Improve performance of follow recommendation scheduler (#16159)

Express follow_recommendations in terms of account_summaries rather than
accounts, integrate filters that are unconditionally used, and materialize
the resulting view.

This should result in the bulk of the computation being performed only once
instead of **once per recommendation language**.

2 files changed, 7 insertions, 4 deletions

diff --git a/app/models/follow_recommendation.rb b/app/models/follow_recommendation.rb
index 6670b6560..1ed6dc49b 100644
--- a/app/models/follow_recommendation.rb
+++ b/app/models/follow_recommendation.rb
@@ -14,9 +14,11 @@ class FollowRecommendation < ApplicationRecord
   belongs_to :account_summary, foreign_key: :account_id
   belongs_to :account, foreign_key: :account_id
 
-  scope :safe, -> { joins(:account_summary).merge(AccountSummary.safe) }
   scope :localized, ->(locale) { joins(:account_summary).merge(AccountSummary.localized(locale)) }
-  scope :filtered, -> { joins(:account_summary).merge(AccountSummary.filtered) }
+
+  def self.refresh
+    Scenic.database.refresh_materialized_view(table_name, concurrently: true, cascade: false)
+  end
 
   def readonly?
     true
diff --git a/app/workers/scheduler/follow_recommendations_scheduler.rb b/app/workers/scheduler/follow_recommendations_scheduler.rb
index 0a0286496..cb1e15961 100644
--- a/app/workers/scheduler/follow_recommendations_scheduler.rb
+++ b/app/workers/scheduler/follow_recommendations_scheduler.rb
@@ -14,13 +14,14 @@ class Scheduler::FollowRecommendationsScheduler
   def perform
     # Maintaining a materialized view speeds-up subsequent queries significantly
     AccountSummary.refresh
+    FollowRecommendation.refresh
 
-    fallback_recommendations = FollowRecommendation.safe.filtered.limit(SET_SIZE).index_by(&:account_id)
+    fallback_recommendations = FollowRecommendation.limit(SET_SIZE).index_by(&:account_id)
 
     I18n.available_locales.each do |locale|
       recommendations = begin
         if AccountSummary.safe.filtered.localized(locale).exists? # We can skip the work if no accounts with that language exist
-          FollowRecommendation.safe.filtered.localized(locale).limit(SET_SIZE).index_by(&:account_id)
+          FollowRecommendation.localized(locale).limit(SET_SIZE).index_by(&:account_id)
         else
           {}
         end