Now that all tests pass for me (I'm not yet running any mysql tests): It seems to me we run the same tests twice:

...
test/test_postgresql.py::postgresqlDBTest::testFilteringStringSort PASSED [ 63%]
test/test_postgresql.py::postgresqlDBTest::testFilteringTransitiveLinkIssue PASSED [ 63%]
test/test_postgresql.py::postgresqlDBTest::testFilteringTransitiveLinkSort PASSED [ 63%]
test/test_postgresql.py::postgresqlDBTest::testFilteringTransitiveLinkSortNull PASSED [ 63%]
test/test_postgresql.py::postgresqlDBTest::testFilteringTransitiveLinkUser PASSED [ 63%]
test/test_postgresql.py::postgresqlDBTest::testFilteringTransitiveLinkUserLimit PASSED [ 63%]
test/test_postgresql.py::postgresqlDBTest::testFilteringTransitiveMultilink PASSED [ 63%]
test/test_postgresql.py::postgresqlDBTest::testFilteringTransitiveMultilinkSort PASSED [ 63%]
...
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringStringSort PASSED [ 69%]
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringTransitiveLinkIssue PASSED [ 69%]
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringTransitiveLinkSort PASSED [ 69%]
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringTransitiveLinkSortNull PASSED [ 69%]
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringTransitiveLinkUser PASSED [ 69%]
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringTransitiveLinkUserLimit PASSED [ 69%]
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringTransitiveMultilink PASSED [ 69%]
test/test_postgresql.py::postgresqlDBTestSchema::testFilteringTransitiveMultilinkSort PASSED [ 69%]

Do they have any added value when run twice, are there subtle differences in parameters?
They take a lot of time, so we might benefit from removing dupes.