DEV: Migrate notifications#id to bigint (#28444)

The `notifications.id` column is the most probable column to run out of
values. This is because it is an `int` column that has only 2147483647
values and many notifications are generated on a regular basis in an
active community. This commit migrates the column to `bigint`.

These migrations do not use `ALTER TABLE ... COLUMN ... TYPE` in order
to avoid the `ACCESS EXCLUSIVE` lock on the entire table. Instead, they
create a new `bigint` column, copy the values to the new column and
then sets the new column as primary key.

Related columns (see `user_badges`, `shelved_notifications`) will
be migrated in a follow-up commit.

app/models/notification.rb

@@ -407,7 +407,7 @@ end
 #
 # Table name: notifications
 #
-#  id                :integer          not null, primary key
+#  old_id            :integer
 #  notification_type :integer          not null
 #  user_id           :integer          not null
 #  data              :string(1000)     not null
@@ -418,6 +418,7 @@ end
 #  post_number       :integer
 #  post_action_id    :integer
 #  high_priority     :boolean          default(FALSE), not null
+#  id                :bigint           not null, primary key
 #
 # Indexes
 #

db/migrate/20240820123401_alter_notifications_id_sequence_to_bigint.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class AlterNotificationsIdSequenceToBigint < ActiveRecord::Migration[7.0]
+  def up
+    execute "ALTER SEQUENCE notifications_id_seq AS bigint"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20240820123402_add_big_int_notifications_id.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+class AddBigIntNotificationsId < ActiveRecord::Migration[7.0]
+  def up
+    # Short-circuit if the table has been migrated already
+    result =
+      execute(
+        "SELECT data_type FROM information_schema.columns WHERE table_name = 'notifications' AND column_name = 'id' LIMIT 1",
+      )
+    data_type = result[0]["data_type"]
+    return if data_type.downcase == "bigint"
+
+    # Create new column
+    execute "ALTER TABLE notifications ADD COLUMN new_id BIGINT NOT NULL DEFAULT 0"
+
+    # Mirror new `id` values to `new_id`
+    execute <<~SQL.squish
+      CREATE FUNCTION mirror_notifications_id()
+      RETURNS trigger AS
+      $$
+      BEGIN
+        NEW.new_id = NEW.id;
+        RETURN NEW;
+      END;
+      $$
+      LANGUAGE plpgsql
+    SQL
+
+    execute <<~SQL.squish
+      CREATE TRIGGER notifications_new_id_trigger BEFORE INSERT ON notifications
+      FOR EACH ROW EXECUTE PROCEDURE mirror_notifications_id()
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20240820123403_copy_notifications_id_values.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+class CopyNotificationsIdValues < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def up
+    # Short-circuit if the table has been migrated already
+    result =
+      execute(
+        "SELECT data_type FROM information_schema.columns WHERE table_name = 'notifications' AND column_name = 'id' LIMIT 1",
+      )
+    data_type = result[0]["data_type"]
+    return if data_type.downcase == "bigint"
+
+    min_id, max_id = execute("SELECT MIN(id), MAX(id) FROM notifications")[0].values
+    batch_size = 10_000
+
+    (min_id..max_id).step(batch_size) { |start_id| execute <<~SQL } if min_id && max_id
+        UPDATE notifications
+        SET new_id = id
+        WHERE id >= #{start_id} AND id < #{start_id + batch_size} AND new_id != id
+      SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20240820123404_copy_notifications_id_indexes.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+class CopyNotificationsIdIndexes < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def up
+    # Short-circuit if the table has been migrated already
+    result =
+      execute(
+        "SELECT data_type FROM information_schema.columns WHERE table_name = 'notifications' AND column_name = 'id' LIMIT 1",
+      )
+    data_type = result[0]["data_type"]
+    return if data_type.downcase == "bigint"
+
+    # Copy existing indexes and suffix them with `_bigint`
+    results =
+      execute(
+        "SELECT indexname, indexdef FROM pg_indexes WHERE tablename = 'notifications' AND indexdef SIMILAR TO '%\\mid\\M%'",
+      )
+    results.each do |res|
+      indexname, indexdef = res["indexname"], res["indexdef"]
+
+      indexdef = indexdef.gsub(/\b#{indexname}\b/, "#{indexname}_bigint")
+      indexdef =
+        indexdef.gsub(
+          /\bCREATE (UNIQUE )?INDEX\b/,
+          "CREATE \\1INDEX CONCURRENTLY",
+        ) if !Rails.env.test?
+      indexdef = indexdef.gsub(/\bid\b/, "new_id")
+
+      execute "DROP INDEX #{Rails.env.test? ? "" : "CONCURRENTLY"} IF EXISTS #{indexname}_bigint"
+      execute(indexdef)
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20240820123405_swap_big_int_notifications_id.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+class SwapBigIntNotificationsId < ActiveRecord::Migration[7.0]
+  def up
+    # Short-circuit if the table has been migrated already
+    result =
+      execute(
+        "SELECT data_type FROM information_schema.columns WHERE table_name = 'notifications' AND column_name = 'id' LIMIT 1",
+      )
+    data_type = result[0]["data_type"]
+    return if data_type.downcase == "bigint"
+
+    # Necessary to rename and drop columns
+    Migration::SafeMigrate.disable!
+
+    # Drop trigger and function used to replicate new values
+    execute "DROP TRIGGER notifications_new_id_trigger ON notifications"
+    execute "DROP FUNCTION mirror_notifications_id()"
+
+    # Move sequence to new column
+    execute "ALTER TABLE notifications ALTER COLUMN id DROP DEFAULT"
+    execute "ALTER TABLE notifications ALTER COLUMN new_id SET DEFAULT nextval('notifications_id_seq'::regclass)"
+    execute "ALTER SEQUENCE notifications_id_seq OWNED BY notifications.new_id"
+
+    # Swap columns
+    execute "ALTER TABLE notifications RENAME COLUMN id TO old_id"
+    execute "ALTER TABLE notifications RENAME COLUMN new_id TO id"
+
+    # Recreate primary key
+    execute "ALTER TABLE notifications DROP CONSTRAINT notifications_pkey"
+    execute "ALTER TABLE notifications ADD CONSTRAINT notifications_pkey PRIMARY KEY USING INDEX notifications_pkey_bigint"
+
+    # Keep old column and mark it as read only
+    execute "ALTER TABLE notifications ALTER COLUMN old_id DROP NOT NULL"
+    Migration::ColumnDropper.mark_readonly(:notifications, :old_id)
+  ensure
+    Migration::SafeMigrate.enable!
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20240820123406_drop_old_notification_id_indexes.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class DropOldNotificationIdIndexes < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def up
+    # Drop old indexes
+    results =
+      execute(
+        "SELECT indexname, indexdef FROM pg_indexes WHERE tablename = 'notifications' AND indexdef SIMILAR TO '%\\mold_id\\M%'",
+      )
+    results.each do |res|
+      indexname, indexdef = res["indexname"], res["indexdef"]
+      execute "DROP INDEX #{Rails.env.test? ? "" : "CONCURRENTLY"} IF EXISTS #{indexname}"
+    end
+
+    # Remove `_bigint` suffix from indexes
+    results =
+      execute(
+        "SELECT indexname, indexdef FROM pg_indexes WHERE tablename = 'notifications' AND indexdef SIMILAR TO '%\\mid\\M%'",
+      )
+    results.each do |res|
+      indexname, indexdef = res["indexname"], res["indexdef"]
+      if indexname.include?("_bigint")
+        execute "ALTER INDEX #{indexname} RENAME TO #{indexname.gsub(/_bigint$/, "")}"
+      end
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end