Query executed during pg_dump leads to excessive memory usage

Hi there,

A database cluster (PostgreSQL 12.4 running on Amazon Aurora @
db.r5.xlarge) with a single database of mine consists of 1,656,618 rows in
pg_class. Using pg_dump on that database leads to excessive memory usage
and sometimes even a kill by signal 9:

2021-09-18 16:51:24 UTC::@:[29787]:LOG: Aurora Runtime process (PID 29794)
was terminated by signal 9: Killed
2021-09-18 16:51:25 UTC::@:[29787]:LOG: terminating any other active
server processes
2021-09-18 16:51:27 UTC::@:[29787]:FATAL: Can't handle storage runtime
process crash
2021-09-18 16:51:31 UTC::@:[29787]:LOG: database system is shut down

The query that is being fired by pg_dump is the following:
SELECT t.tableoid, t.oid, t.typname, t.typnamespace, (SELECT
pg_catalog.array_agg(acl ORDER BY row_n) FROM (SELECT acl, row_n FROM
pg_catalog.unnest(coalesce(t.typacl,pg_catalog.acldefault('T',t.typowner)))
WITH ORDINALITY AS perm(acl,row_n) WHERE NOT EXISTS ( SELECT 1 FROM
pg_catalog.unnest(coalesce(pip.initprivs,pg_catalog.acldefault('T',t.typowner)))
AS init(init_acl) WHERE acl = init_acl)) as foo) AS typacl, (SELECT
pg_catalog.array_agg(acl ORDER BY row_n) FROM (SELECT acl, row_n FROM
pg_catalog.unnest(coalesce(pip.initprivs,pg_catalog.acldefault('T',t.typowner)))
WITH ORDINALITY AS initp(acl,row_n) WHERE NOT EXISTS ( SELECT 1 FROM
pg_catalog.unnest(coalesce(t.typacl,pg_catalog.acldefault('T',t.typowner)))
AS permp(orig_acl) WHERE acl = orig_acl)) as foo) AS rtypacl, NULL AS
inittypacl, NULL AS initrtypacl, (SELECT rolname FROM pg_catalog.pg_roles
WHERE oid = t.typowner) AS rolname, t.typelem, t.typrelid, CASE WHEN
t.typrelid = 0 THEN ' '::"char" ELSE (SELECT relkind FROM pg_class WHERE
oid = t.typrelid) END AS typrelkind, t.typtype, t.typisdefined,
t.typname[0] = '_' AND t.typelem != 0 AND (SELECT typarray FROM pg_type te
WHERE oid = t.typelem) = t.oid AS isarray FROM pg_type t LEFT JOIN
pg_init_privs pip ON (t.oid = pip.objoid AND pip.classoid =
'pg_type'::regclass AND pip.objsubid = 0);

The query plan looks like this. It takes almost 13 minutes(!) to execute
that query:
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------
Hash Left Join (cost=4.65..8147153.76 rows=1017962 width=280) (actual
time=2.526..106999.294 rows=1026902 loops=1)
Hash Cond: (t.oid = pip.objoid)
-> Seq Scan on pg_type t (cost=0.00..36409.62 rows=1017962 width=122)
(actual time=0.008..8836.693 rows=1026902 loops=1)
-> Hash (cost=4.64..4.64 rows=1 width=45) (actual time=2.342..41.972
rows=0 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 8kB
-> Seq Scan on pg_init_privs pip (cost=0.00..4.64 rows=1
width=45) (actual time=2.341..22.109 rows=0 loops=1)
Filter: ((classoid = '1247'::oid) AND (objsubid = 0))
Rows Removed by Filter: 176
SubPlan 1
-> Aggregate (cost=0.38..0.39 rows=1 width=32) (actual
time=0.031..0.031 rows=1 loops=1026902)
-> Hash Anti Join (cost=0.24..0.37 rows=1 width=20) (actual
time=0.008..0.008 rows=0 loops=1026902)
Hash Cond: (perm.acl = init.init_acl)
-> Function Scan on unnest perm (cost=0.01..0.11 rows=10
width=20) (actual time=0.001..0.001 rows=2 loops=1026902)
-> Hash (cost=0.11..0.11 rows=10 width=12) (actual
time=0.002..0.002 rows=2 loops=1026902)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Function Scan on unnest init (cost=0.01..0.11
rows=10 width=12) (actual time=0.001..0.001 rows=2 loops=1026902)
SubPlan 2
-> Aggregate (cost=0.38..0.39 rows=1 width=32) (actual
time=0.050..0.050 rows=1 loops=1026902)
-> Hash Anti Join (cost=0.24..0.37 rows=1 width=20) (actual
time=0.008..0.008 rows=0 loops=1026902)
Hash Cond: (initp.acl = permp.orig_acl)
-> Function Scan on unnest initp (cost=0.01..0.11
rows=10 width=20) (actual time=0.001..0.001 rows=2 loops=1026902)
-> Hash (cost=0.11..0.11 rows=10 width=12) (actual
time=0.002..0.002 rows=2 loops=1026902)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Function Scan on unnest permp (cost=0.01..0.11
rows=10 width=12) (actual time=0.001..0.001 rows=2 loops=1026902)
SubPlan 3
-> Index Scan using pg_authid_oid_index on pg_authid
(cost=0.28..2.29 rows=1 width=64) (actual time=0.002..0.002 rows=1
loops=1026902)
Index Cond: (oid = t.typowner)
SubPlan 4
-> Index Scan using pg_class_oid_index on pg_class (cost=0.43..2.45
rows=1 width=1) (actual time=0.003..0.003 rows=1 loops=671368)
Index Cond: (oid = t.typrelid)
SubPlan 5
-> Index Scan using pg_type_oid_index on pg_type te (cost=0.42..2.44
rows=1 width=4) (actual time=0.020..0.020 rows=1 loops=355428)
Index Cond: (oid = t.typelem)
Planning Time: 0.535 ms
Execution Time: 774011.175 ms
(35 rows)

The high number of rows in pg_class result from more than ~550 schemata,
each containing more than 600 tables. It's part of a multi tenant setup
where each tenant lives in its own schema.

I began to move schemata to another database cluster to reduce the number
of rows in pg_class but I'm having a hard time doing so as a call to
pg_dump might result in a database restart.

Is there anything I can do to improve that situation? Next thing that comes
to my mind is to distribute those ~550 schemata over 5 to 6 databases in
one database cluster instead of having one single database.

Best regards
Ulf