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# 判斷哪些資料表目前適合進行自動資料清理
<a name="Appendix.PostgreSQL.CommonDBATasks.Autovacuum.EligibleTables"></a>

通常有一或兩個資料表需要清理。表單中 `relfrozenxid` 的值大於 `autovacuum_freeze_max_age` 中交易的值，總是會成為自動資料清理的目標。否則，如果元組數因為最後一個 VACUUM 超出清理閾值而過時，則會清理資料表。

[自動資料清理閾值](https://www.postgresql.org/docs/current/static/routine-vacuuming.html#AUTOVACUUM)已定義如下：

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

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

當您連線到資料庫時，執行以下查詢可查看自動資料清理功能認為符合清理條件的資料表清單。

```
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;
```