Add fallback for PostgreSQL without upsert in CopyStatusStats (#8903)

Fix #8590

db/migrate/20180812173710_copy_status_stats.rb

@@ -3,6 +3,28 @@ class CopyStatusStats < ActiveRecord::Migration[5.2]
 
   def up
     safety_assured do
+      if supports_upsert?
+        up_fast
+      else
+        up_slow
+      end
+    end
+  end
+
+  def down
+    # Nothing
+  end
+
+  private
+
+  def supports_upsert?
+    version = select_one("SELECT current_setting('server_version_num') AS v")['v'].to_i
+    version >= 90500
+  end
+
+  def up_fast
+    say 'Upsert is available, importing counters using the fast method'
+
     Status.unscoped.select('id').find_in_batches(batch_size: 5_000) do |statuses|
       execute <<-SQL.squish
         INSERT INTO status_stats (status_id, reblogs_count, favourites_count, created_at, updated_at)
@@ -14,9 +36,19 @@ class CopyStatusStats < ActiveRecord::Migration[5.2]
       SQL
     end
   end
-  end
 
-  def down
-    # Nothing
+  def up_slow
+    say 'Upsert is not available in PostgreSQL below 9.5, falling back to slow import of counters'
+
+    # We cannot use bulk INSERT or overarching transactions here because of possible
+    # uniqueness violations that we need to skip over
+    Status.unscoped.select('id, reblogs_count, favourites_count, created_at, updated_at').find_each do |status|
+      begin
+        params = [[nil, status.id], [nil, status.reblogs_count], [nil, status.favourites_count], [nil, status.created_at], [nil, status.updated_at]]
+        exec_insert('INSERT INTO status_stats (status_id, reblogs_count, favourites_count, created_at, updated_at) VALUES ($1, $2, $3, $4, $5)', nil, params)
+      rescue ActiveRecord::RecordNotUnique
+        next
+      end
+    end
   end
 end