Prune database partitions older than a retention period

When a monthly-partitioned model is specified with
`partition_by :some_column, retain_for: 2.months`
this feature will detach partitions that are more than two months in the
past. A week later, a second worker will drop the tables that were
previously detached.

Changelog: added

app/models/postgresql/detached_partition.rb

+# frozen_string_literal: true
+
+module Postgresql
+  class DetachedPartition < ApplicationRecord
+    scope :ready_to_drop, -> { where('drop_after < ?', Time.current) }
+  end
+end

app/workers/all_queues.yml

@@ -247,6 +247,15 @@
   :idempotent: true
   :tags:
   - :exclude_from_kubernetes
+- :name: cronjob:database_drop_detached_partitions
+  :worker_name: Database::DropDetachedPartitionsWorker
+  :feature_category: :database
+  :has_external_dependencies:
+  :urgency: :low
+  :resource_boundary: :unknown
+  :weight: 1
+  :idempotent: true
+  :tags: []
 - :name: cronjob:database_partition_management
   :worker_name: Database::PartitionManagementWorker
   :feature_category: :database

app/workers/database/drop_detached_partitions_worker.rb

+# frozen_string_literal: true
+
+module Database
+  class DropDetachedPartitionsWorker
+    include ApplicationWorker
+    include CronjobQueue # rubocop:disable Scalability/CronWorkerContext
+
+    feature_category :database
+    data_consistency :always
+    idempotent!
+
+    def perform
+      Gitlab::Database::Partitioning::DetachedPartitionDropper.new.perform
+    ensure
+      Gitlab::Database::Partitioning::PartitionMonitoring.new.report_metrics
+    end
+  end
+end

config/feature_flags/development/drop_detached_partitions.yml

+---
+name: drop_detached_partitions
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67056
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/337155
+milestone: '14.2'
+type: development
+group: group::database
+default_enabled: false

config/feature_flags/development/partition_pruning_dry_run.yml → config/feature_flags/development/partition_pruning.yml

 ---
-name: partition_pruning_dry_run
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65093
-rollout_issue_url: 
-milestone: '14.1'
+name: partition_pruning
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67056
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/337153
+milestone: '14.2'
 type: development
 group: group::database
 default_enabled: false

config/initializers/1_settings.rb

@@ -546,6 +546,9 @@ Settings.cron_jobs['update_container_registry_info_worker']['job_class'] = 'Upda
 Settings.cron_jobs['postgres_dynamic_partitions_manager'] ||= Settingslogic.new({})
 Settings.cron_jobs['postgres_dynamic_partitions_manager']['cron'] ||= '21 */6 * * *'
 Settings.cron_jobs['postgres_dynamic_partitions_manager']['job_class'] ||= 'Database::PartitionManagementWorker'
+Settings.cron_jobs['postgres_dynamic_partitions_dropper'] ||= Settingslogic.new({})
+Settings.cron_jobs['postgres_dynamic_partitions_dropper']['cron'] ||= '45 12 * * *'
+Settings.cron_jobs['postgres_dynamic_partitions_dropper']['job_class'] ||= 'Database::DropDetachedPartitionsWorker'
 Settings.cron_jobs['ci_platform_metrics_update_cron_worker'] ||= Settingslogic.new({})
 Settings.cron_jobs['ci_platform_metrics_update_cron_worker']['cron'] ||= '47 9 * * *'
 Settings.cron_jobs['ci_platform_metrics_update_cron_worker']['job_class'] = 'CiPlatformMetricsUpdateCronWorker'

db/migrate/20210628154900_create_detached_partitions_table.rb

+# frozen_string_literal: true
+
+class CreateDetachedPartitionsTable < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  def change
+    create_table_with_constraints :detached_partitions do |t|
+      t.timestamps_with_timezone null: false
+      t.datetime_with_timezone :drop_after, null: false
+      t.text :table_name, null: false
+
+      # Postgres identifier names can be up to 63 bytes
+      # See https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
+      t.text_limit :table_name, 63
+    end
+  end
+end

db/schema_migrations/20210628154900

+136a375fbd7e1faf25e7f53e0677b8525811bd917892efa1430d204453bf2a1d
\ No newline at end of file

db/structure.sql

@@ -12521,6 +12521,24 @@ CREATE SEQUENCE design_user_mentions_id_seq
 
 ALTER SEQUENCE design_user_mentions_id_seq OWNED BY design_user_mentions.id;
 
