How to log slow queries in PostgreSQL

Slow query logging turns intermittent slowness into actionable evidence by recording the SQL statements that exceed an execution-time threshold.

PostgreSQL can emit a duration: log line for slow statements when log_min_duration_statement is set, capturing both the elapsed time and the statement text. A sensible threshold keeps the signal-to-noise ratio high compared to logging every statement.

Slow-statement logs can grow quickly and may include sensitive literals embedded in SQL text, so threshold choice and retention policies matter. Server-wide configuration via ALTER SYSTEM requires superuser privileges and takes effect on reload, while managed services often expose the same parameter via provider settings instead of systemd.

Steps to log slow queries in PostgreSQL:

Set a global slow query threshold using ALTER SYSTEM.
```
$ sudo -u postgres psql -c "ALTER SYSTEM SET log_min_duration_statement = '500ms';"
ALTER SYSTEM
```
Use a higher threshold first when log volume is unknown. Lower it once noisy statements are filtered. Set log_min_duration_statement to -1 to disable slow statement logging.
Reload the PostgreSQL configuration to apply the updated logging threshold.
```
$ sudo -u postgres psql -c "SELECT pg_reload_conf();"
 pg_reload_conf
----------------
 t
(1 row)
```
Related: How to manage the PostgreSQL service with systemctl on Linux

Verify the effective log_min_duration_statement value.

$ sudo -u postgres psql -Atc "SHOW log_min_duration_statement;"
500ms

Run a test statement that exceeds the threshold.

$ sudo -u postgres psql -d postgres -c "SELECT pg_sleep(0.6);"
 pg_sleep
----------

(1 row)

Confirm the test statement appears in the PostgreSQL log with a duration: entry.

$ sudo grep -hF "SELECT pg_sleep(0.6)" /var/log/postgresql/postgresql-2025-12-29.log | tail -n 1
2025-12-29 08:45:36.367 UTC [5666] 69523faf.1622 4 postgres@postgres [local] app=psql LOG:  duration: 610.600 ms  statement: SELECT pg_sleep(0.6);

On hosts that write logs to /var/log/postgresql, search those files or the systemd journal instead.