判斷哪些資料表目前適合進行自動資料清理

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，如果自上次VACUUM超過真空閾值以來已過時的組群數量，則會將資料表清空。

自動資料清理閾值已定義如下：

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， 是 ， 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;