How to fix Could not open relation with OID in PostgreSQL

ERROR: could not open relation with OID 16391

SELECT oid, relname, reltype, relkind 
FROM pg_class 
WHERE oid = 16391;

sudo systemctl restart postgresql
# or
sudo service postgresql restart

-- List current connections
SELECT pid, usename, application_name, state FROM pg_stat_activity 
WHERE pid <> pg_backend_pid();

-- Terminate non-critical connections
SELECT pg_terminate_backend(pid) FROM pg_stat_activity 
WHERE pid <> pg_backend_pid() AND state = 'idle';

-- Check replication status on primary
SELECT slot_name, slot_type, active, confirmed_flush_lsn 
FROM pg_replication_slots;

-- View replication lag
SELECT client_addr, client_hostname, usename, application_name, 
       pg_lsn_diff(pg_current_wal_lsn(), replay_lsn) AS bytes_behind 
FROM pg_stat_replication;

-- Problematic: may fail with OID errors
SELECT schemaname, tablename, pg_size_pretty(pg_total_relation_size(oid)) 
FROM pg_tables 
LEFT JOIN pg_class ON relname = tablename;

-- Better: uses names directly, adds TOAST filter
SELECT schemaname, tablename, 
       pg_size_pretty(pg_total_relation_size(to_regclass(schemaname||"."||tablename)))
FROM pg_tables 
WHERE schemaname NOT IN ("pg_catalog", "information_schema");

-- For disk usage without TOAST issues
SELECT schemaname, tablename 
FROM pg_tables 
WHERE schemaname NOT IN ("pg_catalog", "information_schema") 
AND oid NOT IN (SELECT reltoastrelid FROM pg_class WHERE reltoastrelid != 0);

-- Look for orphaned references in pg_index
SELECT indexrelid, indrelid FROM pg_index 
WHERE indexrelid = 16391 AND indexrelid NOT IN (SELECT oid FROM pg_class);

-- Check pg_depend for broken references
SELECT objid, deptype FROM pg_depend 
WHERE objid = 16391 AND objid NOT IN (SELECT oid FROM pg_class);

-- If your query fails with OID error, simply retry
RETRY QUERY; -- Your original query