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SQL Server數(shù)據(jù)遷移至PostgreSQL出錯的解釋以及解決方案

更新時間：2014年09月22日 10:04:07 投稿：hebedich

最近對SQL Server到PostgreSQL的數(shù)據(jù)遷移時出現(xiàn)了問題，返回的錯誤為：invalid byte sequence for encoding "UTF8": 0x00。經(jīng)查證pg源代碼，該問題引起的原因是sql server的字符類型字段中含有空字符\0，該字符在pg中不支持。

問題重現(xiàn)：

1、PG客戶端:

postgres=# create table text_test (id int,info text);
CREATE TABLE
postgres=# insert into text_test values (1,E'\0x00');
ERROR: invalid byte sequence for encoding "UTF8": 0x00

2、SQL Server產(chǎn)生數(shù)據(jù)

create table test_varchar(id int,name varchar(20));
insert into test_varchar values (1, 'name' + char(0));
insert into test_varchar values (1, 'name' + '');

然后通過java程序進行獲取數(shù)據(jù)并插入到PG，同樣會得到錯誤信息：

invalid byte sequence for encoding "UTF8": 0x00

首先我們認為此為gb2312轉(zhuǎn)化到UTF8時，發(fā)生了無法轉(zhuǎn)化的錯誤。經(jīng)查UTF8是變長的, 1-6個字節(jié)。他的編碼規(guī)則如下：

Bits	Last code point	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5	Byte 6
7	U+007F	0xxxxxxx
11	U+07FF	110xxxxx	10xxxxxx
16	U+FFFF	1110xxxx	10xxxxxx	10xxxxxx
21	U+1FFFFF	11110xxx	10xxxxxx	10xxxxxx	10xxxxxx
26	U+3FFFFFF	111110xx	10xxxxxx	10xxxxxx	10xxxxxx	10xxxxxx
31	U+7FFFFFFF	1111110x	10xxxxxx	10xxxxxx	10xxxxxx	10xxxxxx	10xxxxxx

而0x00是符合UTF8規(guī)則的。這就使我們非常詫異。然后我們發(fā)現(xiàn)有兩點繼而確認了問題：
1、

PostgreSQL doesn't support storing NULL (\0x00) characters in text fields (this is obviously different from the database NULL value, which is fully supported).

If you need to store the NULL character, you must use a bytea field - which should store anything you want, but won't support text operations on it.

Given that PostgreSQL doesn't support it in text values, there's no good way to get it to remove it. You could import your data into bytea and later convert it to text using a special function (in perl or something, maybe?), but it's likely going to be easier to do that in preprocessing before you load it.

Source:http://stackoverflow.com/questions/1347646/postgres-error-on-insert-error-invalid-byte-sequence-for-encoding-utf8-0x0

2、

Terminating character	Indicated by
Tab	\t This is the default field terminator.
Newline character	\n This is the default row terminator.
Carriage return/line feed	\r
Backslash1	\\
Null terminator (nonvisible terminator)2	\0
Any printable character (control characters are not printable, except null, tab, newline, and carriage return)	(*, A, t, l, and so on)
String of up to 10 printable characters, including some or all of the terminators listed earlier	(\t, end, !!!!!!!!!!, \t—\n, and so on)

Source:http://msdn.microsoft.com/en-us/library/ms191485.aspx

由此我們確定，是pg對null的處理和SQL Server處理是不相同的，所以在這里出現(xiàn)了錯誤。

而導(dǎo)致這一問題的PG具體代碼如下（src/backend/utils/mb/wchar.c的pg_verify_mbstr_len）：

if (!IS_HIGHBIT_SET(*mbstr))
    {
      if (*mbstr != '\0')
      {
        mb_len++;
        mbstr++;
        len--;
        continue;
      }
      if (noError)
        return -1;
      report_invalid_encoding(encoding, mbstr, len);
    }

#define IS_HIGHBIT_SET(ch)   ((unsigned char)(ch) & HIGHBIT)
#define HIGHBIT         (0x80)

report_invalid_encoding函數(shù)是將錯誤信息返回，也就是

invalid byte sequence for encoding "UTF8": 0x00
而真正導(dǎo)致這一問題的就是：
!IS_HIGHBIT_SET(*mbstr)當*mbstr為0x00時進入判斷，然后進而判斷*mbstr是否為\0,當為\0時，直接進入函數(shù)report_invalid_encoding報錯。

所以出現(xiàn)此問題的原因是PG和SQL Server對null的處理是不相同的。

處理方案：

1、將SQL Server源數(shù)據(jù)進行修改方法，

UPDATE: This seems to work:
 
Select * from TABLE
where UNICODE(SUBSTRING(naughtyField, LEN(naughtyField), 1)) = 0
So:
 
Update TABLE
SET naughtyField = SUBSTRING(naughtyField, 1, LEN(naughtyField) - 1)
where UNICODE(SUBSTRING(naughtyField, LEN(naughtyField), 1)) = 0
Source:http://stackoverflow.com/questions/3533320/sql-server-remove-end-string-character-0-from-data

2、對應(yīng)用進行修改，獲取到SQL Server數(shù)據(jù)時，將數(shù)據(jù)進行轉(zhuǎn)化，和第一種方法異曲同工。

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