PostgreSQL 按拼音排序 - convert to GBK/EUC_CN coding

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

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 ('*德华'), ('***')) t(id) order by id;
   id
--------
 ***
 *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 ('*德华'), ('***')) t(id) order by byteain(textout(convert(id,'UTF-8','GBK')));
   id
--------
 *德华
 ***
(2 rows)

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

使用字段或排序collate语法纠正排序顺序

使用binary存储格式排序，只能通过编码来修正排序顺序。

除此之外，我们还可以在不改编码的情况下，使用字段或者order by的collate语法来修正排序顺序。

例子

select * from pg_collation ;

设置列级collate
create table a (c1 text collate "zh_CN.utf8");

修改列collate,会导致rewrite table
alter table a alter c1 type text COLLATE  "zh_CN.utf8";

设置排序级collate
test=# select * from a order by c1 collate "C";
   c1
--------
 ***
 *德华
(2 rows)
test=# select * from a order by c1 collate "zh_CN.utf8";
   c1
--------
 *德华
 ***
(2 rows)

设置operator collate
test=# select * from a where c1 > '***' collate "C";
   c1
--------
 *德华
(1 row)
test=# select * from a where c1 > '***' collate "zh_CN.utf8";
 c1
----
(0 rows)

设置库级collate
postgres=# create database test with template template0 encoding 'UTF8' lc_collate 'zh_CN.utf8';
postgres=# \c test
You are now connected to database "test" as user "postgres".
test=# create table a (c1 text);
CREATE TABLE
test=# insert into a values ('*德华'),('***');
INSERT 0 2
test=# select * from a order by c1;
   c1
--------
 *德华
 ***
(2 rows)

注意索引和创建索引时的collate必须一致，才能使用该索引
postgres=# create table a(c1 text);
CREATE TABLE
postgres=# create index idxa on a(c1 collate "zh_CN.utf8");
CREATE INDEX
postgres=# explain select * from a order by c1;
                           QUERY PLAN
----------------------------------------------------------------
 Sort  (cost=10000000094.38..10000000097.78 rows=1360 width=32)
   Sort Key: c1
   ->  Seq Scan on a  (cost=0.00..23.60 rows=1360 width=32)
(3 rows)

postgres=# explain select * from a order by c1 collate "zh_CN.utf8";
                               QUERY PLAN
------------------------------------------------------------------------
 Index Only Scan using idxa on a  (cost=0.15..31.55 rows=1360 width=64)
(1 row)