Absurdly long queries with Postgres 11 during unit tests

This is heads up in case anyone sees something similar:

I have managed to trigger this degenerate query case in two completely different Django 2.2 projects.   In production with a normal sized dataset, the query time is fine.  But during unit testing with a small subset of the data, the queries took a long time.  A LONG time.

In the most recent case each query took 100x longer.  200+ seconds instead of 2 seconds.  The unit test dataset isn't very large because it's a unit test.  

I think I may have first seen this when I upgraded the project to postgres 11.

Manually vacuuming between tests resolves the issue.  (Yes, autovacuum is on by default -- and isn't the db created from scratch for each 'manage test' invocation?)

This is how I did it:

def _vacuum():

    # Some unit test queries seem to take a much longer time.

    # Let's try vacuuming.

    # https://stackoverflow.com/a/13955271/2077386

    with connection.cursor() as cursor:

        logger.info("Vacuum: begin")

        cursor.execute("VACUUM ANALYZE")

        logger.info("Vacuum: complete")

class VacuumMixin:

    @classmethod

    def setUpClass(cls):

        _vacuum()

        return super().setUpClass()

    @classmethod

    def tearDownClass(cls):

        ret = super().tearDownClass()

        _vacuum()

        return ret

If anyone else sees this, please let me know.  Maybe we can further RCA it.

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