Is it possible to import a shape file containing multipolygons into single polygon in PostGIS? Whenever I try importing a shape file of a polygon, it is stored as a multipolygon (as opposed to a single polygon) in a geom column. Thus, I am unable to extract it as a single polygon value from the multipolygon.
All helpful suggestions much appreciated
You can use ST_GeometryN together with ST_NumGeometries and the generate_series function to obtain what you need.
Let's assume you have the table from Jakub's example:
CREATE TABLE multi AS( SELECT 1 as id, 2 as test, ST_GeomFromText('MULTIPOLYGON(((0 0,4 0,4 4,0 4,0 0)),((1 1,2 1,2 2,1 2,1 1)), ((-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)))') AS geom ); This one contains a multipolygon, an id and another column.
To get each single polygon from the table including all other attributes try something like:
SELECT id, test, ST_GeometryN(poli, generate_series(1, ST_NumGeometries(geom))) AS geom FROM multi "id" and "test" are the values for each row in the original table. generate_series creates a series of numbers from 1 to the number of geometries in each row.
Therefore you will split each multi geometry in its separate single geometry parts and the values in the other columns remain the same.
Just replace the columns and table in the example with the columns from your exported shapefile and you will get the table with the single polygons.
Hope this answers your question.
I used ST_DUMP to convert a table of multipolygon geometries in PostgreSQL to a new table with polygon geometries and other columns of data.
CREATE TABLE poly AS --poly will be the new polygon table WITH dump AS ( SELECT id, test, --columns from your multipolygon table (ST_DUMP(geometry)).geom AS geometry FROM multi --the name of your multipolygon table ) SELECT id, test, geometry::geometry(Polygon,4326) --type cast using SRID from multipolygon FROM dump; Update: I think this could be accomplished much easier with this query.
CREATE TABLE polygon_table AS SELECT id, example_column, (ST_DUMP(geom)).geom::geometry(Polygon,4326) AS geom FROM multipolygon_table Import into a staging table and then use ST_DUMP to brake the multigeom into individual pieces and use that to populate the destination table.
You import all the data you need into a staging table (let's call it multi) and then use ST_DUMP to break the mutligeometry into single geometries:
WITH multi AS( SELECT 1 as id, 2 as test, ST_GeomFromText('MULTIPOLYGON(((0 0,4 0,4 4,0 4,0 0)),((1 1,2 1,2 2,1 2,1 1)), ((-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)))') as poli ) ,dump AS( SELECT id ,test ,ST_DUMP(poli) as d FROM multi) SELECT id ,test ,(dump.d).path ,ST_AsTEXT((dump.d).geom) FROM dump