Dear OSGEO
Jim Gray´s paper and much more on
this issue is on his site at MS Research.
Storing images on a database gives much
more benefits that simple retrieval of
metadata. Databases offer concurrency control,
data protection, integrity and management features
that simple file systems are lacking.
If you have hundreds of images scattered around
as files, you lack data management. Your metadata
may point to a file that could have been deleted.
In a multi-user environment, file systems do not
prevent different users from updating the same
image. The result may be a data which is inconsistent.
Allow me to reiterate my earlier argument, which is
that FOSS4G should **allow** users the option of storing
raster data in a database. Storing images in a database
is not recommended in each and every situation.
The user should have the option, according to his needs.
The current debate on whether images should be stored
on an RDBMS reminds me of a similar debate during the
early 90s, concerning whether vector data should be
stored in an RDBMS. Remember the days of ARC-INFO?
In mid 90s, our team at INPE tried to use the
Postgres-95 RDBMS to store vector data. The result
was a system with a very slow performance.
The concept was right, but the implementation was
lacking. It was only when PostgreSQL and PostGIS
came of age that we could develop a multi-user
spatial database with good performance.
By the same argument, these are early days of
storing raster data in RDBMS. There are missing
features on the database and the performance may
be slower than file systems. But the concept
is fundamentally correct. I predict that five
years hence this debate will be solved and we
will look at it as a relique of the past.
Best Regards
Gilberto
Christopher Schmidt said:
I don't see anything in that paragraph that indicates that storing the
*image data* in the database is important. (A link to the paper online
or something could change that, of course.) Specifically, I don't think
there's any doubt that if you have many-many files, it makes sense to
store the *queryable image information* -- things like spatial extent,
temporal extent, etc. -- belong in a database. The question is, in the
"data" column, do you store a File Path, or the Image Data? Until/Unless
databases get/have image manipulation tools directly, I can't see the
value of storing the image data itself in the database.
The points above argue against file-system based metadata
storage/retrieval: sorting files by date, searching through index files,
etc., so far as I can tell, but I don't see a compelling argument for
image data in the database above.
Of course, this is assuming that the image data access pattern is the
same "in the database" and "on disk": for example, storing GeoTIFF data,
then using GDAL to parse the string from the database as a GeoTIFF file.
If the database you're using has a different (faster) Image access
algorithm, then of course there can be benefits. However, those same
benefits could presumably be realized with sufficiently complete
libraries for accessing the image externally: If Oracles' Database
product, for example, internally tiles the image, and they had a library
to access the image in the same way, presumably you could store those
bits on disk as well. However, if that library depends internally on a
database, then integration of all points into the same database might
help in some ways.
In any case, I think there's obvious reasons to store your image
metadata in a database -- and *using the same tools for accessing the
images*, I don't think we've yet seen a compelling argument for storing
image blobs in the database. Of course, all things are not equal :)
If your database has built in MrSID support, for example, you could
imagine using Database Storage for Images, because you'd get the
automatic compression combined with the querying -- but that's not about
the Database Specifically, just the image storage/reading library that
comes along with it.
Regards,
-- Christopher Schmidt Web Developer
--
===========================================
Dr.Gilberto Camara
Director General
National Institute for Space Research (INPE)
Sao Jose dos Campos, Brazil
voice: +55-12-3945-6035
fax: +55-12-3921-6455
web: http://www.dpi.inpe.br/gilberto
blog: http://techne-episteme.blogspot.com/
============================================
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