Multiple but not all executions of the same Postgres SELECT query are getting locked endlessly

I am using Postgresql version **9.2.18** on Centos Version **CentOS Linux release 7.3.1611 (Core)** In postgres there is a select query on a single table which is getting executed multiple times per minute. For this query around first 20 or 21 executions are getting locked endlessly where as other executions are running fine. When I running below command to check the queries running for more than a minute than these queries are not showing:

pid, pg_stat_activity.query_start, now() - pg_stat_activity.query_start AS query_time,  query,  state from pg_stat_activity WHERE (now() - pg_stat_activity.query_start) > interval '1 minutes';

But on running the query

* FROM pg_locks;

Following is the result (pasted few lines):

locktype  | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction |  pid  |      mode       | g
ranted | fastpath
------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+-----------------+--
-------+----------

 relation   |    16385 |  4614041 |      |       |            |               |         |       |          | 23/15040           | 55538 | AccessShareLock | t
       | t

 relation   |    16385 |  4233898 |      |       |            |               |         |       |          | 23/15040           | 55538 | AccessShareLock | t
       | t

 relation   |    16385 |  4207899 |      |       |            |               |         |       |          | 23/15040           | 55538 | AccessShareLock | t
       | t

 relation   |    16385 |   682008 |      |       |            |               |         |       |          | 23/15040           | 55538 | AccessShareLock | t
       | t

 relation   |    16385 |    17702 |      |       |            |               |         |       |          | 23/15040           | 55538 | AccessShareLock | t
       | t

 relation   |    16385 |    17309 |      |       |            |               |         |       |          | 23/15040           | 55538 | AccessShareLock | t
       | t

 virtualxid |          |          |      |       | 23/15040   |               |         |       |          | 23/15040           | 55538 | ExclusiveLock   | t
       | t

 relation   |    16385 |  4614041 |      |       |            |               |         |       |          | 22/15040           | 55537 | AccessShareLock | t
       | t

 relation   |    16385 |  4233898 |      |       |            |               |         |       |          | 22/15040           | 55537 | AccessShareLock | t
       | t

 relation   |    16385 |  4207899 |      |       |            |               |         |       |          | 22/15040           | 55537 | AccessShareLock | t
       | t

 relation   |    16385 |   682008 |      |       |            |               |         |       |          | 22/15040           | 55537 | AccessShareLock | t
       | t

 relation   |    16385 |    17702 |      |       |            |               |         |       |          | 22/15040           | 55537 | AccessShareLock | t
       | t

 relation   |    16385 |    17309 |      |       |            |               |         |       |          | 22/15040           | 55537 | AccessShareLock | t
       | t

 virtualxid |          |          |      |       | 22/15040   |               |         |       |          | 22/15040           | 55537 | ExclusiveLock   | t
       | t

 relation   |    16385 |  4614041 |      |       |            |               |         |       |          | 21/15040           | 55536 | AccessShareLock | t
       | t

 relation   |    16385 |  4233898 |      |       |            |               |         |       |          | 21/15040           | 55536 | AccessShareLock | t
       | t

 relation   |    16385 |  4207899 |      |       |            |               |         |       |          | 21/15040           | 55536 | AccessShareLock | t
       | t

 relation   |    16385 |   682008 |      |       |            |               |         |       |          | 21/15040           | 55536 | AccessShareLock | t
       | t

 relation   |    16385 |    17702 |      |       |            |               |         |       |          | 21/15040           | 55536 | AccessShareLock | t
       | t

 relation   |    16385 |    17309 |      |       |            |               |         |       |          | 21/15040           | 55536 | AccessShareLock | t
       | t

 virtualxid |          |          |      |       | 21/15040   |               |         |       |          | 21/15040           | 55536 | ExclusiveLock   | t
       | t

