WITH max_age AS (


SELECT 2000000000 as max_old_xid


, setting AS autovacuum_freeze_max_age


FROM pg_catalog.pg_settings


WHERE name = 'autovacuum_freeze_max_age' )


, per_database_stats AS (


SELECT datname


, m.max_old_xid::int


, m.autovacuum_freeze_max_age::int


, age(d.datfrozenxid) AS oldest_current_xid


FROM pg_catalog.pg_database d


JOIN max_age m ON (true)


WHERE d.datallowconn )


SELECT max(oldest_current_xid) AS oldest_current_xid


, max(ROUND(100*(oldest_current_xid/max_old_xid::float))) AS percent_towards_wraparound


, max(ROUND(100*(oldest_current_xid/autovacuum_freeze_max_age::float))) AS percent_towards_emergency_autovac


FROM per_database_stats;

percent_towards_wraparound ,percent_towards_emergency_autovac 的指标是啥含义，没看明白？

2 ----txid溢出后，在继续增长，当事务id的差值超过20亿，pg就无法判断那个事务在前，那个事务在后

这里这个20亿，怎么得到的 ？ 还有这句话如何理解（无法判断那个事务在前，那个事务在后）