# 현재 Autovacuum을 수행할 수 있는 테이블 결정
<a name="Appendix.PostgreSQL.CommonDBATasks.Autovacuum.EligibleTables"></a>

vacuum을 수행해야 하는 테이블이 하나이거나 두 개인 경우가 많습니다. `relfrozenxid` 값이 `autovacuum_freeze_max_age`의 트랜잭션 수보다 큰 테이블은 항상 Autovacuum의 대상이 됩니다. 그렇지 않은 경우 VACUUM이 "vacuum 임계값"을 초과하여 튜플 수가 더 이상 사용되지 않는 경우 테이블이 vacuum됩니다.

[autovacuum 임계값](https://www.postgresql.org/docs/current/static/routine-vacuuming.html#AUTOVACUUM)은 다음과 같이 정의되어 있습니다.

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

여기서 `vacuum base threshold`는 `autovacuum_vacuum_threshold`이고, `vacuum scale factor`는 `autovacuum_vacuum_scale_factor`이며, `number of tuples`는 `pg_class.reltuples`입니다.

데이터베이스에 연결되어 있는 상태에서 다음 쿼리를 실행하여 autovacuum이 vacuum 가능한 대상으로 분류하는 테이블 목록을 확인합니다.

```
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;
```