+CREATE TABLE detached_partitions (
+    id bigint NOT NULL,
+    created_at timestamp with time zone NOT NULL,
+    updated_at timestamp with time zone NOT NULL,
+    drop_after timestamp with time zone NOT NULL,
+    table_name text NOT NULL,
+    CONSTRAINT check_aecee24ba3 CHECK ((char_length(table_name) <= 63))
+);
+
+CREATE SEQUENCE detached_partitions_id_seq
+    START WITH 1
+    INCREMENT BY 1
+    NO MINVALUE
+    NO MAXVALUE
+    CACHE 1;
+
+ALTER SEQUENCE detached_partitions_id_seq OWNED BY detached_partitions.id;
+
 CREATE TABLE diff_note_positions (
     id bigint NOT NULL,
     note_id bigint NOT NULL,
@@ -20217,6 +20235,8 @@ ALTER TABLE ONLY design_management_versions ALTER COLUMN id SET DEFAULT nextval(
 
 ALTER TABLE ONLY design_user_mentions ALTER COLUMN id SET DEFAULT nextval('design_user_mentions_id_seq'::regclass);
 
+ALTER TABLE ONLY detached_partitions ALTER COLUMN id SET DEFAULT nextval('detached_partitions_id_seq'::regclass);
+
 ALTER TABLE ONLY diff_note_positions ALTER COLUMN id SET DEFAULT nextval('diff_note_positions_id_seq'::regclass);
 
 ALTER TABLE ONLY dora_daily_metrics ALTER COLUMN id SET DEFAULT nextval('dora_daily_metrics_id_seq'::regclass);
@@ -21533,6 +21553,9 @@ ALTER TABLE ONLY design_management_versions
 ALTER TABLE ONLY design_user_mentions
     ADD CONSTRAINT design_user_mentions_pkey PRIMARY KEY (id);
 
+ALTER TABLE ONLY detached_partitions
+    ADD CONSTRAINT detached_partitions_pkey PRIMARY KEY (id);
+
 ALTER TABLE ONLY diff_note_positions
     ADD CONSTRAINT diff_note_positions_pkey PRIMARY KEY (id);
 

lib/gitlab/database/partitioning/detached_partition_dropper.rb

+# frozen_string_literal: true
+module Gitlab
+  module Database
+    module Partitioning
+      class DetachedPartitionDropper
+        def perform
+          return unless Feature.enabled?(:drop_detached_partitions, default_enabled: :yaml)
+
+          Gitlab::AppLogger.info(message: "Checking for previously detached partitions to drop")
+          Postgresql::DetachedPartition.ready_to_drop.find_each do |detached_partition|
+            conn.transaction do
+              # Another process may have already dropped the table and deleted this entry
+              next unless (detached_partition = Postgresql::DetachedPartition.lock.find_by(id: detached_partition.id))
+
+              unless check_partition_detached?(detached_partition)
+                Gitlab::AppLogger.error(message: "Attempt to drop attached database partition", partition_name: detached_partition.table_name)
+                detached_partition.destroy!
+                next
+              end
+
+              drop_one(detached_partition)
+            end
+          rescue StandardError => e
+            Gitlab::AppLogger.error(message: "Failed to drop previously detached partition",
+                                    partition_name: detached_partition.table_name,
+                                    exception_class: e.class,
+                                    exception_message: e.message)
+          end
+        end
+
+        private
+
+        def drop_one(detached_partition)
+          conn.transaction do
+            conn.execute(<<~SQL)
+              DROP TABLE #{Gitlab::Database::DYNAMIC_PARTITIONS_SCHEMA}.#{conn.quote_table_name(detached_partition.table_name)}
+            SQL
+
+            detached_partition.destroy!
+          end
+          Gitlab::AppLogger.info(message: "Dropped previously detached partition", partition_name: detached_partition.table_name)
+        end
+
+        def check_partition_detached?(detached_partition)
+          # PostgresPartition checks the pg_inherits view, so our partition will only show here if it's still attached
+          # and thus should not be dropped
+          !PostgresPartition.for_identifier("#{Gitlab::Database::DYNAMIC_PARTITIONS_SCHEMA}.#{detached_partition.table_name}").exists?
+        end
+
+        def conn
+          @conn ||= ApplicationRecord.connection
+        end
+      end
+    end
+  end
+end

