# 确定哪些表当前符合 Autovacuum 条件
<a name="Appendix.PostgreSQL.CommonDBATasks.Autovacuum.EligibleTables"></a>

通常，它是需要执行 vacuum 操作的一个或两个表。其 `relfrozenxid` 值大于 `autovacuum_freeze_max_age` 中的事务数的表始终是 Autovacuum 的目标。否则，如果元组数因上一个 VACUUM 超出 vacuum 阈值而变得过时，则对表执行 vacuum 操作。

[Autovacuum 阈值](https://www.postgresql.org/docs/current/static/routine-vacuuming.html#AUTOVACUUM)的定义如下：

```
Vacuum-threshold = vacuum-base-threshold + vacuum-scale-factor * number-of-tuples
```

其中，`vacuum base threshold` 为 `autovacuum_vacuum_threshold`，`vacuum scale factor` 为 `autovacuum_vacuum_scale_factor`，而 `number of tuples` 为 `pg_class.reltuples`。

在连接到数据库时，运行以下查询可查看 autovacuum 认为有资格执行 vacuum 操作的表的列表。

```
WITH vbt AS (SELECT setting AS autovacuum_vacuum_threshold FROM 
pg_settings WHERE name = 'autovacuum_vacuum_threshold'),
vsf AS (SELECT setting AS autovacuum_vacuum_scale_factor FROM 
pg_settings WHERE name = 'autovacuum_vacuum_scale_factor'), 
fma AS (SELECT setting AS autovacuum_freeze_max_age FROM pg_settings WHERE name = 'autovacuum_freeze_max_age'),
sto AS (select opt_oid, split_part(setting, '=', 1) as param,
split_part(setting, '=', 2) as value from (select oid opt_oid, unnest(reloptions) setting from pg_class) opt)
SELECT '"'||ns.nspname||'"."'||c.relname||'"' as relation,
pg_size_pretty(pg_table_size(c.oid)) as table_size,
age(relfrozenxid) as xid_age,
coalesce(cfma.value::float, autovacuum_freeze_max_age::float) autovacuum_freeze_max_age,
(coalesce(cvbt.value::float, autovacuum_vacuum_threshold::float) +
coalesce(cvsf.value::float,autovacuum_vacuum_scale_factor::float) * c.reltuples)
AS autovacuum_vacuum_tuples, n_dead_tup as dead_tuples FROM
pg_class c join pg_namespace ns on ns.oid = c.relnamespace 
join pg_stat_all_tables stat on stat.relid = c.oid join vbt on (1=1) join vsf on (1=1) join fma on (1=1)
left join sto cvbt on cvbt.param = 'autovacuum_vacuum_threshold' and c.oid = cvbt.opt_oid 
left join sto cvsf on cvsf.param = 'autovacuum_vacuum_scale_factor' and c.oid = cvsf.opt_oid
left join sto cfma on cfma.param = 'autovacuum_freeze_max_age' and c.oid = cfma.opt_oid
WHERE c.relkind = 'r' and nspname <> 'pg_catalog'
AND (age(relfrozenxid) >= coalesce(cfma.value::float, autovacuum_freeze_max_age::float)
OR coalesce(cvbt.value::float, autovacuum_vacuum_threshold::float) + 
coalesce(cvsf.value::float,autovacuum_vacuum_scale_factor::float) * 
c.reltuples <= n_dead_tup)
ORDER BY age(relfrozenxid) DESC LIMIT 50;
```