On 15/08/2020 16:22, Dheeraj Chand
wrote:
‘’’
When importing a shapefile or
other vector data, only the attrib tables get saved to some
database: sqlite by default, or PostgrSQL if you have
configured for that backend. But the geometry is still kept in
the GRASS vector format.
‘’’
1. How would one configure
that? Please assume that I am unfamiliar and uncomfortable
with GRASS when explaining, but able to read Java and Python
man pages with ease, also comfortable with PSQL and PostGIS.
In that case you might want to first go thru some tutorials on
using GRASS. We're here to help if encounter anything that is
unclear, or not working as you expected.
All GRASS commands can be called thru the GRASS-python bindings,
so that might be easiest for you. But do go into the beginning
tutorials first.
The GRASS module that sets the backend database connection is
`db.connect`. Have a look at the man page:
https://grass.osgeo.org/grass78/manuals/db.connect.html
You would choose the driver parameter as "pg", then set the
database and schema as required. This comes after running
`db.login` one time to save your DB auth credentials.
2. Would we get all the
speed benefits of PSQL, would it be used for computations
whenever possible?
Not sure how to answer here. GRASS in general sends SQL commands
back to the DB backend for any undates. If, for example, you had a
point vector of cities, with two columns "population" and
"number_hospitals" in the attribute table, and you wanted to
calculate number of hospitals per 1000 people, then you would
construct a regular SQL query that would be sent to the backend.
So I guess that the speed would be determined by PostgreSQL.
3. Is there a way to easily
move from GRASS and PSQL geometry and topology models when
doing this?
Sorry to repeat again: GRASS maintains all geometry (and
topology) within its own internal vector data structure. NO
PostGIS involved here. But You could easily export GRASS vectors
to a PostGIS database using the `v.out.postgis` module.
https://grass.osgeo.org/grass78/manuals/v.out.postgis.html
To pull a PostGIS vector table into GRASS would require either
`v.in.ogr` or `v.import`
Sent from my iPhone
On Sat, 15 Aug 2020, Micha Silver wrote:
But again, don't confuse -
this is NOT PostGIS, and GRASS does not
need/use PostGIS for geometry.
GRASS geometry is always independent of any
external geospatial format.
Micha,
Thanks for clarifying; I must have mis-understood what
I read. I assumed the
geometry was kept by GRASS and didn't know why PostGIS
was mentioned ... and
I don't recall just where I read all this.
It's the other way around:
When you export GRASS map layers, you can, as
you know, choose to save out
to several formats: shp, Geopackage (the
current default) or to
PostGIS. PostGIS is the best choice when you need
multiuser access to the
geospatial data. But you point out that you're the
only user, so why would you
need the overhead of PostGIS?
Ah, so. I don't.
To repeat, you can set the
default for saving attribute tables to
PostgreSQL, but do not try to
save a GRASS layer to PostGIS in the same
database! That will definitely
lead to trouble. If you want/need a PostGIS
instance for some reason
independent of GRASS, then keep it totally
separated from GRASS. i.e. at
least in a separate schema or even separate
database.
No, I want the attribute data in postgres so I need to
learn to make that
the default.
The main reasons for choosing
PostgreSQL as your database backend would be
1. to allow fancy SQL queries
on the database tables
2. huge, complex data tables
or triggers
3. multiuser access to the
attribute tables
My reason is keeping these data in the same format as
other project data.
But keep in mind that the
default sqlite database is quite powerful, and you would
have to look very deeply to find a PostgreSQL feature
that is missing in sqlite.
Yes, I've been using SQlite as long as I have
PostgreSQL.
Thanks,
Rich
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--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918
https://orcid.org/0000-0002-1128-1325
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