lib/gitlab/database/partitioning/monthly_strategy.rb

@@ -86,7 +86,7 @@ module Gitlab
         end
 
         def pruning_old_partitions?
-          Feature.enabled?(:partition_pruning_dry_run) && retain_for.present?
+          retain_for.present?
         end
 
         def oldest_active_date

lib/gitlab/database/partitioning/partition_manager.rb

@@ -4,6 +4,8 @@ module Gitlab
   module Database
     module Partitioning
       class PartitionManager
+        UnsafeToDetachPartitionError = Class.new(StandardError)
+
         def self.register(model)
           raise ArgumentError, "Only models with a #partitioning_strategy can be registered." unless model.respond_to?(:partitioning_strategy)
 
@@ -16,6 +18,7 @@ module Gitlab
 
         LEASE_TIMEOUT = 1.minute
         MANAGEMENT_LEASE_KEY = 'database_partition_management_%s'
+        RETAIN_DETACHED_PARTITIONS_FOR = 1.week
 
         attr_reader :models
 
@@ -35,13 +38,16 @@ module Gitlab
               partitions_to_create = missing_partitions(model)
               create(partitions_to_create) unless partitions_to_create.empty?
 
-              if Feature.enabled?(:partition_pruning_dry_run)
+              if Feature.enabled?(:partition_pruning, default_enabled: :yaml)
                 partitions_to_detach = extra_partitions(model)
                 detach(partitions_to_detach) unless partitions_to_detach.empty?
               end
             end
           rescue StandardError => e
-            Gitlab::AppLogger.error("Failed to create / detach partition(s) for #{model.table_name}: #{e.class}: #{e.message}")
+            Gitlab::AppLogger.error(message: "Failed to create / detach partition(s)",
+                                    table_name: model.table_name,
+                                    exception_class: e.class,
+                                    exception_message: e.message)
           end
         end
 
@@ -54,7 +60,6 @@ module Gitlab
         end
 
         def extra_partitions(model)
-          return [] unless Feature.enabled?(:partition_pruning_dry_run)
           return [] unless connection.table_exists?(model.table_name)
 
           model.partitioning_strategy.extra_partitions
@@ -74,7 +79,9 @@ module Gitlab
               partitions.each do |partition|
                 connection.execute partition.to_sql
 
-                Gitlab::AppLogger.info("Created partition #{partition.partition_name} for table #{partition.table}")
+                Gitlab::AppLogger.info(message: "Created partition",
+                                       partition_name: partition.partition_name,
+                                       table_name: partition.table)
               end
             end
           end
@@ -89,7 +96,24 @@ module Gitlab
         end
 
