os: centos 7.4
db: postgresql 10.10

postgresql 查看 pg_stat_activity 有时会看到 autovacuum：VACUUM xxoo.xxoo(to prevent wraparound) ,这个其实是预防事务ID回卷，原因是 postgresql 采用 32bit 事务id。

版本

# cat /etc/centos-release
CentOS Linux release 7.4.1708 (Core) 
# 
# su - postgres
Last login: Sat Oct 26 22:55:25 CST 2019 on pts/0
$
$ psql -c "select version();"
                                                 version                                                  
----------------------------------------------------------------------------------------------------------
 PostgreSQL 10.10 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36), 64-bit
(1 row)

when

当表的 age 大于 autovacuum_freeze_max_age时，autovacuum 进程回自动对该表进行 freeze。

autovacuum：VACUUM public.tmp_t0 (to prevent wraparound)

默认的 autovacuum_freeze_max_age 值为 200000000

postgres=# select name,setting from pg_settings where name like 'autovacuum%' order by name;
                name                 |  setting  
-------------------------------------+-----------
 autovacuum                          | on
 autovacuum_analyze_scale_factor     | 0.1
 autovacuum_analyze_threshold        | 50
 autovacuum_freeze_max_age           | 200000000
 autovacuum_max_workers              | 3
 autovacuum_multixact_freeze_max_age | 400000000
 autovacuum_naptime                  | 60
 autovacuum_vacuum_cost_delay        | 20
 autovacuum_vacuum_cost_limit        | -1
 autovacuum_vacuum_scale_factor      | 0.2
 autovacuum_vacuum_threshold         | 50
 autovacuum_work_mem                 | -1
(12 rows)

SELECT c.oid::regclass as table_name,
       greatest(age(c.relfrozenxid),age(t.relfrozenxid)) as age
from pg_class c
     LEFT OUTER JOIN pg_class t ON c.reltoastrelid = t.oid
WHERE c.relkind IN ('r', 'm')
order by 2 desc
;

SELECT datname, age(datfrozenxid) FROM pg_database;

参考:
https://www.postgresql.org/docs/10/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND