Transaction ID wraparound approaching — emergency prevention

The age(datfrozenxid) value for one or more databases is approaching the 2-billion-transaction safety threshold — failure to freeze will eventually cause PostgreSQL to enter read-only mode to prevent data corruption.

Diagnose it

-- XID age by database (run as superuser on each database cluster):
SELECT datname,
       age(datfrozenxid)                          AS xid_age,
       2147483647 - age(datfrozenxid)             AS xids_remaining,
       datfrozenxid
FROM pg_database
ORDER BY xid_age DESC;

-- Per-table view (on the specific database):
SELECT n.nspname || '.' || c.relname             AS table_name,
       age(c.relfrozenxid)                       AS table_xid_age,
       pg_size_pretty(pg_total_relation_size(c.oid)) AS total_size
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind = 'r'
  AND age(c.relfrozenxid) > 1500000000
ORDER BY table_xid_age DESC;

Guidance thresholds: alert at 1.5B (ages above this trigger PostgreSQL warnings
in the log); emergency VACUUM at 1.9B; PostgreSQL forces shutdown at 2.1B to prevent
data loss.

Why it happens

PostgreSQL transaction IDs are 32-bit unsigned integers. After 2 billion transactions, they
wrap around. To prevent this from causing data corruption, VACUUM must periodically
freeze old tuple versions, replacing their real XID with a special frozen XID that
is always considered visible. The datfrozenxid in pg_database
records the oldest non-frozen XID in the database.