Extract counters from accounts table to account_stats table (#9295)

db/migrate/20181116165755_create_account_stats.rb

@@ -0,0 +1,12 @@
+class CreateAccountStats < ActiveRecord::Migration[5.2]
+  def change
+    create_table :account_stats do |t|
+      t.belongs_to :account, null: false, foreign_key: { on_delete: :cascade }, index: { unique: true }
+      t.bigint :statuses_count, null: false, default: 0
+      t.bigint :following_count, null: false, default: 0
+      t.bigint :followers_count, null: false, default: 0
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20181116173541_copy_account_stats.rb

@@ -0,0 +1,54 @@
+class CopyAccountStats < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  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'
+
+    Account.unscoped.select('id').find_in_batches(batch_size: 5_000) do |accounts|
+      execute <<-SQL.squish
+        INSERT INTO account_stats (account_id, statuses_count, following_count, followers_count, created_at, updated_at)
+        SELECT id, statuses_count, following_count, followers_count, created_at, updated_at
+        FROM accounts
+        WHERE id IN (#{accounts.map(&:id).join(', ')})
+        ON CONFLICT (account_id) DO UPDATE
+        SET statuses_count = EXCLUDED.statuses_count, following_count = EXCLUDED.following_count, followers_count = EXCLUDED.followers_count
+      SQL
+    end
+  end
+
+  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
+    Account.unscoped.select('id, statuses_count, following_count, followers_count, created_at, updated_at').find_each do |account|
+      begin
+        params = [[nil, account.id], [nil, account.statuses_count], [nil, account.following_count], [nil, account.followers_count], [nil, account.created_at], [nil, account.updated_at]]
+        exec_insert('INSERT INTO account_stats (account_id, statuses_count, following_count, followers_count, created_at, updated_at) VALUES ($1, $2, $3, $4, $5, $6)', nil, params)
+      rescue ActiveRecord::RecordNotUnique
+        next
+      end
+    end
+  end
+end