Description of problem:
The vacuum process is not able to remove the dead rows as they are held by an unfinished transaction from ovirt-dwh.

    vacuumdb -d ovirt_engine_history -t vm_disk_samples_history -v
    vacuumdb: vacuuming database "ovirt_engine_history"
    INFO:  vacuuming "public.vm_disk_samples_history"
    INFO:  index "idx_vm_disk_history_datetime_samples" now contains 63428 row versions in 176 pages
    DETAIL:  0 index row versions were removed.
    0 index pages have been deleted, 0 are currently reusable.
    CPU 0.00s/0.00u sec elapsed 0.00 sec.
    INFO:  index "idx_vm_disk_configuration_version_samples" now contains 63428 row versions in 181 pages
    DETAIL:  0 index row versions were removed.
    0 index pages have been deleted, 0 are currently reusable.
    CPU 0.00s/0.00u sec elapsed 0.00 sec.
    INFO:  index "vm_disk_samples_history_vm_disk_id_idx" now contains 63428 row versions in 253 pages
    DETAIL:  0 index row versions were removed.
    0 index pages have been deleted, 0 are currently reusable.
    CPU 0.00s/0.00u sec elapsed 0.00 sec.
    INFO:  index "vm_disk_samples_history_pkey" now contains 63428 row versions in 176 pages
    DETAIL:  0 index row versions were removed.
    0 index pages have been deleted, 0 are currently reusable.
    CPU 0.00s/0.00u sec elapsed 0.00 sec.
    INFO:  "vm_disk_samples_history": found 0 removable, 47958 nonremovable row versions in 738 out of 976 pages
==> DETAIL:  45888 dead row versions cannot be removed yet.
    There were 0 unused item pointers.
    Skipped 0 pages due to buffer pins.
    0 pages are entirely empty.
    CPU 0.00s/0.01u sec elapsed 0.01 sec.

Here is the idle in transaction connection:
    select pid, query from pg_stat_activity where state='idle in transaction' and datname='ovirt_engine_history'

      pid  |                                       query                                        
    -------+------------------------------------------------------------------------------------
     17061 | SELECT 'continueAgg', '1'                                                         +
           | FROM history_configuration                                                        +
           | WHERE var_name = 'lastHourAggr'                                                   +
           | AND var_datetime < '2019-02-08 06:00:00.000000+0000'                              +
           | 
     17062 | SELECT DISTINCT 'continueAgg', '1'                                                +
           | FROM history_configuration                                                        +
           | WHERE var_name = 'lastDayAggr'                                                    +
           |         AND var_datetime < '2019-02-07 00:00:00.000000+0000'                      +
           |         AND                                                                       +
           |                 CASE                                                              +
           |                         WHEN                                                      +
           |                                 var_datetime = '2019-02-07 00:00:00.000000+0000'  +
           |                                 AND 'true' =                                      +
           |                                         (                                         +
           |                                                 SELECT var_value                  +
           |                                                 FROM history_configuration        +
           |                                                 WHERE var_name = 'HourlyAggFailed'+
           |                                         )                                         +
           |                                 THEN FALSE                                        +
           |                         ELSE TRUE                                                 +
           |                 END                                                               +
           | 
    (2 rows)


Version-Release number of selected component (if applicable):
    ovirt-engine-dwh-4.2.4.3-1.el7ev.noarch

How reproducible:
    Check for the idle in transaction connections

Steps to Reproduce:
    select pid, query from pg_stat_activity where state='idle in transaction' and datname='ovirt_engine_history'


Actual results:
    The connection prevents the garbage collection

Expected results:
    The trnasaction is closed as soon as the aggregation is finished.

Additional info:
    After killing the process 17062, the dead rows are removed. The transaction can cause significant growth of the DB in some busy environments.