在oracle中将blob（image）转换为varchar

Question

我试图从oracle db读取blob（图像）并在html中显示它。图像大于缓冲区大小，因此我必须先将其拆分，然后附加所有子字符串。我的方法如下（将有一个循环来通过blob）：

SELECT utl_raw.cast_to_varchar2(dbms_lob.substr(FILE_CONTENTS,2000,1)) as mystring from doc where file_name='test.png'

问题是转换后的字符串看起来像scrambled

我没有指定转换的char_set，这可能是原因吗？如果是这样，我怎么知道使用哪一个？

谢谢。

Answer 1

这是一个将BLOB转换为Base64字符串的函数：

FUNCTION EncodeBASE64(InBlob IN BLOB) RETURN CLOB IS

    BlobLen INTEGER := DBMS_LOB.GETLENGTH(InBlob);
    read_offset INTEGER := 1;

    amount INTEGER := 1440; -- must be a whole multiple of 3
    -- size of a whole multiple of 48 is beneficial to get NEW_LINE after each 64 characters 
    buffer RAW(1440);
    res CLOB := EMPTY_CLOB();

BEGIN

    IF InBlob IS NULL OR NVL(BlobLen, 0) = 0 THEN 
        RETURN NULL;
    ELSIF BlobLen <= 24000 THEN
        RETURN UTL_RAW.CAST_TO_VARCHAR2(UTL_ENCODE.BASE64_ENCODE(InBlob));
    ELSE
        -- UTL_ENCODE.BASE64_ENCODE is limited to 32k, process in chunks if bigger
        LOOP
            EXIT WHEN read_offset >= BlobLen;
            DBMS_LOB.READ(InBlob, amount, read_offset, buffer);
            res := res || UTL_RAW.CAST_TO_VARCHAR2(UTL_ENCODE.BASE64_ENCODE(buffer));       
            read_offset := read_offset + amount;
        END LOOP;
    END IF;
    RETURN res;

END EncodeBASE64;