         def detach_one_partition(partition)
-          Gitlab::AppLogger.info("Planning to detach #{partition.partition_name} for table #{partition.table}")
+          assert_partition_detachable!(partition)
+
+          connection.execute partition.to_detach_sql
+
+          Postgresql::DetachedPartition.create!(table_name: partition.partition_name,
+                                                drop_after: RETAIN_DETACHED_PARTITIONS_FOR.from_now)
+
+          Gitlab::AppLogger.info(message: "Detached Partition",
+                                 partition_name: partition.partition_name,
+                                 table_name: partition.table)
+        end
+
+        def assert_partition_detachable!(partition)
+          parent_table_identifier = "#{connection.current_schema}.#{partition.table}"
+
+          if (example_fk = PostgresForeignKey.by_referenced_table_identifier(parent_table_identifier).first)
+            raise UnsafeToDetachPartitionError, "Cannot detach #{partition.partition_name}, it would block while checking foreign key #{example_fk.name} on #{example_fk.constrained_table_identifier}"
+          end
         end
 
         def with_lock_retries(&block)

lib/gitlab/database/partitioning/partition_monitoring.rb

@@ -16,6 +16,7 @@ module Gitlab
 
             gauge_present.set({ table: model.table_name }, strategy.current_partitions.size)
             gauge_missing.set({ table: model.table_name }, strategy.missing_partitions.size)
+            gauge_extra.set({ table: model.table_name }, strategy.extra_partitions.size)
           end
         end
 
@@ -28,6 +29,10 @@ module Gitlab
         def gauge_missing
           @gauge_missing ||= Gitlab::Metrics.gauge(:db_partitions_missing, 'Number of database partitions currently expected, but not present')
         end
+
+        def gauge_extra
+          @gauge_extra ||= Gitlab::Metrics.gauge(:db_partitions_extra, 'Number of database partitions currently attached to tables, but outside of their retention window and scheduled to be dropped')
+        end
       end
     end
   end

lib/gitlab/database/partitioning/time_partition.rb

@@ -47,6 +47,13 @@ module Gitlab
           SQL
         end
 
+        def to_detach_sql
+          <<~SQL
+            ALTER TABLE #{conn.quote_table_name(table)}
+            DETACH PARTITION #{fully_qualified_partition}
+          SQL
+        end
+
         def ==(other)
           table == other.table && partition_name == other.partition_name && from == other.from && to == other.to
         end

lib/gitlab/database/postgres_partition.rb

@@ -7,10 +7,14 @@ module Gitlab
 
       belongs_to :postgres_partitioned_table, foreign_key: 'parent_identifier', primary_key: 'identifier'
 
-      scope :by_identifier, ->(identifier) do
+      scope :for_identifier, ->(identifier) do
         raise ArgumentError, "Partition name is not fully qualified with a schema: #{identifier}" unless identifier =~ /^\w+\.\w+$/
 
-        find(identifier)
+        where(primary_key => identifier)
+      end
+
+      scope :by_identifier, ->(identifier) do
+        for_identifier(identifier).first!
       end
 
       scope :for_parent_table, ->(name) { where("parent_identifier = concat(current_schema(), '.', ?)", name).order(:name) }

