Add migration helper to swap partitioned tables

Add a new migration helper that completes the last step of a
partitioning migration: swapping the partitioned copy with the original.
In addition to renaming the tables, it also renames the primary keys and
moves the primary key sequences appropriately.

lib/gitlab/database/partitioning/replace_table.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module Partitioning
+      class ReplaceTable
+        DELIMITER = ";\n\n"
+
+        attr_reader :original_table, :replacement_table, :replaced_table, :primary_key_column,
+          :sequence, :original_primary_key, :replacement_primary_key, :replaced_primary_key
+
+        def initialize(original_table, replacement_table, replaced_table, primary_key_column)
+          @original_table = original_table
+          @replacement_table = replacement_table
+          @replaced_table = replaced_table
+          @primary_key_column = primary_key_column
+
+          @sequence = default_sequence(original_table, primary_key_column)
+          @original_primary_key = default_primary_key(original_table)
+          @replacement_primary_key = default_primary_key(replacement_table)
+          @replaced_primary_key = default_primary_key(replaced_table)
+        end
+
+        def perform
+          yield sql_to_replace_table if block_given?
+
+          execute(sql_to_replace_table)
+        end
+
+        private
+
+        delegate :execute, :quote_table_name, :quote_column_name, to: :connection
+        def connection
+          @connection ||= ActiveRecord::Base.connection
+        end
+
+        def default_sequence(table, column)
+          "#{table}_#{column}_seq"
+        end
+
+        def default_primary_key(table)
+          "#{table}_pkey"
+        end
+
+        def sql_to_replace_table
+          @sql_to_replace_table ||= [
+            drop_default_sql(original_table, primary_key_column),
+            set_default_sql(replacement_table, primary_key_column, "nextval('#{quote_table_name(sequence)}'::regclass)"),
+
+            change_sequence_owner_sql(sequence, replacement_table, primary_key_column),
+
+            rename_table_sql(original_table, replaced_table),
+            rename_constraint_sql(replaced_table, original_primary_key, replaced_primary_key),
+
+            rename_table_sql(replacement_table, original_table),
+            rename_constraint_sql(original_table, replacement_primary_key, original_primary_key)
+          ].join(DELIMITER)
+        end
+
+        def drop_default_sql(table, column)
+          "ALTER TABLE #{quote_table_name(table)} ALTER COLUMN #{quote_column_name(column)} DROP DEFAULT"
+        end
+
+        def set_default_sql(table, column, expression)
+          "ALTER TABLE #{quote_table_name(table)} ALTER COLUMN #{quote_column_name(column)} SET DEFAULT #{expression}"
+        end
+
+        def change_sequence_owner_sql(sequence, table, column)
+          "ALTER SEQUENCE #{quote_table_name(sequence)} OWNED BY #{quote_table_name(table)}.#{quote_column_name(column)}"
+        end
+
+        def rename_table_sql(old_name, new_name)
+          "ALTER TABLE #{quote_table_name(old_name)} RENAME TO #{quote_table_name(new_name)}"
+        end
+
+        def rename_constraint_sql(table, old_name, new_name)
+          "ALTER TABLE #{quote_table_name(table)} RENAME CONSTRAINT #{quote_column_name(old_name)} TO #{quote_column_name(new_name)}"
+        end
+      end
+    end
+  end
+end

lib/gitlab/database/partitioning_migration_helpers/table_management_helpers.rb

@@ -66,8 +66,11 @@ module Gitlab
             create_range_partitioned_copy(table_name, partitioned_table_name, partition_column, primary_key)
             create_daterange_partitions(partitioned_table_name, partition_column.name, min_date, max_date)
           end
+
+          with_lock_retries do
             create_trigger_to_sync_tables(table_name, partitioned_table_name, primary_key)
           end
+        end
 
         # Clean up a partitioned copy of an existing table. First, deletes the database function and trigger that were
         # used to copy writes to the partitioned table, then removes the partitioned table (also removing partitions).
@@ -81,13 +84,9 @@ module Gitlab
           assert_not_in_transaction_block(scope: ERROR_SCOPE)
 
           with_lock_retries do
-            trigger_name = make_sync_trigger_name(table_name)
-            drop_trigger(table_name, trigger_name)
+            drop_sync_trigger(table_name)
           end
 
