Clicking pghero under Mastodon Administration menu, the interface shows the Query stats must be enabled for slow queries orange warning, in the interface that looks like this:

A screenshot of the pghero interface notifying the user that Query stats must be enabled for slow queries and that Query stats are available but not enabled with the Enable button beneath the message.

After clicking the blue Enable button, instead of a success, the error The database user does not have permission to enable query stats is shown in the top row or the interface as a red ribbon:

The database user does not have permission to enable query stats

A small hint toward resolving this error could be found in the pghero#7 which mentions running a SQL statement under user postgres like this:

CREATE extension pg_stat_statements;

To run such a command, establish an access to the postgres database first. If running Mastodon under docker-compose navigate to the directory where docker-compose.yml is located and run the following:

sudo docker exec -it mastodon_db_1 psql -h localhost -U postgres

When in psql, run these commands:

postgres=# \c mastodon;
You are now connected to database "mastodon" as user "postgres".
mastodon=# CREATE extension pg_stat_statements;
mastodon=# exit

Refreshing pghero interface nows outputs a verbose message Make Query Stats available by adding the following lines to postgresql.conf and then restart the server for changes to take effect:

A screenshot of the pghero interface displaying a hint about configuring postgres to enable query statistics.

Still assuming the above guide, the postgres/ folder is in the same one as docker-completely.yml. Edit the file postgres/postgresql.conf there and add/uncomment the exact same lines from the screenshot above:

shared_preload_libraries = 'pg_stat_statements'
pg_stat_statements.track = all

I have put these under the sections CLIENT CONNECTION DEFAULTS and STATISTICS respectively. The shared_preload_libraries have a comment there stating also # (change requires restart), so do the following:

sudo docker-compose down
sudo docker-compose up -d

Refreshing pghero interface now shows a green success line stating No slow queries instead:

No slow queries text in green background

Although I am not sure at this point what is this configuration actually good for, I have found out how to get rid of the warning and here's the guide. Enjoy!