spec/lib/gitlab/database/partitioning/detached_partition_dropper_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::Partitioning::DetachedPartitionDropper do
+  include Database::TableSchemaHelpers
+
+  let(:connection) { ActiveRecord::Base.connection }
+
+  def expect_partition_present(name)
+    aggregate_failures do
+      expect(table_oid(name)).not_to be_nil
+      expect(Postgresql::DetachedPartition.find_by(table_name: name)).not_to be_nil
+    end
+  end
+
+  def expect_partition_removed(name)
+    aggregate_failures do
+      expect(table_oid(name)).to be_nil
+      expect(Postgresql::DetachedPartition.find_by(table_name: name)).to be_nil
+    end
+  end
+
+  before do
+    connection.execute(<<~SQL)
+      CREATE TABLE parent_table (
+         id bigserial not null,
+         created_at timestamptz not null,
+         primary key (id, created_at)
+       ) PARTITION BY RANGE(created_at)
+    SQL
+  end
+
+  def create_partition(name:, table: 'parent_table', from:, to:, attached:, drop_after:)
+    from = from.beginning_of_month
+    to = to.beginning_of_month
+    full_name = "#{Gitlab::Database::DYNAMIC_PARTITIONS_SCHEMA}.#{name}"
+    connection.execute(<<~SQL)
+      CREATE TABLE #{full_name}
+      PARTITION OF #{table}
+      FOR VALUES FROM ('#{from.strftime('%Y-%m-%d')}') TO ('#{to.strftime('%Y-%m-%d')}')
+    SQL
+
+    unless attached
+      connection.execute(<<~SQL)
+        ALTER TABLE #{table} DETACH PARTITION #{full_name}
+      SQL
+    end
+
+    Postgresql::DetachedPartition.create!(table_name: name,
+                                          drop_after: drop_after)
+  end
+
+  describe '#perform' do
+    context 'when the partition should not be dropped yet' do
+      it 'does not drop the partition' do
+        create_partition(name: 'test_partition',
+                         from: 2.months.ago, to: 1.month.ago,
+                         attached: false,
+                         drop_after: 1.day.from_now)
+
+        subject.perform
+
+        expect_partition_present('test_partition')
+      end
+    end
+
+    context 'with a partition to drop' do
+      before do
+        create_partition(name: 'test_partition',
+                         from: 2.months.ago,
+                         to: 1.month.ago.beginning_of_month,
+                         attached: false,
+                         drop_after: 1.second.ago)
+      end
+
+      it 'drops the partition' do
+        subject.perform
+
+        expect(table_oid('test_partition')).to be_nil
+      end
+
+      context 'when the drop_detached_partitions feature flag is disabled' do
+        before do
+          stub_feature_flags(drop_detached_partitions: false)
+        end
+        it 'does not drop the partition' do
+          subject.perform
+
+          expect(table_oid('test_partition')).not_to be_nil
+        end
+      end
+
+      context 'when another process drops the table while the first waits for a lock' do
+        it 'skips the table' do
+          # Rspec's receive_method_chain does not support .and_wrap_original, so we need to nest here.
+          expect(Postgresql::DetachedPartition).to receive(:lock).and_wrap_original do |lock_meth|
+            locked = lock_meth.call
+            expect(locked).to receive(:find_by).and_wrap_original do |find_meth, *find_args|
+              # Another process drops the table then deletes this entry
+              Postgresql::DetachedPartition.where(*find_args).delete_all
+              find_meth.call(*find_args)
+            end
+
+            locked
+          end
+
+          expect(subject).not_to receive(:drop_one)
+
+          subject.perform
+        end
+      end
+    end
+
+    context 'when the partition to drop is still attached to its table' do
+      before do
+        create_partition(name: 'test_partition',
+                         from: 2.months.ago,
+                         to: 1.month.ago.beginning_of_month,
+                         attached: true,
+                         drop_after: 1.second.ago)
+      end
+
+      it 'does not drop the partition, but does remove the DetachedPartition entry' do
+        subject.perform
+        aggregate_failures do
+          expect(table_oid('test_partition')).not_to be_nil
+          expect(Postgresql::DetachedPartition.find_by(table_name: 'test_partition')).to be_nil
+        end
+      end
+
+      it 'removes the detached_partition entry' do
+        detached_partition = Postgresql::DetachedPartition.find_by!(table_name: 'test_partition')
+
+        subject.perform
+
+        expect(Postgresql::DetachedPartition.exists?(id: detached_partition.id)).to be_falsey
+      end
+    end
+
+    context 'with multiple partitions to drop' do
+      before do
+        create_partition(name: 'partition_1',
+                         from: 3.months.ago,
+                         to: 2.months.ago,
+                         attached: false,
+                         drop_after: 1.second.ago)
+
+        create_partition(name: 'partition_2',
+                         from: 2.months.ago,
+                         to: 1.month.ago,
+                         attached: false,
+                         drop_after: 1.second.ago)
+      end
+
+      it 'drops both partitions' do
+        subject.perform
+
+        expect_partition_removed('partition_1')
+        expect_partition_removed('partition_2')
+      end
+
+      context 'when the first drop returns an error' do
+        it 'still drops the second partition' do
+          expect(subject).to receive(:drop_one).ordered.and_raise('injected error')
+          expect(subject).to receive(:drop_one).ordered.and_call_original
+
+          subject.perform
+
+          # We don't know which partition we tried to drop first, so the tests here have to work with either one
+          expect(Postgresql::DetachedPartition.count).to eq(1)
+          errored_partition_name = Postgresql::DetachedPartition.first!.table_name
+
+          dropped_partition_name = (%w[partition_1 partition_2] - [errored_partition_name]).first
+          expect_partition_present(errored_partition_name)
+          expect_partition_removed(dropped_partition_name)
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/database/partitioning/monthly_strategy_spec.rb

