Useful diagnostic SQL queries ​

Start with read-only observations that identify which database and requests are involved. Through the server, use the system qualifier for diagnostic tables; an unqualified application table with the same name is a different object.

SELECT query_id, user, elapsed, query_kind
FROM system.processes
ORDER BY elapsed DESC
LIMIT 10;

This snapshot shows requests still active, including response delivery. A request can finish between observations. Query history answers the separate question of what completed or failed:

SELECT event_time_text, query_id, exception_code, exception
FROM system.query_log
WHERE type IN ('ExceptionBeforeStart', 'ExceptionWhileProcessing')
ORDER BY event_time DESC
LIMIT 10;

Check effective database limits before assuming a request used built-in defaults:

SELECT name, value, changed
FROM system.settings
WHERE changed
ORDER BY name;

Save the query ID, exact error and relevant setting values with the report. Redact credentials and private SQL values before sharing output. Query logging may omit short requests when a duration threshold is configured, so absence from history alone does not prove that a request never arrived.

See system tables and query logging.