values ('刘德华'), ('刘少奇')) t(id) order by byteain(textout(convert(id,'UTF-8','GBK')));
   id   
--------
 刘德华
 刘少奇
(2 rows)


9.x
postgres=# select * from (values ('刘德华'), ('刘少奇')) t(id) order by convert(id::bytea,'UTF-8','GBK');
   id   
--------
 刘德华
 刘少奇
(2 rows)

注意多音字

中文有一些多音字，比如重庆(chongqing), 但是编码时它可能是按zhong编码的，所以看这个例子。

postgres=> select * from (values ('中山'), ('重庆')) t(id) order by byteain(textout(convert(id,'UTF-8','GBK')));
  id  
------
 中山
 重庆
(2 rows)

索引

表达式索引即可，使用immutable function.

代码

8.x

postgres=> \df+ convert
                                                                                        List of functions
   Schema   |  Name   | Result data type | Argument data types |  Type  | Data access | Volatility |   Owner   | Language | Source code |                       Description                       
------------+---------+------------------+---------------------+--------+-------------+------------+-----------+----------+-------------+---------------------------------------------------------
 pg_catalog | convert | text             | text, name          | normal | no sql      | stable     | xxx | internal | pg_convert  | convert string with specified destination encoding name
 pg_catalog | convert | text             | text, name, name    | normal | no sql      | stable     | xxx | internal | pg_convert2 | convert string with specified encoding names
(2 rows)


9.x

postgres=# \df+ convert
                                                                                                List of functions
   Schema   |  Name   | Result data type | Argument data types |  Type  | Volatility | Parallel |  Owner   | Security | Access privileges | Language | Source code |                 Description                  
------------+---------+------------------+---------------------+--------+------------+----------+----------+----------+-------------------+----------+-------------+----------------------------------------------
 pg_catalog | convert | bytea            | bytea, name, name   | normal | stable     | safe     | postgres | invoker  |                   | internal | pg_convert  | convert string with specified encoding names
(1 row)

src/backend/utils/mb/mbutils.c

/*
 * Convert string between two arbitrary encodings.
 *
 * BYTEA convert(BYTEA string, NAME src_encoding_name, NAME dest_encoding_name)
 */
Datum
pg_convert(PG_FUNCTION_ARGS)
{
        bytea      *string = PG_GETARG_BYTEA_PP(0);
        char       *src_encoding_name = NameStr(*PG_GETARG_NAME(1));
        int                     src_encoding = pg_char_to_encoding(src_encoding_name);
        char       *dest_encoding_name = NameStr(*PG_GETARG_NAME(2));
        int                     dest_encoding = pg_char_to_encoding(dest_encoding_name);
        const char *src_str;
        char       *dest_str;
        bytea      *retval;
        int                     len;

        if (src_encoding < 0)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("invalid source encoding name \"%s\"",
                                                src_encoding_name)));
        if (dest_encoding < 0)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("invalid destination encoding name \"%s\"",
                                                dest_encoding_name)));

        /* make sure that source string is valid */
        len = VARSIZE_ANY_EXHDR(string);
        src_str = VARDATA_ANY(string);
        pg_verify_mbstr_len(src_encoding, src_str, len, false);

        /* perform conversion */
        dest_str = (char *) pg_do_encoding_conversion((unsigned char *) src_str,
                                                                                                  len,
                                                                                                  src_encoding,
                                                                                                  dest_encoding);

        /* update len if conversion actually happened */
        if (dest_str != src_str)
                len = strlen(dest_str);

        /*
         * build bytea data type structure.
         */
        retval = (bytea *) palloc(len + VARHDRSZ);
        SET_VARSIZE(retval, len + VARHDRSZ);
        memcpy(VARDATA(retval), dest_str, len);

        if (dest_str != src_str)
                pfree(dest_str);

        /* free memory if allocated by the toaster */
        PG_FREE_IF_COPY(string, 0);

        PG_RETURN_BYTEA_P(retval);
}

参考

1. https://www.postgresql.org/docs/9.6/static/multibyte.html#AEN39011

2. https://www.postgresql.org/docs/9.6/static/sql-createconversion.html

3. https://www.postgresql.org/docs/9.6/static/catalog-pg-conversion.html