DuckDB implements a logging mechanism that provides users with detailed information about events such as query execution, performance metrics, and system events.

Basics

The DuckDB logging mechanism can be enabled or disabled using a special function, enable_logging. Logs are stored in a special view named duckdb_logs, which can be queried like any standard table.

Example:

CALL enable_logging();
-- Run some queries...
SELECT * FROM duckdb_logs;

To disable logging, run

CALL disable_logging();

To clear the current log, run

CALL truncate_duckdb_logs();

Log Level

DuckDB supports different logging levels that control the verbosity of the logs:

• ERROR: Only logs error messages
• WARN: Logs warnings and errors
• INFO: Logs general information, warnings and errors (default)
• DEBUG: Logs detailed debugging information
• TRACE: Logs very detailed tracing information

The log level can be set using:

CALL enable_logging(level='debug');

Log Types

In DuckDB, log messages can have an associated log type. Log types allow two main things:

• Fine-grained control over log message generation
• Support for structured logging

Logging-Specific Types

To log only messages of a specific type:

CALL enable_logging('HTTP');

The above function will automatically set the correct log level, and will add the HTTP type to the enabled_log_types settings. This ensures only log messages of the 'HTTP' type will be written to the log.

To enable multiple log types, simply pass:

CALL enable_logging(['HTTP', 'QueryLog']);~~~~

Structured Logging

Some log types like HTTP will have an associated message schema. To make DuckDB automatically parse the message, use the duckdb_logs_parsed() macro. For example:

SELECT request.headers FROM duckdb_logs_parsed('HTTP');

To view the schema of each structure log type simply run:

DESCRIBE FROM duckdb_logs_parsed('HTTP');

List of Available Log Types

This is a (non-exhaustive) list of the available log types in DuckDB.

Log Storages

By default, DuckDB logs to an in-memory log storage (memory). DuckDB supports different types of log storage. Currently the following log storage types are implemented in core DuckDB

Note that the duckdb_logs view is automatically updated to target the currently active log storage. This means that switching the log storage may influence what is returned by the duckdb_logs function.

Logging to stdout

CALL enable_logging(storage='stdout');

Logging to file

CALL enable_logging(storage='file', storage_config={'path': 'path/to/store/logs'});

or using the equivalent shorthand

CALL enable_logging(storage_path='path/to/store/logs');

Advanced Usage

Normalized vs denormalized logging

DuckDB's log storages can log in two ways: normalized vs denormalized.

In denormalized logging, the log context information is appended directly to each log entry, while in normalized logging the log entries are stored separately with context_ids referencing the context information.

For file storage, you can switch between normalized and denormalized by providing a path ending in .csv (for normalized) or without .csv (for denormalized). For file logging, denormalized is generally recommended since this increases performance and reduces the total size of the logs. To configure normalization of file log storage:

-- normalized: creates `/tmp/duckdb_log_contexts.csv` and `/tmp/duckdb_log_entries.csv`
CALL enable_logging(storage_path='/tmp');
-- denormalized: creates `/tmp/logs.csv`
CALL enable_logging(storage_path='/tmp/logs.csv');

Note that the difference between normalized and denormalized is typically hidden from users through the 'duckdb_logs' function, which automatically joins normalized tables into a single unified result. To illustrate, both configurations above will be queryable using FROM duckdb_logs; and will produce identical results.

Buffer Size

The log storage in DuckDB implements a buffering mechanism to optimize logging performance. This implementation introduces a potential delay between message logging and storage writing. This delay can obscure the actual message writing time, which is particularly problematic when debugging crashes, as messages generated immediately before a crash might not be written. To address this, the buffer size can be configured as follows:

CALL enable_logging(storage_config={'buffer_size': 0});

or using the equivalent shorthand:

CALL enable_logging(storage_buffer_size=0);

Note that the default buffer size is different for different log storages:

So for example, if you want to increase your stdout logging performance, simply enable buffering to greatly (>10x) speed up your logging:

CALL enable_logging(storage='stdout', storage_buffer_size=2048);

Or imagine you are debugging a crash in DuckDB and you want to use the file logger to understand what's going on: Simply disable the buffering using:

CALL enable_logging(storage_path='/tmp/mylogs', storage_buffer_size=2048);

Syntactic sugar

DuckDB contains some syntactic sugar to make common paths:

The following statements are all equal

-- regular invocation 
CALL enable_logging(storage='file', storage_config={'path': 'path/to/store/logs'});
-- using shorthand for common path storage config param 
CALL enable_logging(storage='file', storage_path='path/to/store/logs');
-- omitting `storage='file'` -> is implied from presence of `storage_config`
CALL enable_logging(storage_config={'path': 'path/to/store/logs'});