log_filename

Definition PG 18 manual

When logging_collector is enabled, this parameter sets the file names of the created log files. The value is treated as a strftime pattern, so %-escapes can be used to specify time-varying file names. (Note that if there are any time-zone-dependent %-escapes, the computation is done in the zone specified by log_timezone.) The supported %-escapes are similar to those listed in the Open Group's strftime specification. Note that the system's strftime is not used directly, so platform-specific (nonstandard) extensions do not work. The default is postgresql-%Y-%m-%d_%H%M%S.log.

If you specify a file name without escapes, you should plan to use a log rotation utility to avoid eventually filling the entire disk. In releases prior to 8.4, if no % escapes were present, PostgreSQL would append the epoch of the new log file's creation time, but this is no longer the case.

If CSV-format output is enabled in log_destination, .csv will be appended to the timestamped log file name to create the file name for CSV-format output. (If log_filename ends in .log, the suffix is replaced instead.)

If JSON-format output is enabled in log_destination, .json will be appended to the timestamped log file name to create the file name for JSON-format output. (If log_filename ends in .log, the suffix is replaced instead.)

This parameter can only be set in the postgresql.conf file or on the server command line.

How it works

log_filename sets the file name pattern for log files. logging_collector expands strftime escapes using log_timezone whenever it opens a new file, so repeating names must be coordinated with rotation and truncation.

log_filename is a SIGHUP-context setting: a configuration reload activates the new server value without a restart; subsequent operations that consult it use the refreshed value.

The active path is composed from log_destination, logging_collector or syslog/eventlog, file naming and permissions, rotation triggers, and external shipping or retention.

Operational considerations

Editing log_filename without reloading configuration and verifying the effective value and subsequent behavior.

Combining incompatible destination, collector, filename, and rotation assumptions and then losing or overwriting logs.

Failing to monitor a full or unwritable log target, which can block logging or database activity depending on the path.

Using a repeating strftime name without matching rotation and truncation policy, causing append growth or overwrite surprises.

Workload guidance

OLAP: Choose a predictable period boundary for large analytical logs and ensure downstream ingestion closes the previous file before the pattern repeats; do not rely on size rotation with a non-unique name.

OLTP: Use a filename pattern whose uniqueness matches rotation policy. If names repeat, combine time-based rotation, log_truncate_on_rotation, and confirmed shipping so a reused name cannot mix periods or overwrite uncollected data.

SMALL: A bounded weekday or date pattern is reasonable only with matching retention and free-space monitoring. Keep enough time components to prevent accidental collisions after restart or manual rotation.