PostgreSQL,SQL_ASCII转码问题.

首先不建议使用SQL_ASCII作为服务端的编码。
因为你必须知道存进去的是什么编码，才能解出来。并且有些编码是存不进去的（例如编码中用到了sql_ascii中的转义码的）

插入数据：
sql_ascii作为服务端编码时

1. 服务端存储的是客户端的编码，不进行转码。
2. 当客户端编码不是sql_ascii时，有可能能存进去，也有可能存不进去。（编码中用到了sql_ascii中的转义码的存不进去）

sql_ascii作为客户端编码时

1. 服务端不进行编码合法性检测，服务端不进行编码转换，直接存数据库中.

取数据：
sql_ascii作为服务端编码时

1. 当需要提取存储在sql_ascii编码数据库中的数据时，必须以原编码去取，否则可能出现乱码。

sql_ascii作为客户端编码时

1. 直接返回，不进行编码检测和转换.

合法性检测代码如下：

/*
 * Convert any encoding to server encoding.
 *
 * See the notes about string conversion functions at the top of this file.
 *
 * Unlike the other string conversion functions, this will apply validation
 * even if encoding == DatabaseEncoding->encoding.  This is because this is
 * used to process data coming in from outside the database, and we never
 * want to just assume validity.
 */
char *
pg_any_to_server(const char *s, int len, int encoding)
{
        if (len <= 0)
                return (char *) s;              /* empty string is always valid */

        if (encoding == DatabaseEncoding->encoding ||
                encoding == PG_SQL_ASCII)
        {
                /*
                 * No conversion is needed, but we must still validate the data.
                 */
                (void) pg_verify_mbstr(DatabaseEncoding->encoding, s, len, false);
                return (char *) s;
        }

        if (DatabaseEncoding->encoding == PG_SQL_ASCII)
        {
                /*
                 * No conversion is possible, but we must still validate the data,
                 * because the client-side code might have done string escaping using
                 * the selected client_encoding.  If the client encoding is ASCII-safe
                 * then we just do a straight validation under that encoding.  For an
                 * ASCII-unsafe encoding we have a problem: we dare not pass such data
                 * to the parser but we have no way to convert it.  We compromise by
                 * rejecting the data if it contains any non-ASCII characters.
                 */
                if (PG_VALID_BE_ENCODING(encoding))
                        (void) pg_verify_mbstr(encoding, s, len, false);
                else
                {
                        int                     i;

                        for (i = 0; i < len; i++)
                        {
                                if (s[i] == '\0' || IS_HIGHBIT_SET(s[i]))
                                        ereport(ERROR,
                                                        (errcode(ERRCODE_CHARACTER_NOT_IN_REPERTOIRE),
                                         errmsg("invalid byte value for encoding \"%s\": 0x%02x",
                                                        pg_enc2name_tbl[PG_SQL_ASCII].name,
                                                        (unsigned char) s[i])));
                        }
                }
                return (char *) s;
        }

        /* Fast path if we can use cached conversion function */
        if (encoding == ClientEncoding->encoding)
                return perform_default_encoding_conversion(s, len, true);

        /* General case ... will not work outside transactions */
        return (char *) pg_do_encoding_conversion((unsigned char *) s,
                                                                                          len,
                                                                                          encoding,
                                                                                          DatabaseEncoding->encoding);
}