Optimize FeedManager#unmerge, and slightly optimize FeedManager#merge

app/lib/feed_manager.rb

@@ -50,9 +50,15 @@ class FeedManager
 
   def merge_into_timeline(from_account, into_account)
     timeline_key = key(:home, into_account.id)
+    query        = from_account.statuses.limit(MAX_ITEMS)
+
+    if redis.zcard(timeline_key) >= FeedManager::MAX_ITEMS
+      oldest_home_score = redis.zrange(timeline_key, 0, 0, with_scores: true)&.first&.last&.to_i || 0
+      query = query.where('id > ?', oldest_home_score)
+    end
 
     redis.pipelined do
-      from_account.statuses.limit(MAX_ITEMS).each do |status|
+      query.each do |status|
         next if status.direct_visibility? || filter?(:home, status, into_account)
         redis.zadd(timeline_key, status.id, status.id)
       end
@@ -63,8 +69,9 @@ class FeedManager
 
   def unmerge_from_timeline(from_account, into_account)
     timeline_key = key(:home, into_account.id)
+    oldest_home_score = redis.zrange(timeline_key, 0, 0, with_scores: true)&.first&.last&.to_i || 0
 
-    from_account.statuses.select('id').find_in_batches do |statuses|
+    from_account.statuses.select('id').where('id > ?', oldest_home_score).find_in_batches do |statuses|
       redis.pipelined do
         statuses.each do |status|
           redis.zrem(timeline_key, status.id)