get_random_community_id: add filters for nsfw and private, use algorithm that doesn't scan the entire table

crates/api/src/community/random.rs

@@ -27,7 +27,7 @@ pub async fn get_random_community(
   let local_user = local_user_view.as_ref().map(|u| &u.local_user);
 
   let random_community_id =
-    Community::get_random_community_id(&mut context.pool(), &data.type_).await?;
+    Community::get_random_community_id(&mut context.pool(), &data.type_, data.show_nsfw).await?;
 
   let is_mod_or_admin = is_mod_or_admin_opt(
     &mut context.pool(),

crates/api_common/src/community.rs

@@ -274,6 +274,8 @@ pub struct TransferCommunity {
 pub struct GetRandomCommunity {
   #[cfg_attr(feature = "full", ts(optional))]
   pub type_: Option<ListingType>,
+  #[cfg_attr(feature = "full", ts(optional))]
+  pub show_nsfw: Option<bool>,
 }
 
 #[skip_serializing_none]

crates/db_schema/src/impls/community.rs

@@ -22,12 +22,13 @@ use crate::{
   utils::{
     action_query,
     find_action,
-    functions::{coalesce, lower, random},
+    functions::{coalesce, coalesce_2_nullable, lower, random_smallint},
     get_conn,
     now,
     uplete,
     DbPool,
   },
+  CommunityVisibility,
   ListingType,
   SubscribedType,
 };
@@ -209,23 +210,58 @@ impl Community {
   pub async fn get_random_community_id(
     pool: &mut DbPool<'_>,
     type_: &Option<ListingType>,
+    show_nsfw: Option<bool>,
   ) -> Result<CommunityId, Error> {
     let conn = &mut get_conn(pool).await?;
 
-    let mut query = community::table
-      .filter(not(community::deleted))
-      .filter(not(community::removed))
-      .into_boxed();
+    // This is based on the random page selection algorithm in MediaWiki. It assigns a random number
+    // X to each item. To pick a random one, it generates a random number Y and gets the item with
+    // the lowest X value where X >= Y.
+    //
+    // https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/specials/SpecialRandomPage.php;763c5f084101676ab1bc52862e1ffbd24585a365
+    //
+    // The difference is we also regenerate the item's assigned number when the item is picked.
+    // Without this, items would have permanent variations in the probability of being picked.
+    // Additionally, in each group of multiple items that are assigned the same random number (a
+    // more likely occurence with `smallint`), there would be only one item that ever gets
+    // picked.
 
-    if let Some(ListingType::Local) = type_ {
-      query = query.filter(community::local);
-    }
+    let try_pick = || {
+      let mut query = community::table
+        .filter(not(
+          community::deleted
+            .or(community::removed)
+            .or(community::visibility.eq(CommunityVisibility::Private)),
+        ))
+        .order(community::random_number.asc())
+        .select(community::id)
+        .into_boxed();
 
-    query
-      .select(community::id)
-      .order(random())
-      .limit(1)
-      .first::<CommunityId>(conn)
+      if let Some(ListingType::Local) = type_ {
+        query = query.filter(community::local);
+      }
+
+      if !show_nsfw.unwrap_or(false) {
+        query = query.filter(not(community::nsfw));
+      }
+
+      query
+    };
+
+    diesel::update(community::table)
+      .filter(
+        community::id.nullable().eq(coalesce_2_nullable(
+          try_pick()
+            .filter(community::random_number.ge(random_smallint()))
+            .single_value(),
+          // Wrap to the beginning if the generated number is higher than all
+          // `community::random_number` values, just like in the MediaWiki algorithm
+          try_pick().single_value(),
+        )),
+      )
+      .set(community::random_number.eq(random_smallint()))
+      .returning(community::id)
+      .get_result::<CommunityId>(conn)
       .await
   }
 }
@@ -570,6 +606,7 @@ mod tests {
       posting_restricted_to_mods: false,
       instance_id: inserted_instance.id,
       visibility: CommunityVisibility::Public,
+      random_number: inserted_community.random_number,
     };
 
