Hi list,

I'm trying to document the Firebird/Interbase connector library for the
FreePascal language.

I came across BLOB segment size and understand that, when writing BLOBs,
you need to write in chunks smaller than or equal to the segment size.

Have looked at Firebird language reference update, IB 6 language
reference, and grepped through the doc directory of a FB 2.5 install.

The IB 6 language reference mentions ...
BLOB [SUB_TYPE {int | subtype_name}] [SEGMENT SIZE int]
but doesn't explain what the segment size actually stands for.

Presumably having a larger segment size might improve performance for
reading/writing large BLOBs, but might lead to bigger storage
requirements per BLOB?

It also says BLOB segment size is limited to 64k.

I did find this post by Helen Borrie:

http://tech.groups.yahoo.com/group/firebird-support/message/94611

> From Firebird's perspective, can any amount of data be written
>to a blob in a single operation? Without segmenting it?

Broadly, yes. In the modern era the segmenting of blobs occurs at the server
side, according to some algorithm determined by page size and probably also
whether blob data is stored on blob pages or data pages. It is not
affected by
any segment size you define for your blob column so you can safely
forget about
it. By default the segment size will be 80 bytes but the engine doesn't care
about that, either.

Excerpt of the code to write blobs (hope I got the right part):
while BlobBytesWritten < (BlobSize-BlobSegmentSize) do
begin
isc_put_segment(@FStatus[0], @blobHandle, BlobSegmentSize,
@s[(i*BlobSegmentSize)+1]);
inc(BlobBytesWritten,BlobSegmentSize);
inc(i);
end;
if BlobBytesWritten <> BlobSize then
isc_put_segment(@FStatus[0], @blobHandle,
BlobSize-BlobBytesWritten, @s[(i*BlobSegmentSize)+1]);


My questions:
1. Do Firebird users need to know more about segment size (and if so,
what ;) ), or is it indeed just a relic of the past?
2. If relevant, is there some default segment size?
3. If relevant, has the max segment size changed since IB6?
4. Seeing Helen's post, can the code be changed to just output the
entire BLOB in one go or have I misunderstood?

Thanks,
Reinier