@@ -237,16 +237,6 @@ RSpec.describe Gitlab::Database::Partitioning::MonthlyStrategy do
 
           expect(subject).to contain_exactly(min_value_to_may)
         end
-
-        context 'when the feature flag is toggled off' do
-          before do
-            stub_feature_flags(partition_pruning_dry_run: false)
-          end
-
-          it 'is empty' do
-            expect(subject).to eq([])
-          end
-        end
       end
 
       context 'with a time retention policy of 2 months' do
@@ -258,16 +248,6 @@ RSpec.describe Gitlab::Database::Partitioning::MonthlyStrategy do
                                Gitlab::Database::Partitioning::TimePartition.new(model.table_name, '2020-05-01', '2020-06-01', partition_name: 'partitioned_test_202005')
           )
         end
-
-        context 'when the feature flag is toggled off' do
-          before do
-            stub_feature_flags(partition_pruning_dry_run: false)
-          end
-
-          it 'is empty' do
-            expect(subject).to eq([])
-          end
-        end
       end
     end
   end

spec/lib/gitlab/database/partitioning/partition_manager_spec.rb

@@ -4,9 +4,14 @@ require 'spec_helper'
 
 RSpec.describe Gitlab::Database::Partitioning::PartitionManager do
   include Database::PartitioningHelpers
-  include Database::TableSchemaHelpers
   include ExclusiveLeaseHelpers
 
+  def has_partition(model, month)
+    Gitlab::Database::PostgresPartition.for_parent_table(model.table_name).any? do |partition|
+      Gitlab::Database::Partitioning::TimePartition.from_sql(model.table_name, partition.name, partition.condition).from == month
+    end
+  end
+
   describe '.register' do
     let(:model) { double(partitioning_strategy: nil) }
 
@@ -111,14 +116,14 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionManager do
 
     let(:extra_partitions) do
       [
-        instance_double(Gitlab::Database::Partitioning::TimePartition, table: table, partition_name: 'foo1'),
-        instance_double(Gitlab::Database::Partitioning::TimePartition, table: table, partition_name: 'foo2')
+        instance_double(Gitlab::Database::Partitioning::TimePartition, table: table, partition_name: 'foo1', to_detach_sql: 'SELECT 1'),
+        instance_double(Gitlab::Database::Partitioning::TimePartition, table: table, partition_name: 'foo2', to_detach_sql: 'SELECT 2')
       ]
     end
 
-    context 'with the partition_pruning_dry_run feature flag enabled' do
+    context 'with the partition_pruning feature flag enabled' do
       before do
-        stub_feature_flags(partition_pruning_dry_run: true)
+        stub_feature_flags(partition_pruning: true)
       end
 
       it 'detaches each extra partition' do
@@ -146,9 +151,9 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionManager do
       end
     end
 
