The query performance is very low + correlated subquery - oracle11g

Question

The query's aim is to get reserved rooms status is 3 or going to reserve in 30 to 45 minutes status in 2 or unreserved status in 1. reservation rooms are in RESEENH table and each reservation is in ORD_NOARCHIVE table which has begintime and endtime of reservation. So for each reservation room this query checks whether reservation is available at current time also its checks the meeting room parents and children. if children is reserved then parents are are blocked.
it takes 10 secs to fetch first 50 records.
with cte as
(
SELECT DISTINCT R.syscode,
R.behcode,
R.syscode AS FK_RESERVATIONUNIT, (
CASE
WHEN R.TYPE = 3 THEN '1'
WHEN R.TYPE = 1 THEN '2'
ELSE NULL
END ) AS LOCATION_TYPE,
R.sysobjalg AS FK_PROPERTY,
MP.syscode AS FK_MEASUREMENTPOINT,
MP.fk_plc_occupancy_state AS FK_PLC_OCCUPANCY_STATE,
F.syscode AS FK_FLOOR,
R.transitiontime,
r.type,
r.is_compoundreservationunit,
r.is_archived,
MP.fk_person,
os.transitionperiod
FROM reseenh R
--left outer join ordtrantbl RSS
--ON RSS.reservationunisyscode = R.syscode
left outer join objond F
ON F.syscode = R.fk_floor
left outer join pln_measurementpoint MP
ON MP.fk_reservationunit = R.syscode
AND MP.is_primary_measurement_point = 'T',
pln_ordersetting os
)
select cte.syscode,cte.behcode,cte.FK_RESERVATIONUNIT,
(CASE
WHEN O.begindatetime_user IS NULL THEN '1' --GREEN
WHEN O.begindatetime_user - (Nvl(cte.transitiontime, ( cte.transitionperiod ))/1440 ) > current_date THEN '2' -- ORANGE
WHEN O.begindatetime_user + (Nvl(cte.transitiontime, ( cte.transitionperiod )) /1440 ) > current_date THEN '3' -- RED
ELSE '3'
END ) AS LOCAVAILABILITY_STATUS_CODE,
cte.LOCATION_TYPE,
cte.FK_PROPERTY,
Coalesce(O.sysmelder, cte.fk_person) AS FK_PERSON,
O.syscode AS FK_ORDER,
O.ref_bostate_userdefined AS FK_ORDER_STATE_USER,
O.fk_bostate AS FK_ORDER_STATE_SYSTEM,
FK_MEASUREMENTPOINT,FK_PLC_OCCUPANCY_STATE,FK_FLOOR
from cte left outer join ord_noarchive O on O.syscode in
( SELECT MAX(ord.syscode) KEEP (DENSE_RANK FIRST ORDER BY ord.begindatetime_user) OVER (PARTITION BY ord.sysreseenh )
FROM ord_noarchive ORD
WHERE ( ( (
current_date >= ( ORD.begindatetime_user - ( Nvl(cte.transitiontime, ( cte.transitionperiod ))/1440) )
AND (
current_date - ( Nvl(cte.transitiontime, (cte.transitionperiod )) / 1440 ) ) <=ORD.enddatetime_user )
OR ( (
current_date + ( (
CASE
WHEN (
cte.TYPE = 1 ) THEN 30
ELSE 45
END ) / 1440 ) ) >= ( ORD.begindatetime_user - (Nvl(cte.transitiontime, ( cte.transitionperiod))/1440 ) )
AND (
current_date - ( Nvl(cte.transitiontime, ( cte.transitionperiod )) / 1440 ) ) < ORD.enddatetime_user ) )
AND ORD.sysreseenh IN
(
SELECT fk_reservationunit_parent
FROM pln_reservationunit_rut
WHERE fk_reservationunit_child IN
(
SELECT fk_reservationunit_child
FROM pln_reservationunit_rut
WHERE cte.is_compoundreservationunit = 'T'
AND fk_reservationunit_parent = cte.syscode)
UNION
SELECT cte.syscode
FROM dual
UNION
SELECT
CASE
WHEN cte.is_compoundreservationunit = 'T' THEN fk_reservationunit_child
ELSE fk_reservationunit_parent
END
FROM pln_reservationunit_rut
WHERE (
cte.is_compoundreservationunit = 'T'
AND fk_reservationunit_parent = cte.syscode )
OR (
cte.is_compoundreservationunit = 'F'
AND fk_reservationunit_child = cte.syscode ))
AND ORD.fk_bostate IN
(
SELECT syscode
FROM pln_bostate
WHERE pnname IN ( 'Requested',
'Made',
'AdministrativelyCompleted' )
AND fk_bodefinition = ref_bodefinition)
AND ORD.sysreseenh = O.sysreseenh
))
WHERE cte.is_archived = 'F'
AND cte.TYPE IN ( 1,
3 )
AND cte.fk_floor=495

Answer 1

No time to analyze the details of a very complex query, but my overall sense is that you are trying to do too many things at once. Separate the tasks. Check on room availability in one query, and check on the children/parent thing in a separate query.
Also, you could analyze the execution plan for the query and see what it bogging it down. My suspicion (again without time to really try to understand your query) is that at some point the mixing of tasks is translating into a many-to-many relationship where you have an intermediate result that is a cross product of rows between some of the tables in your query.