feat(ingest): support Postgres PostGIS extensions (#2136)

metadata-ingestion/README.md

@@ -211,6 +211,8 @@ Extracts:
 
 Extra requirements: `pip install psycopg2-binary` or `pip install psycopg2`
 
+If you're using PostGIS extensions for Postgres, also use `pip install GeoAlchemy2`.
+
 ```yml
 source:
   type: postgres

metadata-ingestion/setup.cfg

@@ -30,6 +30,8 @@ ignore_missing_imports = yes
 ignore_missing_imports = yes
 [mypy-ldap.*]
 ignore_missing_imports = yes
+[mypy-geoalchemy2.*]
+ignore_missing_imports = yes
 
 [isort]
 profile = black

metadata-ingestion/src/datahub/ingestion/source/postgres.py

@@ -1,5 +1,15 @@
 from .sql_common import BasicSQLAlchemyConfig, SQLAlchemySource
 
+try:
+    # GeoAlchemy adds support for PostGIS extensions in SQLAlchemy. In order to
+    # activate it, we must import it so that it can hook into SQLAlchemy. While
+    # we don't use the Geometry type that we import, we do care about the side
+    # effects of the import. For more details, see here:
+    # https://geoalchemy-2.readthedocs.io/en/latest/core_tutorial.html#reflecting-tables.
+    from geoalchemy2 import Geometry  # noqa
+except ImportError:
+    pass
+
 
 class PostgresConfig(BasicSQLAlchemyConfig):
     # defaults