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From Suresh Thalamati <suresh.thalam...@gmail.com>
Subject (DERBY-378) implementing import/export of large objects...
Date Thu, 26 Oct 2006 18:55:21 GMT

Since Jira e-mail is down, posting the following directly to 
derby-dev, not really that important, but did not want it to be lost 
when Jira system decides to sent all the e-mail once.

Currently derby does not allow users to perform import/export using 
the system procedures if the table contains a clobs/blob data types. I 
think enhancing derby to allow import/export of clob/blob data  will 
be useful to the users.

Some thoughts on how to implement import/export of large objects in 
Derby :

Currently Derby supports import/export using delimited file format. I 
think same format can be used to perform import/export of large object 
data also. Most of the issues are common to both import and export, 
for the obvious reason that one should be able to import the data into 
a derby database, if it was exported from Derby.

Large Objects can be written to the same export file given by the user 
or can be written to another(external) file and store the reference in 
the main export file.

Following sections discuss issues specific to each data type:


o Clobs data is exported similar to the other character data types,
   except that it can be written to an external file also.

o Double delimiters are used while writing the clob data to the export
file, if the data contains the delimiters. i.e for example
if data is  'He said "it is a nice day"', it will be written to the
file as ' He said ""it is a nice day"" '.

o If the clob data is written to an external file and then the 
reference is stored in the main export file,  double delimiters are 
not required.


o Blob data is written to export file as it is stored in the database, 
it is not converted to hex or any another format. character code-set 
conversion is not done for the binary data.

o If the data contains the delimiters , then it will be a problem if 
stored in the same file with other data types. It may not be such a 
good idea to attempt to interpret binary data to find the delimiters 
inside the data and inject double delimiters into the data. That 
leaves us with following two options :

   1) Allow import/export of blob data only when they it is  stored in 
an external file.

   2) Write the blob data to the export file along with other data 
types during export, assuming the blob data does not contain any 
delimiters and throw an error if on import if it finds delimiters 
inside the data, by interpreting it using the same code-set as the 
other character data.

o When the blob data is written to an external file then no need to 
scan data for delimiters during export/import.

Handling large objects in an external file:

Advantage of import/export of clob/blob data using an extern file that
different from the the main export file is that the data need not be
interpreted or modified if there are delimiters inside the large 
object data. Import/export of large object will perform better because 
is not scanned. Main import/export file will contain the reference to 
where the lobs are located.

Large object reference File Format:

Large object location reference will contain the file name, starting 
offset of the large object inside the file and the length of the file. 
Format will be something like : fileName.ext.StartOffSet.Length, as 
string, For example pictures.dat.100.999

Reference will be stored in the main export file. Data in the main 
export file will look like the following:

3, "pictures.dat.999.9999", "Robert"

What will be the large objects file name during export ? :

There are two possible options:

1) Let the Derby generate the file name to store lobs, by appending 
some string like "_lobdata" to the export file name given by the user. 
For example if user had given "employees.del" , large object file name 
will be employees_lobdata.del.

Advantage of this approach might be no need to define new procedures. 
In this case, by default all the large object data will always written 
to an external file.

Disadvantage is if the user wants the all the data for a table in one 
file for some reason, for example if there are only few clobs in the 
table then there is no option. And also if by chance the file already 
exists, then export will throw an error and the user has to move the 
existing file ..etc..

2) Let the user specify the file name where the blob/clob will be 
written. To support this, Derby needs to add four new import/export 
procedures, because the user need to pass the external file name 
during export and to indicate that lobs are in an external file during 
import the.

All the import/export procedures will be appended with 
"LOBS_IN_EXTFILE" and export procedures will have an extra argument 
that will take the file name,where the blobs are written.

New Export Procedures:

New IMPORT Procedures:


In this approach old procedures will fail to import/export or
write the large object data along with other columns data. I am 
inclined towards the  second approach, except for the fact that there 
another 4 new system procedures.

If maintaining backward compatibility for import/export procedures is
not necessary, then just new arguments are needed for the existing 
procedures. But if some users are using it in some application, then 
it will break. I am genereally hesitant to change the existing 
procedure signatures, may be it does not matter for import/export.

To summarize:

  1) Large objects are stored along with other data or in an external

  2) Binary data (Blob data type) is not modified even if the data 
contains delimiters inside the data during export.

  3) Import will fail gracefully or get confused and throw weird 
errors, if it finds delimiters inside blobs data if stored along with 
  other data in the import file.

  4) Four new procedure are required to allow users to specify where to
     read/write the large object data.

Any suggestions or comments ?


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