[YAML]: Update postgres IT test and readme

sdks/python/apache_beam/yaml/README.md

@@ -43,3 +43,28 @@ https://beam.apache.org/documentation/sdks/yaml/
 
 For information about contributing to Beam YAML see
 https://docs.google.com/document/d/19zswPXxxBxlAUmswYPUtSc-IVAu1qWvpjo1ZSDMRbu0
+
+## Integration Tests
+
+The integration_tests.py dynamically creates test methods based on the yaml
+files provided in the `tests` and `extended_tests` directories and runs the
+pipeline. It also contains context managers for setting up test environments for
+both precommit tests (e.g. `tests` folder) and postcommit tests
+(e.g. `extended_tests` folder).
+
+To run the precommit tests:
+
+```bash
+pytest -v integration_tests.py
+
+or
+
+pytest -v integration_tests.py::<yaml_file_name_without_extension>Test
+```
+
+To run the postcommit tests:
+
+```bash
+pytest -v integration_tests.py --test_files_dir="extended_tests"
+```
+

sdks/python/apache_beam/yaml/extended_tests/postgres.yaml

@@ -35,7 +35,7 @@ pipelines:
             url: "{TEMP_DB}"
             query: "INSERT INTO tmp_table (value, rank) VALUES(?,?)"
 
-  # Postgres read pipeline
+  # Postgres read pipeline with driver_class_name
   - pipeline:
       type: chain
       transforms:
@@ -50,4 +50,20 @@ pipelines:
               - {value: 123, rank: 0}
               - {value: 456, rank: 1}
               - {value: 789, rank: 2}
-
+
+  # Postgres read pipeline without driver_class_name
+  # This is to prevent https://github.com/apache/beam/issues/35122 from
+  # happening again.
+  - pipeline:
+      type: chain
+      transforms:
+        - type: ReadFromPostgres
+          config:
+            url: "{TEMP_DB}"
+            query: "SELECT * FROM tmp_table"
+        - type: AssertEqual
+          config:
+            elements:
+              - {value: 123, rank: 0}
+              - {value: 456, rank: 1}
+              - {value: 789, rank: 2}