-          function_name = make_sync_function_name(table_name)
-          drop_function(function_name)
-
           partitioned_table_name = make_partitioned_table_name(table_name)
           drop_table(partitioned_table_name)
         end
@@ -177,6 +176,52 @@ module Gitlab
           end
         end
 
+        # Replaces a non-partitioned table with its partitioned copy. This is the final step in a partitioning
+        # migration, which makes the partitioned table ready for use by the application. The partitioned copy should be
+        # replaced with the original table in such a way that it appears seamless to any database clients. The replaced
+        # table will be renamed to "#{replaced_table}_archived"
+        #
+        # **NOTE** This method should only be used after all other migration steps have completed successfully.
+        # There are several limitations to this method that MUST be handled before, or during, the swap migration:
+        #
+        # - Secondary indexes and foreign keys are not automatically recreated on the partitioned table.
+        # - Some types of constraints (UNIQUE and EXCLUDE) which rely on indexes, will not automatically be recreated
+        #   on the partitioned table, since the underlying index will not be present.
+        # - Foreign keys referencing the original non-partitioned table, would also need to be updated to reference the
+        #   partitioned table, but unfortunately this is not supported in PG11.
+        # - Views referencing the original table will not be automatically updated to reference the partitioned table.
+        #
+        # Example:
+        #
+        #   replace_with_partitioned_table :audit_events
+        #
+        def replace_with_partitioned_table(table_name)
+          assert_table_is_allowed(table_name)
+
+          partitioned_table_name = make_partitioned_table_name(table_name)
+          archived_table_name = make_archived_table_name(table_name)
+          primary_key_name = connection.primary_key(table_name)
+
+          replace_table(table_name, partitioned_table_name, archived_table_name, primary_key_name)
+        end
+
+        # Rolls back a migration that replaced a non-partitioned table with its partitioned copy. This can be used to
+        # restore the original non-partitioned table in the event of an unexpected issue.
+        #
+        # Example:
+        #
+        #   rollback_replace_with_partitioned_table :audit_events
+        #
+        def rollback_replace_with_partitioned_table(table_name)
+          assert_table_is_allowed(table_name)
+
+          partitioned_table_name = make_partitioned_table_name(table_name)
+          archived_table_name = make_archived_table_name(table_name)
+          primary_key_name = connection.primary_key(archived_table_name)
+
+          replace_table(table_name, archived_table_name, partitioned_table_name, primary_key_name)
+        end
+
         private
 
         def assert_table_is_allowed(table_name)
@@ -190,6 +235,10 @@ module Gitlab
           tmp_table_name("#{table}_part")
         end
 
+        def make_archived_table_name(table)
+          "#{table}_archived"
+        end
+
         def make_sync_function_name(table)
           object_name(table, 'table_sync_function')
         end
@@ -270,12 +319,18 @@ module Gitlab
           function_name = make_sync_function_name(source_table_name)
           trigger_name = make_sync_trigger_name(source_table_name)
 
-          with_lock_retries do
           create_sync_function(function_name, partitioned_table_name, unique_key)
           create_comment('FUNCTION', function_name, "Partitioning migration: table sync for #{source_table_name} table")
 
           create_sync_trigger(source_table_name, trigger_name, function_name)
         end
+
+        def drop_sync_trigger(source_table_name)
+          trigger_name = make_sync_trigger_name(source_table_name)
+          drop_trigger(source_table_name, trigger_name)
+
+          function_name = make_sync_function_name(source_table_name)
+          drop_function(function_name)
         end
 
         def create_sync_function(name, partitioned_table_name, unique_key)
@@ -358,6 +413,21 @@ module Gitlab
             end
           end
         end
+
+        def replace_table(original_table_name, replacement_table_name, replaced_table_name, primary_key_name)
+          replace_table = Gitlab::Database::Partitioning::ReplaceTable.new(original_table_name,
+              replacement_table_name, replaced_table_name, primary_key_name)
+
+          with_lock_retries do
+            drop_sync_trigger(original_table_name)
+
+            replace_table.perform do |sql|
+              say("replace_table(\"#{sql}\")")
+            end
+
+            create_trigger_to_sync_tables(original_table_name, replaced_table_name, primary_key_name)
+          end
+        end
       end
     end
   end

