Adding PostgreSQL DB slow query log

source/deploy/high-scale-troubleshoot.rst

@@ -12,4 +12,13 @@ My system keeps hanging when I search for a message in Mattermost
 
 First, check how many messages have been posted on your system, including deleted posts and posts made using automations.
 
-Go to the **System Console > Reporting > Site Statistics** and review the **Total Posts** figure reported. If this figure is above 3,000,000 posts, we recommend deploying Elasticsearch alongside your Mattermost server for improved search performance. Follow our guides to `deploy an Elasticsearch server </scale/elasticsearch.html>`__.
+Go to the **System Console > Reporting > Site Statistics** and review the **Total Posts** figure reported. If this figure is above 3,000,000 posts, we recommend deploying Elasticsearch alongside your Mattermost server for improved search performance. Follow our guides to `deploy an Elasticsearch server </scale/elasticsearch.html>`__.
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+How to troubleshoot server-side performance issues without Prometheus or Grafana?
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+Enable slow query logging in PostgreSQL and leave it enabled to gather data over time. PostgreSQL's slow query log helps you identify queries that take longer than a specified amount of time. The slow query log isn't enabled by default, and must be enabled manually.
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+To enable slow query logging globally, change the following line in ``postgresql.conf``: ``log_min_duration_statement = 1000``, then restart PostgreSQL. When you this value to ``1000``, PostgreSQL considers queries that take longer than 1 second to be slow queries and logs them in the log file. Alternatively, you can run ``SELECT pg_reload_conf();`` to reload the configuration without a restart.
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+To enable slow query logging for a specific database, use ``ALTER DATABASE`` to change the configuration parameter for a single database.