[AIRFLOW-269] Add some unit tests for PostgreSQL

[sekikn]

Dear Airflow Maintainers,

Please accept this PR that addresses the following issues:

• https://issues.apache.org/jira/browse/AIRFLOW-269

Testing Done:

• ran ./run_unit_tests.sh tests.operators:PostgresTest and confirmed all 4 tests passed

There is only one unit test for PostgreSQL for now.
This patch add some tests as well as MySQL.

[criccomini]

Looks like tests are failing

root: INFO: CREATE TABLE IF NOT EXISTS test_postgres_to_postgres (LIKE INFORMATION_SCHEMA.TABLES)
root: ERROR: inherited relation "tables" is not a table
Traceback (most recent call last):
  File "/home/travis/build/apache/incubator-airflow/airflow/models.py", line 1324, in run
    result = task_copy.execute(context=context)
  File "/home/travis/build/apache/incubator-airflow/airflow/operators/generic_transfer.py", line 75, in execute
    destination_hook.run(self.preoperator)
  File "/home/travis/build/apache/incubator-airflow/airflow/hooks/dbapi_hook.py", line 145, in run
    cur.execute(s)
ProgrammingError: inherited relation "tables" is not a table

[sekikn]

I've missed that error since it doesn't occur in my local environment. Thanks for the comment! I'm not sure its cause for now, but I'll investigate it.

[sekikn]

After some experiments, I've figured out the cause. information_schema.tables is a view in PostgreSQL, and PostgreSQL 9.1 probably doesn't support copying tables by "CREATE TABLE LIKE" from views.
The following sentence appears in 9.2 document for the first time, and not in 9.1.

The LIKE clause can also be used to copy columns from views, foreign tables, or composite types.

Possible solutions are:

1. specifying PostgreSQL version greater than 9.1 in .travis.yml
2. instead of a view, using a real table (such as pg_class, information_schema.sql_languages or something) as a copying source

@criccomini What version of PostgreSQL does Airflow officially support? If 9.1 is not supported, I prefer the first option for consistency with MySQL's test case.

[bolkedebruin]

9.1 is relative old, so I am ok with a more recent version.

[sekikn]

Thanks! I've just set postgresql version to 9.2 on Travis CI and made the PR squashed.

[sekikn]

The latest CI failure is irrelevant to this issue. The unit test in question succeeded as follows:

INFO  [root] DROP TABLE IF EXISTS test_postgres_to_postgres
INFO  [root] CREATE TABLE IF NOT EXISTS test_postgres_to_postgres (LIKE INFORMATION_SCHEMA.TABLES)
INFO  [root] Inserting rows into postgres_default
INFO  [root] Using connection to: localhost
INFO  [root] Done loading. Loaded a total of 100 rows
ok

[criccomini]

LGTM

Thanks so much for adding tests. Really valuable.