I'm trying to manage certain operations depending on PostGIS version and schema.

To get PostGIS version and schemas list, I'm used to execute SQL statements, respectively:

SELECT PostGIS_full_version();
SELECT nspname FROM pg_catalog.pg_namespace;

So I'm trying to get the same information using Python GDAL:

# Python 2.7.12 and GDAL 2.1.3
conn_settings = "PG: host={} port={} dbname={} user={} password={}"\
                .format(host, port, db_name, user, password)

conn = ogr.Open(str(conn_settings))

# prepare SQL requests
sql_version = str("SELECT PostGIS_full_version();")
sql_schemas = str("select nspname from pg_catalog.pg_namespace;")

# executing SQL
pg_version = conn.ExecuteSQL(sql_version)
pg_schemas = conn.ExecuteSQL(sql_schemas)

>>> <osgeo.ogr.Layer; proxy of <Swig Object of type 'OGRLayerShadow *' at 0x08A54C08> >

>>> <class 'osgeo.ogr.Layer'>

>>> ['AlterFieldDefn', 'Clip', 'CommitTransaction', 'CreateFeature', 'CreateField', 'CreateFields', 'CreateGeomField', 'DeleteFeature', 'DeleteField', 'Dereference', 'Erase', 'FindFieldIndex', 'GetDescription', 'GetExtent', 'GetFIDColumn', 'GetFeature', 'GetFeatureCount', 'GetFeaturesRead', 'GetGeomType', 'GetGeometryColumn', 'GetLayerDefn', 'GetMetadata', 'GetMetadataDomainList', 'GetMetadataItem', 'GetMetadata_Dict', 'GetMetadata_List', 'GetName', 'GetNextFeature', 'GetRefCount', 'GetSpatialFilter', 'GetSpatialRef', 'GetStyleTable', 'Identity', 'Intersection', 'Reference', 'ReorderField', 'ReorderFields', 'ResetReading', 'RollbackTransaction', 'SetAttributeFilter', 'SetDescription', 'SetFeature', 'SetIgnoredFields', 'SetMetadata', 'SetMetadataItem', 'SetNextByIndex', 'SetSpatialFilter', 'SetSpatialFilterRect', 'SetStyleTable', 'StartTransaction', 'SymDifference', 'SyncToDisk', 'TestCapability', 'Union', 'Update', '__bool__', '__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__getitem__', '__hash__', '__init__', '__iter__', '__len__', '__module__', '__new__', '__nonzero__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'next', 'schema', 'this', 'thisown']

>>> {}

Is it possible to get SQL result as a string object (or dict...)?

  • Can you edit your question to add an example of how you'd like to see the result, and an example of what you're getting now? – Midavalo Mar 19 '17 at 18:37

OGR's ExecuteSQL only returns a Layer handle, as your question demonstrates, so it is really limited for general-purpose SQL work. To use OGR, it's important to understand the data model of DataSourceLayerFeatureField.

For example:

ly = conn.ExecuteSQL(sql_version)
feat = ly.GetNextFeature()  # first and only row
version = feat.GetField(0)  # first and only field index

However, you will find this may not work for general-purpose queries. I normally recommend psycopg2 for most general-purpose queries.

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