possible query optimization for random large datasets

src/inc/routes/apiv2/index.mjs

@@ -25,8 +25,7 @@ export default router => {
       const hasSession = !!req.session;
       const modequery = mime.startsWith("audio") ? lib.getMode(0) : lib.getMode(req.session?.mode ?? 0);
 
-      const rows = await db`
+      const baseQuery = db`
-        select "items".*
         from "items"
         ${isFav
           ? db`join "favorites" on "favorites".item_id = "items".id join "user" as fu on fu.id = "favorites".user_id`
@@ -41,8 +40,26 @@ export default router => {
           ${isFav ? db`and fu."user" = ${user}` : db`and items.username ilike ${user}`}
           ${tag ? db`and tags.normalized ilike '%' || slugify(${tag}) || '%'` : db``}
           ${!hasSession && globalfilter ? db`and not exists (select 1 from tags_assign where item_id = items.id and (${db.unsafe(globalfilter)}))` : db``}
-        order by random()
+      `;
-        limit 1
+
+      const count = await db`
+        select count(*) as total
+        ${baseQuery}
+      `;
+
+      if (count[0].total == 0) {
+        return res.json({
+          success: false,
+          items: []
+        });
+      }
+
+      const offset = Math.floor(Math.random() * count[0].total);
+
+      const rows = await db`
+        select "items".*
+        ${baseQuery}
+        limit 1 offset ${offset}
       `;
 
       return res.json({