**This result is showing the locked processes which are the locked queries. Next the result of command

-ef | grep SELECT

is as follows:**

postgres  55519    997 47 07:09 ?        01:52:23 postgres: pgdbuser dam_stage 15.206.133.156(33248) SELECT
postgres  55520    997 47 07:09 ?        01:52:16 postgres: pgdbuser dam_stage 15.206.133.156(33250) SELECT
postgres  55521    997 47 07:09 ?        01:52:16 postgres: pgdbuser dam_stage 15.206.133.156(33254) SELECT
postgres  55522    997 47 07:09 ?        01:52:18 postgres: pgdbuser dam_stage 15.206.133.156(33258) SELECT
postgres  55523    997 47 07:09 ?        01:52:18 postgres: pgdbuser dam_stage 15.206.133.156(33256) SELECT
postgres  55524    997 47 07:09 ?        01:52:27 postgres: pgdbuser dam_stage 15.206.133.156(33252) SELECT
postgres  55525    997 47 07:09 ?        01:52:23 postgres: pgdbuser dam_stage 15.206.133.156(33260) SELECT
postgres  55526    997 47 07:09 ?        01:52:16 postgres: pgdbuser dam_stage 15.206.133.156(33264) SELECT
postgres  55527    997 47 07:09 ?        01:52:19 postgres: pgdbuser dam_stage 15.206.133.156(33262) SELECT
postgres  55528    997 47 07:09 ?        01:52:21 postgres: pgdbuser dam_stage 15.206.133.156(33268) SELECT
postgres  55529    997 47 07:09 ?        01:52:26 postgres: pgdbuser dam_stage 15.206.133.156(33270) SELECT
postgres  55530    997 48 07:09 ?        01:52:37 postgres: pgdbuser dam_stage 15.206.133.156(33276) SELECT
postgres  55531    997 47 07:09 ?        01:52:21 postgres: pgdbuser dam_stage 15.206.133.156(33266) SELECT
postgres  55532    997 47 07:09 ?        01:52:24 postgres: pgdbuser dam_stage 15.206.133.156(33272) SELECT
postgres  55533    997 47 07:09 ?        01:52:23 postgres: pgdbuser dam_stage 15.206.133.156(33274) SELECT
postgres  55534    997 47 07:09 ?        01:52:17 postgres: pgdbuser dam_stage 15.206.133.156(33278) SELECT
postgres  55535    997 47 07:09 ?        01:52:19 postgres: pgdbuser dam_stage 15.206.133.156(33284) SELECT
postgres  55536    997 47 07:09 ?        01:52:32 postgres: pgdbuser dam_stage 15.206.133.156(33282) SELECT
postgres  55537    997 47 07:09 ?        01:52:20 postgres: pgdbuser dam_stage 15.206.133.156(33280) SELECT
postgres  55538    997 47 07:09 ?        01:52:20 postgres: pgdbuser dam_stage 15.206.133.156(33286) SELECT
root      81425  79781  0 11:03 pts/2    00:00:00 grep --color=auto SELECT

**If I kill these queries/processes then they are replaced by new instances of same queries.** Please suggest what could be the cause of this and how to rectify this issue? **EDIT:** The query is like this *SELECT "id", "reference_code", "completed_at", "data_json" FROM "queue" WHERE "status" = 1 AND "reference_type" LIKE '%sorted%' ORDER BY "completed_at" DESC LIMIT 1* **These processes are also being shown in the top command like this(pid's are different as previous one were killed):**

PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND

 82894 postgres  20   0  236748  39812  37396 R  49.8  0.1 678:23.38 postgres

 82890 postgres  20   0  236748  39800  37384 S  49.2  0.1 677:12.88 postgres

 82892 postgres  20   0  236748  39764  37348 R  49.2  0.1 677:09.95 postgres

 82895 postgres  20   0  236748  39840  37424 S  49.2  0.1 677:23.80 postgres

 82896 postgres  20   0  236748  39744  37328 R  49.2  0.1 679:36.06 postgres

 82899 postgres  20   0  236748  39816  37400 R  49.2  0.1 676:39.73 postgres

 82900 postgres  20   0  236748  39800  37384 R  49.2  0.1 677:25.23 postgres

 82901 postgres  20   0  236748  39800  37384 S  49.2  0.1 677:28.81 postgres

 82902 postgres  20   0  236748  39776  37360 S  49.2  0.1 677:27.31 postgres

 82903 postgres  20   0  236748  39788  37372 S  49.2  0.1 676:54.77 postgres

 82906 postgres  20   0  236748  39904  37488 S  49.2  0.1 677:55.78 postgres

 82907 postgres  20   0  236748  39896  37480 S  49.2  0.1 677:51.72 postgres

 82908 postgres  20   0  236748  39780  37364 S  49.2  0.1 677:27.25 postgres

 82891 postgres  20   0  236748  39844  37428 R  48.8  0.1 677:20.41 postgres

 82893 postgres  20   0  236748  39812  37396 R  48.8  0.1 677:22.60 postgres

 82898 postgres  20   0  236748  39800  37384 R  48.8  0.1 677:06.41 postgres

 82904 postgres  20   0  236748  39800  37384 S  48.8  0.1 677:45.52 postgres

 82905 postgres  20   0  236748  39696  37280 S  48.8  0.1 677:42.04 postgres

 82909 postgres  20   0  236748  39884  37468 R  48.8  0.1 678:16.44 postgres

 82897 postgres  20   0  236748  39708  37292 S  48.5  0.1 677:20.17 postgres