Nested Loops

Can somebody help me avoid nested loops in below query:
--
ap_poc_db=# explain (analyze,buffers)
ap_poc_db-# select site_id, account_id FROM ap.site_exposure se
ap_poc_db-# WHERE se.portfolio_id=-1191836
ap_poc_db-# AND EXISTS (select 1 from ap.catevent_flood_sc_split sp where sp.migration_sourcename= 'KatRisk_SC_Flood_2015_v9' AND ST_Intersects(se.shape, sp.shape))
ap_poc_db-# group by site_id, account_id;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Group (cost=23479854.04..23479880.06 rows=206 width=16) (actual time=1387.825..1389.134 rows=1532 loops=1)
Group Key: se.site_id, se.account_id
Buffers: shared hit=172041
-> Gather Merge (cost=23479854.04..23479879.04 rows=205 width=16) (actual time=1387.823..1388.676 rows=1532 loops=1)
Workers Planned: 5
Workers Launched: 5
Buffers: shared hit=172041
-> Group (cost=23478853.96..23478854.27 rows=41 width=16) (actual time=1346.044..1346.176 rows=255 loops=6)
Group Key: se.site_id, se.account_id
Buffers: shared hit=864280
-> Sort (cost=23478853.96..23478854.07 rows=41 width=16) (actual time=1346.041..1346.079 rows=255 loops=6)
Sort Key: se.site_id, se.account_id
Sort Method: quicksort Memory: 37kB
Buffers: shared hit=864280
-> Nested Loop Semi Join (cost=4.53..23478852.87 rows=41 width=16) (actual time=34.772..1345.489 rows=255 loops=6)
Buffers: shared hit=864235
-> Append (cost=0.00..156424.56 rows=123645 width=48) (actual time=1.011..204.748 rows=102990 loops=6)
Buffers: shared hit=154879
-> Parallel Seq Scan on site_exposure_1191836 se (cost=0.00..156424.56 rows=123645 width=48) (actual time=1.004..187.702 rows=102990 loops=6)
Filter: (portfolio_id = '-1191836'::integer)
Buffers: shared hit=154879
-> Bitmap Heap Scan on catevent_flood_sc_split sp (cost=4.53..188.54 rows=15 width=492) (actual time=0.007..0.007 rows=0 loops=617937)
Recheck Cond: (se.shape && shape)
Filter: ((migration_sourcename = 'KatRisk_SC_Flood_2015_v9'::bpchar) AND _st_intersects(se.shape, shape))
Rows Removed by Filter: 0
Heap Blocks: exact=1060
Buffers: shared hit=709356
-> Bitmap Index Scan on catevent_flood_sc_split_shape_mig_src_gix (cost=0.00..4.52 rows=45 width=0) (actual time=0.005..0.005 rows=0 loops=617937)
Index Cond: (se.shape && shape)
Buffers: shared hit=691115
Planning time: 116.141 ms
Execution time: 1391.785 ms
(32 rows)

ap_poc_db=#

Thank you in advance!

Regards,
Virendra

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