-    context 'with the partition_pruning_dry_run feature flag disabled' do
+    context 'with the partition_pruning feature flag disabled' do
       before do
-        stub_feature_flags(partition_pruning_dry_run: false)
+        stub_feature_flags(partition_pruning: false)
       end
 
       it 'returns immediately' do
@@ -158,4 +163,128 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionManager do
       end
     end
   end
+
+  describe '#detach_partitions' do
+    around do |ex|
+      travel_to(Date.parse('2021-06-23')) do
+        ex.run
+      end
+    end
+
+    subject { described_class.new([my_model]).sync_partitions }
+
+    let(:connection) { ActiveRecord::Base.connection }
+    let(:my_model) do
+      Class.new(ApplicationRecord) do
+        include PartitionedTable
+
+        self.table_name = 'my_model_example_table'
+
+        partitioned_by :created_at, strategy: :monthly, retain_for: 1.month
+      end
+    end
+
+    before do
+      connection.execute(<<~SQL)
+        CREATE TABLE my_model_example_table
+        (id serial not null, created_at timestamptz not null, primary key (id, created_at))
+        PARTITION BY RANGE (created_at);
+
+        CREATE TABLE #{Gitlab::Database::DYNAMIC_PARTITIONS_SCHEMA}.my_model_example_table_202104
+        PARTITION OF my_model_example_table
+        FOR VALUES FROM ('2021-04-01') TO ('2021-05-01');
+
+        CREATE TABLE #{Gitlab::Database::DYNAMIC_PARTITIONS_SCHEMA}.my_model_example_table_202105
+        PARTITION OF my_model_example_table
+        FOR VALUES FROM ('2021-05-01') TO ('2021-06-01');
+      SQL
+
+      # Also create all future partitions so that the sync is only trying to detach old partitions
+      my_model.partitioning_strategy.missing_partitions.each do |p|
+        connection.execute p.to_sql
+      end
+    end
+
+    def num_tables
+      connection.select_value(<<~SQL)
+        SELECT COUNT(*)
+        FROM pg_class
+        where relkind IN ('r', 'p')
+      SQL
+    end
+
+    it 'detaches exactly one partition' do
+      expect { subject }.to change { find_partitions(my_model.table_name, schema: Gitlab::Database::DYNAMIC_PARTITIONS_SCHEMA).size }.from(9).to(8)
+    end
+
+    it 'detaches the old partition' do
+      expect { subject }.to change { has_partition(my_model, 2.months.ago.beginning_of_month) }.from(true).to(false)
+    end
+
+    it 'deletes zero tables' do
+      expect { subject }.not_to change { num_tables }
+    end
+
+    it 'creates the appropriate PendingPartitionDrop entry' do
+      subject
+
+      pending_drop = Postgresql::DetachedPartition.find_by!(table_name: 'my_model_example_table_202104')
+      expect(pending_drop.drop_after).to eq(Time.current + described_class::RETAIN_DETACHED_PARTITIONS_FOR)
+    end
+
+    # Postgres 11 does not support foreign keys to partitioned tables
+    if Gitlab::Database.main.version.to_f >= 12
+      context 'when the model is the target of a foreign key' do
+        before do
+          connection.execute(<<~SQL)
+        create unique index idx_for_fk ON my_model_example_table(created_at);
+
+        create table referencing_table (
+          id bigserial primary key not null,
+          referencing_created_at timestamptz references my_model_example_table(created_at)
+        );
+          SQL
+        end
+
+        it 'does not detach partitions with a referenced foreign key' do
+          expect { subject }.not_to change { find_partitions(my_model.table_name).size }
+        end
+      end
+    end
+  end
+
+  context 'creating and then detaching partitions for a table' do
+    let(:connection) { ActiveRecord::Base.connection }
+    let(:my_model) do
+      Class.new(ApplicationRecord) do
+        include PartitionedTable
+
+        self.table_name = 'my_model_example_table'
+
+        partitioned_by :created_at, strategy: :monthly, retain_for: 1.month
+      end
+    end
+
+    before do
+      connection.execute(<<~SQL)
+        CREATE TABLE my_model_example_table
+        (id serial not null, created_at timestamptz not null, primary key (id, created_at))
+        PARTITION BY RANGE (created_at);
+      SQL
+    end
+
+    def num_partitions(model)
+      find_partitions(model.table_name, schema: Gitlab::Database::DYNAMIC_PARTITIONS_SCHEMA).size
+    end
+
+    it 'creates partitions for the future then drops the oldest one after a month' do
+      # 1 month for the current month, 1 month for the old month that we're retaining data for, headroom
+      expected_num_partitions = (Gitlab::Database::Partitioning::MonthlyStrategy::HEADROOM + 2.months) / 1.month
+      expect { described_class.new([my_model]).sync_partitions }.to change { num_partitions(my_model) }.from(0).to(expected_num_partitions)
+
+      travel 1.month
+
+      expect { described_class.new([my_model]).sync_partitions }.to change { has_partition(my_model, 2.months.ago.beginning_of_month) }.from(true).to(false).and(change { num_partitions(my_model) }.by(0))
+    end
+  end
 end

