On Apr 8, 2009, at 3:25 PM, Peter Willis wrote:
Hello,
I just ingested a MULTIPOLYGON vector into a PostGIS enabled
database and realized that each vector becomes a unique TABLE
in the database.
Is this really necessary?
Why not use proper relational database techniques and have
all vectors of a specific type go into a single table
with a unique ID for rows that belong to a specific
vector?
Shouldn't I have tables named:
LINES
MULTIPOLYGONS
POLYGONS
POINTS
that link to a table named VECTORS by a unique ID.
From a data modeling perspective, you are viewing the lines,
polygons, points, etc. as entities. When modeling features (lakes,
towns, etc.) they *are* the entity and have attributes. A lake has a
name, depth, area, and so forth and a spatial representation.
For GIS usage it makes sense to model the entity, not its spatial
representation.
PostGIS (as well as Oracle Spatial, Informix DataBlade) use this
scheme which is based on the OpenGIS "Simple Features Specification
for SQL" (see http://www.opengeospatial.org/standards/sfs).
You end up with 200 tables (think layers) with attributes and a
spatial representation. The scheme you suggest would add complexity
not only in the database design, but also client implementation.
%<.............
If we're going to use a database, we should make use of the
facilities provided by a database and stop thinking in terms of
flat files from the 1970s.
I think when viewed from the perspective that features==entities
you'll find that the database is being used appropriately.
If you want more discussion on this issue, I suggest you post to the
PostGIS users list where you'll get the attention of the developers
among others. This isn't really a QGIS issue---we make use of PostGIS'
OGC certified implementation.
Best...
-gary
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Gary Sherman
Chair, QGIS Project Steering Committee
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