Some learning about JOIN filled queries like the SELECT that is used to list post_aggregates table when browsing postings on Lemmy.

https://dba.stackexchange.com/questions/155972/postgres-join-conditions-vs-where-conditions

“Logically, it makes no difference at all whether you place conditions in the join clause of an INNER JOIN or the WHERE clause of the same SELECT. The effect is the same. (Not the case for OUTER JOIN!)”

Learning about join_collapse_limit

  • RoundSparrow @ BTOPM
    1•1 year ago

    https://stackoverflow.com/questions/22339836/postgresql-join-collapse-limit-and-time-for-query-planning

    “I just discovered join_collapse_limit has been preventing the PostgreSQL planner from finding a much better join order. In my case, increasing the limit to 10 (from the default of 8) allowed the planner to improve search time from ~30 secs to ~1 ms, which is much more acceptable.”

    Docs “By default, this variable is set the same as from_collapse_limit, which is appropriate for most uses.”

    Could very well apply to Lemmy!!

    • RoundSparrow @ BTOPM
      1•1 year ago

      I’m thinking for Lemmy…

      SET geqo = on;
SET geqo_threshold = 12;

SET from_collapse_limit = 15;
SET join_collapse_limit = 15;
    • RoundSparrow @ BTOPM
      1•1 year ago

      I think 8 is a concern for Lemmy!

      FROM (((((((((((((“post_aggregates”
      INNER JOIN “person” ON (“post_aggregates”.“creator_id” = “person”.“id”))
      INNER JOIN “community” ON (“post_aggregates”.“community_id” = “community”.“id”))
      LEFT OUTER JOIN “community_person_ban” ON ((“post_aggregates”.“community_id” = “community_person_ban”.“community_id”) AND (“community_person_ban”.“person_id” = “post_aggregates”.“creator_id”))) INNER JOIN “post” ON (“post_aggregates”.“post_id” = “post”.“id”))
      LEFT OUTER JOIN “community_follower” ON ((“post_aggregates”.“community_id” = “community_follower”.“community_id”) AND (“community_follower”.“person_id” = $1)))
      LEFT OUTER JOIN “community_moderator” ON ((“post”.“community_id” = “community_moderator”.“community_id”) AND (“community_moderator”.“person_id” = $2)))
      LEFT OUTER JOIN “post_saved” ON ((“post_aggregates”.“post_id” = “post_saved”.“post_id”) AND (“post_saved”.“person_id” = $3)))
      LEFT OUTER JOIN “post_read” ON ((“post_aggregates”.“post_id” = “post_read”.“post_id”) AND (“post_read”.“person_id” = $4)))
      LEFT OUTER JOIN “person_block” ON ((“post_aggregates”.“creator_id” = “person_block”.“target_id”) AND (“person_block”.“person_id” = $5)))
      LEFT OUTER JOIN “post_like” ON ((“post_aggregates”.“post_id” = “post_like”.“post_id”) AND (“post_like”.“person_id” = $6)))
      LEFT OUTER JOIN “person_post_aggregates” ON ((“post_aggregates”.“post_id” = “person_post_aggregates”.“post_id”) AND (“person_post_aggregates”.“person_id” = $7)))
      LEFT OUTER JOIN “community_block” ON ((“post_aggregates”.“community_id” = “community_block”.“community_id”) AND (“community_block”.“person_id” = $8)))
      LEFT OUTER JOIN “local_user_language” ON ((“post”.“language_id” = “local_user_language”.“language_id”) AND (“local_user_language”.“local_user_id” = $9)))
      WHERE