spec/lib/gitlab/database/partitioning/partition_monitoring_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionMonitoring do
 
     let(:models) { [model] }
     let(:model) { double(partitioning_strategy: partitioning_strategy, table_name: table) }
-    let(:partitioning_strategy) { double(missing_partitions: missing_partitions, current_partitions: current_partitions) }
+    let(:partitioning_strategy) { double(missing_partitions: missing_partitions, current_partitions: current_partitions, extra_partitions: extra_partitions) }
     let(:table) { "some_table" }
 
     let(:missing_partitions) do
@@ -19,6 +19,10 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionMonitoring do
       [double, double]
     end
 
+    let(:extra_partitions) do
+      [double, double, double]
+    end
+
     it 'reports number of present partitions' do
       subject
 
@@ -30,5 +34,11 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionMonitoring do
 
       expect(Gitlab::Metrics.registry.get(:db_partitions_missing).get({ table: table })).to eq(missing_partitions.size)
     end
+
+    it 'reports number of extra partitions' do
+      subject
+
+      expect(Gitlab::Metrics.registry.get(:db_partitions_extra).get({ table: table })).to eq(extra_partitions.size)
+    end
   end
 end

spec/models/postgresql/detached_partition_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Postgresql::DetachedPartition do
+  describe '#ready_to_drop' do
+    let_it_be(:drop_before) { Postgresql::DetachedPartition.create!(drop_after: 1.day.ago, table_name: 'old_table') }
+    let_it_be(:drop_after) { Postgresql::DetachedPartition.create!(drop_after: 1.day.from_now, table_name: 'new_table') }
+
+    it 'includes partitions that should be dropped before now' do
+      expect(Postgresql::DetachedPartition.ready_to_drop.to_a).to include(drop_before)
+    end
+
+    it 'does not include partitions that should be dropped after now' do
+      expect(Postgresql::DetachedPartition.ready_to_drop.to_a).not_to include(drop_after)
+    end
+  end
+end

spec/workers/database/drop_detached_partitions_worker_spec.rb

+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe Database::DropDetachedPartitionsWorker do
+  describe '#perform' do
+    subject { described_class.new.perform }
+
+    let(:dropper) {  instance_double('DropDetachedPartitions', perform: nil) }
+    let(:monitoring) { instance_double('PartitionMonitoring', report_metrics: nil) }
+
+    before do
+      allow(Gitlab::Database::Partitioning::DetachedPartitionDropper).to receive(:new).and_return(dropper)
+      allow(Gitlab::Database::Partitioning::PartitionMonitoring).to receive(:new).and_return(monitoring)
+    end
+
+    it 'delegates to DropPartitionsPendingDrop' do
+      expect(dropper).to receive(:perform)
+
+      subject
+    end
+
+    it 'reports partition metrics' do
+      expect(monitoring).to receive(:report_metrics)
+
+      subject
+    end
+  end
+end