先看3个参数：
#如果表的事务ID年龄大于该值, 则数据库触发freeze,即使未开启autovacuum也会强制触发FREEZE
autovacuum_freeze_max_age | 500000

#在手动或自动垃圾回收时, 如果记录的事务ID年龄大于该值, 将被FREEZE
vacuum_freeze_min_age | 10

#在手动垃圾回收时, 如果表的事务ID年龄大于该值, 将触发FREEZE. 该参数的上限值为 %95 autovacuum_freeze_max_age
vacuum_freeze_table_age | 100000

首先如果pg_class表age(relfrozenxid)大于autovacuum_freeze_max_age，那么数据库系统会自动做vacuum，回收年龄

没有达到500000之前，不做vacuum

hank=>  select oid::regclass,age(relfrozenxid) from pg_class where relname='hank' and relowner=16392;
 oid  |  age   
------+--------
 hank | 495471
(1 row)

达到后，立即做vacuum

hank=>  select oid::regclass,age(relfrozenxid) from pg_class where relname='hank' and relowner=16392;
 oid  |  age   
------+--------
 hank | 504559
(1 row)

可见日志：

2017-08-07 17:23:44.193 CST,,,4774,,5988319d.12a6,2,,2017-08-07 17:23:41 CST,3/5308,0,LOG,00000,"automatic vacuum of table ""hank.hank.hank"": index scans: 0
pages: 0 removed, 1596 remain, 0 skipped due to pins, 0 skipped frozen
tuples: 0 removed, 307278 remain, 0 are dead but not yet removable
buffer usage: 3204 hits, 1 misses, 1278 dirtied
avg read rate: 0.003 MB/s, avg write rate: 3.386 MB/s
system usage: CPU 0.00s/0.05u sec elapsed 2.94 sec",,,,,,,,,""

vacuum后age回到vacuum_freeze_min_age所设置的值，注意这里做的是vacuum,不是vacuum freeze,如果手工做vacuum freeze，age(relfrozenxid)会降到0

hank=>  select oid::regclass,age(relfrozenxid) from pg_class where relname='hank' and relowner=16392;
 oid  | age 
------+-----
 hank |  10
(1 row)

这里手工freeze后，回到0开始计数

hank=> vacuum freeze hank;
VACUUM
hank=>  select oid::regclass,age(relfrozenxid) from pg_class where relname='hank' and relowner=16392;
 oid  | age 
------+-----
 hank |   0
(1 row)

另外对vacuum_freeze_table_age参数说明一下：
普通的vacuum由visibility map决定表的那些page需要被扫描，正常情况下，会跳过没有dead row的page,即使这些page有旧的xid值，所以，正常vacuum并不冻结表中所有行的xid,vacuum_freeze_table_age控制什么时候需要扫描全表，并冻结所有行的xid，如果事物数大于(vacuum_freeze_table_age-vacuum_freeze_min_age),那么就扫描所有可见的，没有冻结的page,如果vacuum_freeze_table_age设置为0，那么每次都会强制扫描所有page。