背景
当用户客户端字符集与服务端字符集不匹配时，写入的多字节字符（例如中文）可能出现乱码。

例子
数据库字符集为sql_ascii，允许存储任意编码字符。

digoal@pg11-320tb-zfs-> psql
psql (11beta4)
Type "help" for help.

postgres=# l+

                                                             List of databases  

Name	Owner	Encoding	Collate	Ctype	Access privileges	Size	Tablespace	Description
postgres	postgres	SQL_ASCII	C	en_US.utf8		140 TB	pg_default	default administrative connection database
template0	postgres	SQL_ASCII	C	en_US.utf8	=c/postgres +	15 MB	pg_default	unmodifiable empty database
				postgres=CTc/postgres
template1	postgres	SQL_ASCII	C	en_US.utf8	=c/postgres +	15 MB	pg_default	default template for new databases

(3 rows)
客户端为utf8编码

digoal@pg11-320tb-zfs-> echo $LANG
en_US.utf8
编辑一个文件，以UTF8编码

vi test.sql

insert into tbl values (1, '你好');
内容如下

digoal@pg11-320tb-zfs-> cat test.sql
insert into tbl values (1, '你好');
编码如下

digoal@pg11-320tb-zfs-> file test.sql
test.sql: UTF-8 Unicode text
转换为GBK，写入数据库

digoal@pg11-320tb-zfs-> iconv --help
Usage: iconv [OPTION...] [FILE...]
Convert encoding of given files from one encoding to another.

Input/Output format specification:
-f, --from-code=NAME encoding of original text
-t, --to-code=NAME encoding for output

Information:
-l, --list list all known coded character sets

Output control:
-c omit invalid characters from output
-o, --output=FILE output file
-s, --silent suppress warnings

  --verbose              print progress information  

-?, --help Give this help list

  --usage                Give a short usage message  

-V, --version Print program version

Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.

For bug reporting instructions, please see:
http://www.gnu.org/software/libc/bugs.html.

digoal@pg11-320tb-zfs-> iconv -f UTF8 -t GBK test.sql|psql -f -
INSERT 0 1
或者这样写入(client_encoding和server都设置为sql_ascii时，不检查编码合法性，直接存入数据库)

digoal@pg11-320tb-zfs-> iconv -f UTF8 -t GBK test.sql -o test.sql.gbk

digoal@pg11-320tb-zfs-> psql
psql (11beta4)
Type "help" for help.

postgres=# set client_encoding =sql_ascii;
SET
postgres=# i ./test.sql.gbk
INSERT 0 1
当设置客户端client_encoding为utf8编码时，由于存入的数据编码不合法，导致查询异常

digoal@pg11-320tb-zfs-> psql
psql (11beta4)
Type "help" for help.

postgres=# set client_encoding =utf8;
SET
postgres=# select * from tbl;
ERROR: invalid byte sequence for encoding "UTF8": 0xc4 0xe3
当client_encoding设置为GBK编码，查询为乱码

postgres=# set client_encoding =gbk;
SET
postgres=# select * from tbl;

id	info
1	?oí

参考
《PostgreSQL 多字节字符集合法性检测》

《[转] SqlServe到PG迁移错误:无效的编码序列"UTF8": 0x00》

《PostgreSQL UTF8 和 GB18030编码map文件不完整的问题》

《PostgreSQL WHY ERROR: invalid byte sequence for encoding "UTF8"》

《PostgreSQL SQL_ASCII encoding introduce》

《PostgreSQL Server Encoding sql_ascii attention》
转自阿里云德哥