Pager usage is off.
                              version                               
--------------------------------------------------------------------
 PostgreSQL 20devel on x86_64-linux, compiled by gcc-14.2.0, 64-bit
(1 row)

        name         | setting 
---------------------+---------
 enable_sort         | on
 hash_mem_multiplier | 2
 work_mem            | 4096
(3 rows)

psql:/repro/repro.sql:12: NOTICE:  00000: table "t0" does not exist, skipping
LOCATION:  DropErrorMsgNonExistent, tablecmds.c:1527
DROP TABLE
CREATE TABLE
== case 1: recursive UNION, inflated estimate (default settings)
You are now connected to database "postgres" as user "postgres".
SET
psql:/repro/repro.sql:23: ERROR:  53200: out of memory
DETAIL:  Failed on request of size 68719476736 in memory context "ExecutorState".
LOCATION:  MemoryContextAllocationFailure, mcxt.c:1208
psql:/repro/repro.sql:27: ERROR:  53200: out of memory
DETAIL:  Failed on request of size 68719476736 in memory context "ExecutorState".
LOCATION:  MemoryContextAllocationFailure, mcxt.c:1208
 peak_rss_mb 
-------------
          24
(1 row)

== control 1: same query, UNION ALL (no hash table)
You are now connected to database "postgres" as user "postgres".
 count 
-------
     0
(1 row)

 peak_rss_mb 
-------------
          21
(1 row)

== case 2: recursive UNION, accurate estimate of 1M rows
You are now connected to database "postgres" as user "postgres".
  count  
---------
 1000000
(1 row)

 peak_rss_mb 
-------------
        1094
(1 row)

== control 2: same rows, UNION ALL
You are now connected to database "postgres" as user "postgres".
  count  
---------
 1000000
(1 row)

 peak_rss_mb 
-------------
          33
(1 row)

== case 3: hashed EXCEPT chosen while disabled (enable_sort = off)
You are now connected to database "postgres" as user "postgres".
SET
SET
psql:/repro/repro.sql:59: ERROR:  53200: out of memory
DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".
LOCATION:  MemoryContextAllocationFailure, mcxt.c:1208
psql:/repro/repro.sql:61: ERROR:  53200: out of memory
DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".
LOCATION:  MemoryContextAllocationFailure, mcxt.c:1208
 peak_rss_mb 
-------------
          24
(1 row)

== control 3: same query, default settings (sorted SetOp)
You are now connected to database "postgres" as user "postgres".
                                         QUERY PLAN                                         
--------------------------------------------------------------------------------------------
 SetOp Except  (cost=15879164969983.26..16407700298108.28 rows=1600000000 width=16)
   ->  Sort  (cost=15879164969983.26..15984871235608.26 rows=42282506250000 width=16)
         Sort Key: a.c0, b.c0, c.c0, d.c0
         ->  Nested Loop  (cost=0.00..528738676754.88 rows=42282506250000 width=16)
               ->  Nested Loop  (cost=0.00..207348588.00 rows=16581375000 width=12)
                     ->  Nested Loop  (cost=0.00..81358.62 rows=6502500 width=8)
                           ->  Seq Scan on t0 a  (cost=0.00..35.50 rows=2550 width=4)
                           ->  Materialize  (cost=0.00..48.25 rows=2550 width=4)
                                 ->  Seq Scan on t0 b  (cost=0.00..35.50 rows=2550 width=4)
                     ->  Materialize  (cost=0.00..48.25 rows=2550 width=4)
                           ->  Seq Scan on t0 c  (cost=0.00..35.50 rows=2550 width=4)
               ->  Materialize  (cost=0.00..48.25 rows=2550 width=4)
                     ->  Seq Scan on t0 d  (cost=0.00..35.50 rows=2550 width=4)
   ->  Result  (cost=0.00..0.01 rows=1 width=16)
(14 rows)

 c0 | c0 | c0 | c0 
----+----+----+----
(0 rows)

 peak_rss_mb 
-------------
          22
(1 row)

== case 4: hashed EXCEPT, default settings, hash-only type (xid)
You are now connected to database "postgres" as user "postgres".
psql:/repro/repro.sql:75: ERROR:  53200: out of memory
DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".
LOCATION:  MemoryContextAllocationFailure, mcxt.c:1208
psql:/repro/repro.sql:77: ERROR:  53200: out of memory
DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".
LOCATION:  MemoryContextAllocationFailure, mcxt.c:1208
 peak_rss_mb 
-------------
          22
(1 row)

== server log (errors and backtraces)
2026-10-15 13:51:49.035 UTC [42] ERROR:  out of memory
2026-10-15 13:51:49.035 UTC [42] DETAIL:  Failed on request of size 68719476736 in memory context "ExecutorState".
	postgres: postgres postgres [local] EXPLAIN(BuildTupleHashTable+0x164) [0x609aed0af5dd]
	postgres: postgres postgres [local] EXPLAIN(ExecInitRecursiveUnion+0x2c7) [0x609aed10b85d]
2026-10-15 13:51:49.038 UTC [42] ERROR:  out of memory
2026-10-15 13:51:49.038 UTC [42] DETAIL:  Failed on request of size 68719476736 in memory context "ExecutorState".
	postgres: postgres postgres [local] SELECT(BuildTupleHashTable+0x164) [0x609aed0af5dd]
	postgres: postgres postgres [local] SELECT(ExecInitRecursiveUnion+0x2c7) [0x609aed10b85d]
2026-10-15 13:51:51.715 UTC [46] ERROR:  out of memory
2026-10-15 13:51:51.715 UTC [46] DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".
	postgres: postgres postgres [local] EXPLAIN(BuildTupleHashTable+0x164) [0x609aed0af5dd]
	postgres: postgres postgres [local] EXPLAIN(ExecInitSetOp+0x314) [0x609aed10f598]
2026-10-15 13:51:51.718 UTC [46] ERROR:  out of memory
2026-10-15 13:51:51.718 UTC [46] DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".
	postgres: postgres postgres [local] SELECT(BuildTupleHashTable+0x164) [0x609aed0af5dd]
	postgres: postgres postgres [local] SELECT(ExecInitSetOp+0x314) [0x609aed10f598]
2026-10-15 13:51:51.729 UTC [48] ERROR:  out of memory
2026-10-15 13:51:51.729 UTC [48] DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".
2026-10-15 13:51:51.731 UTC [48] ERROR:  out of memory
2026-10-15 13:51:51.731 UTC [48] DETAIL:  Failed on request of size 34359738368 in memory context "ExecutorState".