[PATCH] Fix memory leak in memoize for numeric key

Hello, all!

I found a query which consumes a lot of memory and triggers OOM killer.
Memory leak occurs in memoize node for numeric key.
Version postgresql is 14.9. The problem is very
similar https://www.postgresql.org/message-id/17844-d2f6f9e75a622bed@postgresql.org

I attached to the backend with a debugger and set a breakpoint in AllocSetAlloc

(gdb) bt 10
#0 AllocSetAlloc (context=0x5c55086dc2f0, size=12) at aset.c:722
#1 0x00005c5507d886e0 in palloc (size=size(at)entry=12) at mcxt.c:1082
#2 0x00005c5507890bba in detoast_attr (attr=0x715d5daa04c9) at detoast.c:184
#3 0x00005c5507d62375 in pg_detoast_datum (datum=<optimized out>) at fmgr.c:1725
#4 0x00005c5507cc94ea in hash_numeric (fcinfo=<optimized out>) at numeric.c:2554
#5 0x00005c5507d61570 in FunctionCall1Coll (flinfo=flinfo(at)entry=0x5c5508b93d00,
collation=<optimized out>, arg1=<optimized out>) at fmgr.c:1138
#6 0x00005c5507aadc16 in MemoizeHash_hash (key=0x0, tb=<optimized out>) at nodeMemoize.c:199
#7 0x00005c5507aadf22 in memoize_insert (key=0x0, found=<synthetic pointer>,
tb=0x5c5508bb4760) at ../../../src/include/lib/simplehash.h:762
#8 cache_lookup (found=<synthetic pointer>, mstate=0x5c5508b91418) at nodeMemoize.c:519
#9 ExecMemoize (pstate=0x5c5508b91418) at nodeMemoize.c:705

I was able to create reproducible test case on machine with default config
and postgresql 14.9:

CREATE TABLE table1 (
id numeric(38) NOT NULL,
col1 text,
CONSTRAINT id2 PRIMARY KEY (id)
);
CREATE TABLE table2 (
id numeric(38) NOT NULL,
id_table1 numeric(38) NULL,
CONSTRAINT id1 PRIMARY KEY (id)
);
ALTER TABLE table2 ADD CONSTRAINT constr1 FOREIGN KEY (id_table1) REFERENCES table1(id);

INSERT INTO table1 (id, col1)
SELECT id::numeric, id::text
FROM generate_series(3000000000, 3000000000 + 600000) gs(id);

INSERT INTO table2 (id, id_table1)
SELECT id::numeric , (select floor(random() * 600000)::numeric + 3000000000)::numeric
FROM generate_series(1,600000) gs(id);

set max_parallel_workers_per_gather=0;
set enable_hashjoin = off;

EXPLAIN analyze
select sum(q.id_table1)
from (
SELECT t2.*
FROM table1 t1
JOIN table2 t2
ON t2.id_table1 = t1.id) q;

Plan:
Aggregate (cost=25744.90..25744.91 rows=1 width=32) (actual time=380.140..380.142 rows=1 loops=1)
-> Nested Loop (cost=0.43..24244.90 rows=600000 width=9) (actual time=0.063..310.915 rows=600000 loops=1)
-> Seq Scan on table2 t2 (cost=0.00..9244.00 rows=600000 width=9) (actual time=0.009..38.629 rows=600000 loops=1)
-> Memoize (cost=0.43..0.47 rows=1 width=8) (actual time=0.000..0.000 rows=1 loops=600000)
Cache Key: t2.id_table1
Cache Mode: logical
Hits: 599999 Misses: 1 Evictions: 0 Overflows: 0 Memory Usage: 1kB
-> Index Only Scan using id2 on table1 t1 (cost=0.42..0.46 rows=1 width=8) (actual time=0.039..0.040 rows=1 loops=1)
Index Cond: (id = t2.id_table1)
Heap Fetches: 0
Planning Time: 0.445 ms
Execution Time: 380.750 ms

I've attached memoize_memory_leak_numeric_key.patch to address this.

Using test case, here are the memory stats before and after the
fix (taken during ExecEndMemoize by using MemoryContextStatsDetail(TopMemoryContext, 100, 1)).
Before:
ExecutorState: 25209672 total in 15 blocks; 1134432 free (7 chunks); 24075240 used
MemoizeHashTable: 8192 total in 1 blocks; 7480 free (1 chunks); 712 used

After:
ExecutorState: 76616 total in 5 blocks; 1776 free (8 chunks); 74840 used
MemoizeHashTable: 8192 total in 1 blocks; 7480 free (1 chunks); 712 used

Thanks,
Alexei Orlov
al(dot)orlov(at)cft(dot)ru,
aporlov(at)gmail(dot)com