Convert keep dense_rank from Oracle query into pos

I'm trying to convert the following Oracle query into Postgres

select
        this_.GLOBAL_TRANSACTION_ID as y0_,
        this_.BUSINESS_IDENTIFIER as y1_,
        this_.ENVIRONMENT as y2_,
        count(*) as y3_,
        this_.HOST_NAME as y4_,
        listagg(process,
        ', ') within
    group (order by
        date_time) as process,
        min(this_.DATE_TIME) as y6_,
        max(this_.DATE_TIME) as y7_,
        max(status)keep(dense_rank last
    order by
        date_time,
        decode(status,
        'COMPLETED',
        'd',
        'FAILED',
        'c',
        'TERMINATED',
        'b',
        'STARTED',
        'a',
        'z')) as status
    from
        ACTIVITY_MONITOR_TRANSACTION this_
    where
        this_.DATE_TIME between ? and ?
        and 1=1
    group by
        this_.GLOBAL_TRANSACTION_ID,
        this_.BUSINESS_IDENTIFIER,
        this_.ENVIRONMENT,
        this_.HOST_NAME,
        global_transaction_id,
        business_identifier,
        global_transaction_id,
        business_identifier
    order by
        y7_ asc

the problem is I don't know how to convert this block:

max(status)keep(dense_rank last
    order by
        date_time,
        decode(status,
        'COMPLETED',
        'd',
        'FAILED',
        'c',
        'TERMINATED',
        'b',
        'STARTED',
        'a',
        'z')) as status

The aim of this block is to get the latest status, and in case of exact same time (it is possible!) assign the status following the order above.

This is an example of data:
      ID        DATA_TIME          GLOBAL_TRANSACTION_ID   STATUS
===================================================================
 54938456;"2015-04-20 09:39:27";"8d276718-eca7-4fd0-a266 ;"STARTED"
 54938505;"2015-04-20 09:39:27";"8d276718-eca7-4fd0-a266 ;"COMPLETED"
 54938507;"2015-04-20 09:39:27";"8d276718-eca7-4fd0-a266 ;"FAILED"
 54938507;"2015-04-20 09:38:25";"8d276718-eca7-4fd0-a266 ;"FAILED"

The status should be "COMPLETED" so my query should return, among other rows, the following:

 GLOBAL_TRANSACTION_ID    COUNT    (...)       STATUS
=====================================================
 8d276718-eca7-4fd0-a266    4      (...)     COMPLETED

I have tried splitting the query into 2:

select
    this_.GLOBAL_TRANSACTION_ID as y0_,
    this_.BUSINESS_IDENTIFIER as y1_,
    this_.ENVIRONMENT as y2_,
    count(*) as y3_,
    this_.HOST_NAME as y4_,
    array_to_string(array_agg(distinct process),
    ',') as process,
    min(this_.DATE_TIME) as y6_,
    max(this_.DATE_TIME) as y7_,
    max(this_.STATUS) as y8_
from
    ACTIVITY_MONITOR_TRANSACTION this_
where
    this_.DATE_TIME between ? and ?
group by
    this_.GLOBAL_TRANSACTION_ID,
    this_.BUSINESS_IDENTIFIER,
    this_.ENVIRONMENT,
    this_.HOST_NAME,
    global_transaction_id,
    business_identifier
order by
    y7_ desc limit ?

and then

select
    status
from
    activity_monitor_transaction
where
    GLOBAL_TRANSACTION_ID=?
order by
    date_time DESC,
    CASE status
        WHEN 'COMPLETED'THEN 'd'
        WHEN 'FAILED' THEN 'c'
        WHEN 'TERMINATED' THEN 'b'
        WHEN 'STARTED' THEN 'a'
        ELSE 'z'
    END DESC LIMIT 1

But this is causing me performance issues as I have to execute the second query once per row.

This is the table script for postgres:

CREATE TABLE activity_monitor_transaction
(
  id numeric(11,0) NOT NULL,
  date_time timestamp(6) without time zone NOT NULL,
  global_transaction_id character varying(40) NOT NULL,
  repost_flag character(1) NOT NULL DEFAULT 'N'::bpchar,
  environment character varying(20),
  transaction_mode character varying(20),
  status character varying(20),
  step character varying(80),
  event character varying(20),
  event_code character varying(20),
  event_subcode character varying(20),
  summary character varying(200),
  business_identifier character varying(80),
  alternate_business_identifier character varying(80),
  domain character varying(20),
  process character varying(80),
  service_name character varying(80),
  service_version character varying(20),
  detail text,
  app_name character varying(80),
  app_user character varying(20),
  host_name character varying(80),
  thread_name character varying(200),
  CONSTRAINT activity_monitor_transact_pk PRIMARY KEY (id)
  USING INDEX TABLESPACE actmon_data
)

and some data:

insert into ACTIVITY_MONITOR_TRANSACTION values 
(54938456,'2015-04-20 09:39:27','8d276718-eca7-4fd0-a266-d465181f911a','N','Perf','','STARTED','servicereq.p2p.rso.blaze.dedup.in.channel','PROCESS','','','','3100729','51174628','ERP','servicereq-p2p-rso-blaze','servicereq-p2p-rso-blaze','1.0.0-SNAPSHOT','','servicereq-p2p-rso-blaze','CIC','intintprf20','SimpleAsyncTaskExecutor-88177');

insert into ACTIVITY_MONITOR_TRANSACTION values 
(54938505,'2015-04-20 09:45:27','8d276718-eca7-4fd0-a266-d465181f911a','N','Perf','','COMPLETED','servicereq.p2p.rso.blaze.service.out.channel','PROCESS','','','','3100729','51174628','ERP','servicereq-p2p-rso-blaze','servicereq-p2p-rso-blaze','1.0.0-SNAPSHOT','','servicereq-p2p-rso-blaze','CIC','intintprf20','SimpleAsyncTaskExecutor-88177');

insert into ACTIVITY_MONITOR_TRANSACTION values 
(54938507,'2015-04-20 09:45:27','8d276718-eca7-4fd0-a266-d465181f911a','N','Perf','','FAILED','inputChannel','PROCESS','','','','3100729','','ERP','servicereq-p2p-rso-blaze','servicereq-p2p-rso-blaze','1.0.0-SNAPSHOT','','servicereq-p2p-rso-blaze','CIC','intintprf20','SimpleAsyncTaskExecutor-88177');

Is there any way to mimic the keep dense_rank block into postgres in order to have just one query?

You can use PostgreSQL WINDOW FUNCTIONS

-- we only added infos to the activity_monitor_transaction
-- we are free to group by date_time or status
SELECT
  first_value(status) OVER w AS global_transaction_status,
  count(*) OVER w AS global_transaction_count,
  activity_monitor_transaction.*
FROM
  activity_monitor_transaction
WINDOW w AS (
  PARTITION BY global_transaction_id
  ORDER BY date_time DESC, id DESC
  ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
)