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Lemmy Server Performance @lemmy.ml RoundSparrow @lemmy.ml

PostgreSQL for Lemmy instance installers/operators/upgraders

Diving in, I haven't worked with PostgreSQL for 15 years, but sharing random notes and obsrervations

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  • These instructions assume you installed "Lemmy from scratch" on Linux (as opposed to Docker, or FreeBSD instead of Linux, etc).

    Linux shell on server of instance

    Change users to the database Linux account:
    sudo -iu postgres
    Open the PostgreSQL shell client application:
    psql

    query the database server about table locks

    select * from pg_locks;

    • from psql shell, list the lemmy_server tables

      connect to the database named "lemmy":
      \c lemmy
      You are now connected to database "lemmy" as user "postgres".

      List the tables in the database:
      \dt

                        List of relations
 Schema |            Name            | Type  | Owner 
--------+----------------------------+-------+-------
 public | __diesel_schema_migrations | table | lemmy
 public | activity                   | table | lemmy
 public | admin_purge_comment        | table | lemmy
 public | admin_purge_community      | table | lemmy
 public | admin_purge_person         | table | lemmy
 public | admin_purge_post           | table | lemmy
 public | comment                    | table | lemmy
 public | comment_aggregates         | table | lemmy
 public | comment_like               | table | lemmy
 public | comment_reply              | table | lemmy
 public | comment_report             | table | lemmy
 public | comment_saved              | table | lemmy
 public | community                  | table | lemmy
 public | community_aggregates       | table | lemmy
 public | community_block            | table | lemmy
 public | community_follower         | table | lemmy
 public | community_language         | table | lemmy
 public | community_moderator        | table | lemmy
 public | community_person_ban       | table | lemmy
 public | email_verification         | table | lemmy
 public | federation_allowlist       | table | lemmy
 public | federation_blocklist       | table | lemmy
 public | instance                   | table | lemmy
 public | language                   | table | lemmy
 public | local_site                 | table | lemmy
 public | local_site_rate_limit      | table | lemmy
 public | local_user                 | table | lemmy
 public | local_user_language        | table | lemmy
 public | mod_add                    | table | lemmy
 public | mod_add_community          | table | lemmy
 public | mod_ban                    | table | lemmy
 public | mod_ban_from_community     | table | lemmy
 public | mod_feature_post           | table | lemmy
 public | mod_hide_community         | table | lemmy
 public | mod_lock_post              | table | lemmy
 public | mod_remove_comment         | table | lemmy
 public | mod_remove_community       | table | lemmy
 public | mod_remove_post            | table | lemmy
 public | mod_transfer_community     | table | lemmy
 public | password_reset_request     | table | lemmy
 public | person                     | table | lemmy
 public | person_aggregates          | table | lemmy
 public | person_ban                 | table | lemmy
 public | person_block               | table | lemmy
 public | person_follower            | table | lemmy
 public | person_mention             | table | lemmy
 public | person_post_aggregates     | table | lemmy
 public | post                       | table | lemmy
 public | post_aggregates            | table | lemmy
 public | post_like                  | table | lemmy
 public | post_read                  | table | lemmy
 public | post_report                | table | lemmy
 public | post_saved                 | table | lemmy
 public | private_message            | table | lemmy
 public | private_message_report     | table | lemmy
 public | registration_application   | table | lemmy
 public | secret                     | table | lemmy
 public | site                       | table | lemmy
 public | site_aggregates            | table | lemmy
 public | site_language              | table | lemmy
 public | tagline                    | table | lemmy
(61 rows)

      • query to list Community joins that are pending

        SELECT * FROM community_follower WHERE pending='t';

        This kind of query I'd like to work on adding to the server admin screens for operators.

        query to list Communities by name

        SELECT id,instance_id,name,title,local,published FROM community ORDER BY name;

        edit: tickle federation replication

        • why would community joins be pending?

          edit: Ah I see. A better query might be:

          SELECT p.name, c.name, i.domain, p.local, * FROM community_follower
         inner join person p on p.id = community_follower.person_id
         inner join community c on c.id = community_follower.community_id
         inner join instance i on c.instance_id = i.id
         WHERE pending='t';

          which will show you the user, the community they're trying to join, and the instance they're trying to join on. Example:

          (redacted my users in case they don't want it known what they're trying to join.

          • Thank you for sharing the query.

            why would community joins be pending?

            I believe it is a glaring symptom of federation replication failure. Data isn't making it back from the remote server to confirm the join. Either the outbound never made it to the remote, or the remote never made it back to your server. Multiple instances have had users complaining of these federation failures, example: https://lemmy.ml/post/1280517

            You (the end-user) can try to cancel the join of the community and join again to trigger new connection to the server. I would also add the date to the output so you can try to see when these failures are happening (are they all on the same day?)

            What software are you using to view the queries?

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