Re: Query executed during pg_dump leads to excessive memory usage

Em dom., 19 de set. de 2021 às 07:05, Ulf Lohbrügge <
ulf(dot)lohbruegge(at)gmail(dot)com> escreveu:

> 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?
>
Can you try:

1. Limit resource usage by Postgres, with cgroups configuration.
2. pg_dump compression: man pgsql -Z
3. Run vacuum and reindex before?

regards,
Ranier Vilela