# system.query_log

A read-only structured request history. query_id links events from one request with its response and diagnostic records. log_queries and log_queries_min_query_duration_ms control recording.

## Columns

| Name                            | Meaning                                                                    |
| ------------------------------- | -------------------------------------------------------------------------- |
| event_time, event_time_text     | Ordered event time and its text representation.                            |
| type                            | QueryStart, QueryFinish, ExceptionBeforeStart or ExceptionWhileProcessing. |
| query_start_time                | Request start time.                                                        |
| query_duration_ms               | Request duration in milliseconds.                                          |
| query_id                        | Request identifier linking its events.                                     |
| current_database                | Database used by the request.                                              |
| query, query_kind               | SQL text and statement classification.                                     |
| address, port                   | Client endpoint.                                                           |
| user, interface                 | Authenticated identity and TCP/HTTP interface.                             |
| result_rows                     | Returned rows across the request statements.                               |
| exception_code, exception       | Permanent failure code and message; zero and empty text on success.        |
| normalized_query_hash           | Normalized-SQL grouping hash.                                              |
| read_rows, read_bytes           | Read work observed during the request.                                     |
| written_rows, written_bytes     | Write work observed during the request.                                    |
| memory_usage, peak_memory_usage | Peak accounted SQL memory at terminal events.                              |

## Visibility

Events are recorded asynchronously. A direct single-table SELECT from system.query_log waits for previously queued events before reading. Each reading statement takes a fresh history snapshot, including inside a user transaction.

The current request's terminal event appears only after its execution. A nonzero duration threshold skips fast requests and records only qualifying terminal events; disabling logging overrides it.

## Counter interpretation

At terminal events, both memory_usage and peak_memory_usage report peak accounted SQL heap usage. Work later rolled back can still contribute to read/write counters. result_rows counts returned rows across the request's statements.

## Example

```sql
SELECT type, exception_code
FROM system.query_log
WHERE type = 'QueryFinish'
ORDER BY event_time DESC
LIMIT 1
FORMAT csv;
```

```text
"QueryFinish",0
```

Use the system qualifier through the server. See [logging settings](https://coordinatordb.com/docs/reference/settings/session-settings/log-queries), [active requests](https://coordinatordb.com/docs/reference/system-tables/processes) and [query measurements](https://coordinatordb.com/docs/guides/clickhouse/performance-and-monitoring/isolate-query-bottlenecks).