----- Original Message -----
From: Daniel Rail <daniel@...>
Date: Wednesday, June 25, 2003 6:07 am
Subject: Re: [Firebird-Architect] Re: Fw: Performance

> Hello eric_ck_leung,
>
> Wednesday, June 25, 2003, 3:25:26 AM, you wrote:
> > I am wondering if FB allows users to specify any BLOB fields being
> > stored to any external files or storage devices.
>
> Because of the multi-generational architecture of Firebird, it would
> be hard to do this.
>
> > I think if blob fields and other fields share the same storage
> > pages. After prolong inserts, updates and deletes operations, And
> > Blob fields are usually have irregular length, the pages might then
> > be fragement greatly. And the database efficiency might be lowered.
>
> Blobs have their own pages and it's only an 8 byte pointer, that
> points to the blob, that is stored with the fields in a table.
> So, as
> you can see, you wouldn't see fragments as you might expect. A blob
> can cover more than one page depending on the blob size and the page
> size, but those pages are not the same one where the records of the
> table are stored. And as Jim mentioned in a previous message, the
> blob is only retrieved when referenced.
>

Hi Daniel,
it's not full true. Because really BLOB information usually being much
much bigger that scan information always slow down scan process
drastically. 'Read ahead' feature of OS doesn't work in this case as OS
have to issue physical read practically for each DB page.

Alexander

> --
> Best regards,
> Daniel Rail
> Senior System Engineer
> ACCRA Group Inc. (www.accra.ca)
> ACCRA Med Software Inc. (www.filopto.com)
>