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

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::Partitioning::ReplaceTable, '#perform' do
+  include TableSchemaHelpers
+
+  subject { described_class.new(original_table, replacement_table, archived_table, 'id') }
+
+  let(:original_table) { '_test_original_table' }
+  let(:replacement_table) { '_test_replacement_table' }
+  let(:archived_table) { '_test_archived_table' }
+
+  let(:original_sequence) { "#{original_table}_id_seq" }
+
+  let(:original_primary_key) { "#{original_table}_pkey" }
+  let(:replacement_primary_key) { "#{replacement_table}_pkey" }
+  let(:archived_primary_key) { "#{archived_table}_pkey" }
+
+  before do
+    connection.execute(<<~SQL)
+      CREATE TABLE #{original_table} (
+        id serial NOT NULL PRIMARY KEY,
+        original_column text NOT NULL,
+        created_at timestamptz NOT NULL);
+
+      CREATE TABLE #{replacement_table} (
+        id int NOT NULL,
+        replacement_column text NOT NULL,
+        created_at timestamptz NOT NULL,
+        PRIMARY KEY (id, created_at))
+        PARTITION BY RANGE (created_at);
+
+      CREATE TABLE #{replacement_table}_202001 PARTITION OF #{replacement_table}
+      FOR VALUES FROM ('2020-01-01') TO ('2020-02-01');
+    SQL
+  end
+
+  it 'archives the old table and puts the replacement in its place' do
+    expect(table_type(original_table)).to eq('normal')
+    expect(table_type(replacement_table)).to eq('partitioned')
+    expect(table_type(archived_table)).to be_nil
+
+    subject.perform
+
+    expect(table_type(original_table)).to eq('partitioned')
+    expect(table_type(archived_table)).to eq('normal')
+    expect(table_type(replacement_table)).to be_nil
+  end
+
+  it 'transfers the primary key sequence to the replacement table' do
+    expect(sequence_owned_by(original_table, 'id')).to eq(original_sequence)
+    expect(default_expression_for(original_table, 'id')).to eq("nextval('#{original_sequence}'::regclass)")
+
+    expect(sequence_owned_by(replacement_table, 'id')).to be_nil
+    expect(default_expression_for(replacement_table, 'id')).to be_nil
+
+    subject.perform
+
+    expect(sequence_owned_by(original_table, 'id')).to eq(original_sequence)
+    expect(default_expression_for(original_table, 'id')).to eq("nextval('#{original_sequence}'::regclass)")
+    expect(sequence_owned_by(archived_table, 'id')).to be_nil
+    expect(default_expression_for(archived_table, 'id')).to be_nil
+  end
+
+  it 'renames the primary key constraints to match the new table names' do
+    expect(primary_key_constraint_name(original_table)).to eq(original_primary_key)
+    expect(primary_key_constraint_name(replacement_table)).to eq(replacement_primary_key)
+
+    subject.perform
+
+    expect(primary_key_constraint_name(original_table)).to eq(original_primary_key)
+    expect(primary_key_constraint_name(archived_table)).to eq(archived_primary_key)
+  end
+
+  def connection
+    ActiveRecord::Base.connection
+  end
+end

spec/lib/gitlab/database/partitioning_migration_helpers/table_management_helpers_spec.rb

@@ -5,6 +5,7 @@ require 'spec_helper'
 RSpec.describe Gitlab::Database::PartitioningMigrationHelpers::TableManagementHelpers do
   include PartitioningHelpers
   include TriggerHelpers
+  include TableSchemaHelpers
 
   let(:migration) do
     ActiveRecord::Migration.new.extend(described_class)
@@ -629,6 +630,68 @@ RSpec.describe Gitlab::Database::PartitioningMigrationHelpers::TableManagementHe
     end
   end
 