     let community_follower_form = CommunityFollowerForm {

crates/db_schema/src/schema.rs

@@ -220,6 +220,7 @@ diesel::table! {
         visibility -> CommunityVisibility,
         #[max_length = 150]
         description -> Nullable<Varchar>,
+        random_number -> Int2,
     }
 }
 

crates/db_schema/src/source/community.rs

@@ -76,6 +76,8 @@ pub struct Community {
   /// A shorter, one-line description of the site.
   #[cfg_attr(feature = "full", ts(optional))]
   pub description: Option<String>,
+  #[serde(skip)]
+  pub random_number: i16,
 }
 
 #[derive(Debug, Clone, derive_new::new)]

crates/db_schema/src/utils.rs

@@ -545,8 +545,12 @@ pub mod functions {
 
   define_sql_function!(fn random() -> Text);
 
+  define_sql_function!(fn random_smallint() -> SmallInt);
+
   // really this function is variadic, this just adds the two-argument version
   define_sql_function!(fn coalesce<T: diesel::sql_types::SqlType + diesel::sql_types::SingleValue>(x: diesel::sql_types::Nullable<T>, y: T) -> T);
+
+  define_sql_function!(#[sql_name = "coalesce"] fn coalesce_2_nullable<T: diesel::sql_types::SqlType + diesel::sql_types::SingleValue>(x: diesel::sql_types::Nullable<T>, y: diesel::sql_types::Nullable<T>) -> diesel::sql_types::Nullable<T>);
 }
 
 pub const DELETED_REPLACEMENT_TEXT: &str = "*Permanently Deleted*";

crates/db_views/src/comment_report_view.rs

@@ -389,6 +389,7 @@ mod tests {
         featured_url: inserted_community.featured_url,
         instance_id: inserted_instance.id,
         visibility: CommunityVisibility::Public,
+        random_number: inserted_community.random_number,
       },
       creator: Person {
         id: inserted_jessica.id,

crates/db_views/src/comment_view.rs

@@ -1058,6 +1058,7 @@ mod tests {
         moderators_url: data.inserted_community.moderators_url.clone(),
         featured_url: data.inserted_community.featured_url.clone(),
         visibility: CommunityVisibility::Public,
+        random_number: data.inserted_community.random_number,
       },
       counts: CommentAggregates {
         comment_id: data.inserted_comment_0.id,

crates/db_views/src/post_view.rs

@@ -1719,6 +1719,7 @@ mod tests {
         moderators_url: inserted_community.moderators_url.clone(),
         featured_url: inserted_community.featured_url.clone(),
         visibility: CommunityVisibility::Public,
+        random_number: inserted_community.random_number,
       },
       counts: PostAggregates {
         post_id: inserted_post.id,

migrations/2024-12-09-200602_optimize_get_random_community/down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE community
+    DROP COLUMN random_number;
+
+DROP FUNCTION random_smallint;
+

migrations/2024-12-09-200602_optimize_get_random_community/up.sql

@@ -0,0 +1,18 @@
+-- * `smallint` range from https://www.postgresql.org/docs/17/datatype-numeric.html
+-- * built-in `random` function has `VOLATILE` and `PARALLEL RESTRICTED` according to:
+--   * https://www.postgresql.org/docs/current/parallel-safety.html#PARALLEL-LABELING
+--   * https://www.postgresql.org/docs/17/xfunc-volatility.html
+CREATE FUNCTION random_smallint ()
+    RETURNS smallint
+    LANGUAGE sql
+    VOLATILE PARALLEL RESTRICTED RETURN random (
+        -32768, 32767
+);
+
+ALTER TABLE community
+    ADD COLUMN random_number smallint NOT NULL DEFAULT random_smallint ();
+
+CREATE INDEX idx_community_random_number ON community (random_number) INCLUDE (local, nsfw)
+WHERE
+    NOT (deleted OR removed OR visibility = 'Private');
+