+  describe '#replace_with_partitioned_table' do
+    let(:archived_table) { "#{source_table}_archived" }
+
+    before do
+      migration.partition_table_by_date source_table, partition_column, min_date: min_date, max_date: max_date
+    end
+
+    it 'replaces the original table with the partitioned table' do
+      expect(table_type(source_table)).to eq('normal')
+      expect(table_type(partitioned_table)).to eq('partitioned')
+      expect(table_type(archived_table)).to be_nil
+
+      migration.replace_with_partitioned_table source_table
+
+      expect(table_type(source_table)).to eq('partitioned')
+      expect(table_type(archived_table)).to eq('normal')
+      expect(table_type(partitioned_table)).to be_nil
+    end
+
+    it 'moves the trigger from the original table to the new table' do
+      expect_function_to_exist(function_name)
+      expect_valid_function_trigger(source_table, trigger_name, function_name, after: %w[delete insert update])
+
+      migration.replace_with_partitioned_table source_table
+
+      expect_function_to_exist(function_name)
+      expect_valid_function_trigger(source_table, trigger_name, function_name, after: %w[delete insert update])
+    end
+  end
+
+  describe '#rollback_replace_with_partitioned_table' do
+    let(:archived_table) { "#{source_table}_archived" }
+
+    before do
+      migration.partition_table_by_date source_table, partition_column, min_date: min_date, max_date: max_date
+
+      migration.replace_with_partitioned_table source_table
+    end
+
+    it 'replaces the partitioned table with the non-partitioned table' do
+      expect(table_type(source_table)).to eq('partitioned')
+      expect(table_type(archived_table)).to eq('normal')
+      expect(table_type(partitioned_table)).to be_nil
+
+      migration.rollback_replace_with_partitioned_table source_table
+
+      expect(table_type(source_table)).to eq('normal')
+      expect(table_type(partitioned_table)).to eq('partitioned')
+      expect(table_type(archived_table)).to be_nil
+    end
+
+    it 'moves the trigger from the partitioned table to the non-partitioned table' do
+      expect_function_to_exist(function_name)
+      expect_valid_function_trigger(source_table, trigger_name, function_name, after: %w[delete insert update])
+
+      migration.rollback_replace_with_partitioned_table source_table
+
+      expect_function_to_exist(function_name)
+      expect_valid_function_trigger(source_table, trigger_name, function_name, after: %w[delete insert update])
+    end
+  end
+
   def filter_columns_by_name(columns, names)
     columns.reject { |c| names.include?(c.name) }
   end

spec/support/helpers/table_schema_helpers.rb

+# frozen_string_literal: true
+
+module TableSchemaHelpers
+  def table_type(name)
+    connection.select_value(<<~SQL)
+      SELECT
+        CASE class.relkind
+        WHEN 'r' THEN 'normal'
+        WHEN 'p' THEN 'partitioned'
+        ELSE 'other'
+        END as table_type
+      FROM pg_catalog.pg_class class
+      WHERE class.relname = '#{name}'
+    SQL
+  end
+
+  def sequence_owned_by(table_name, column_name)
+    connection.select_value(<<~SQL)
+      SELECT
+        sequence.relname as name
+      FROM pg_catalog.pg_class as sequence
+      INNER JOIN pg_catalog.pg_depend depend
+        ON depend.objid = sequence.relfilenode
+      INNER JOIN pg_catalog.pg_class class
+        ON class.relfilenode = depend.refobjid
+      INNER JOIN pg_catalog.pg_attribute attribute
+        ON attribute.attnum = depend.refobjsubid
+        AND attribute.attrelid = depend.refobjid
+      WHERE class.relname = '#{table_name}'
+        AND attribute.attname = '#{column_name}'
+    SQL
+  end
+
+  def default_expression_for(table_name, column_name)
+    connection.select_value(<<~SQL)
+      SELECT
+        pg_get_expr(attrdef.adbin, attrdef.adrelid) AS default_value
+      FROM pg_catalog.pg_attribute attribute
+      INNER JOIN pg_catalog.pg_attrdef attrdef
+        ON attribute.attrelid = attrdef.adrelid
+        AND attribute.attnum = attrdef.adnum
+      WHERE attribute.attrelid = '#{table_name}'::regclass
+        AND attribute.attname = '#{column_name}'
+    SQL
+  end
+
+  def primary_key_constraint_name(table_name)
+    connection.select_value(<<~SQL)
+      SELECT
+        conname AS constraint_name
+      FROM pg_catalog.pg_constraint
+      WHERE conrelid = '#{table_name}'::regclass
+        AND contype = 'p'
+    SQL
+  end
+end