Remove not-null on type column in audit_events

We are in the process of moving away from STI model in `AuditEvent` and
dropping `type` column. There is only one type (e.g. `SecurityEvent`)
and this redundant column wastes precious storage.

Rollback is an no-op since there would be null values added to the table
after the application stop writing to this field. We will need to create
separate migration to rectify these null values if there is a need to go
back to the original schema.

changelogs/unreleased/229914-remove-constraint-on-audit-event-type.yml

+---
+title: Remove not-null constraint on type column in audit_events
+merge_request: 39192
+author:
+type: other

db/migrate/20200811055018_remove_not_null_constraint_on_type_from_audit_events.rb

+# frozen_string_literal: true
+
+class RemoveNotNullConstraintOnTypeFromAuditEvents < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  # rubocop:disable Migration/WithLockRetriesDisallowedMethod
+  # To avoid deadlock on audit_event and audit_event_part... since there is a trigger to insert record from audit_events
+  # to audit_events_part..., we need to ensure each ALTER TABLE command run in its own transaction.
+  def up
+    with_lock_retries do
+      change_column_null :audit_events_part_5fc467ac26, :type, true
+    end
+
+    with_lock_retries do
+      change_column_null :audit_events, :type, true
+    end
+  end
+  # rubocop:enable Migration/WithLockRetriesDisallowedMethod
+
+  def down
+    # no-op -- null values might be added after this constraint is removed.
+  end
+end

db/schema_migrations/20200811055018

+b7477fbcba166d848e3b1bd7f4a184f50f3b2fafe80bfcf44fd6f4f9979ffcb7
\ No newline at end of file

db/structure.sql

@@ -65,7 +65,7 @@ COMMENT ON FUNCTION public.table_sync_function_2be879775d() IS 'Partitioning mig
 CREATE TABLE public.audit_events_part_5fc467ac26 (
     id bigint NOT NULL,
     author_id integer NOT NULL,
-    type character varying NOT NULL,
+    type character varying,
     entity_id integer NOT NULL,
     entity_type character varying NOT NULL,
     details text,
@@ -9461,7 +9461,7 @@ CREATE TABLE public.ar_internal_metadata (
 CREATE TABLE public.audit_events (
     id integer NOT NULL,
     author_id integer NOT NULL,
-    type character varying NOT NULL,
+    type character varying,
     entity_id integer NOT NULL,
     entity_type character varying NOT NULL,
     details text,

spec/models/audit_event_partitioned_spec.rb

@@ -10,6 +10,13 @@ RSpec.describe AuditEventPartitioned do
     expect(partitioned_table.column_names).to match_array(source_table.column_names)
   end
 
+  it 'has the same null constraints as the source table' do
+    constraints_from_source_table = null_constraints(source_table)
+    constraints_from_partitioned_table = null_constraints(partitioned_table)
+
+    expect(constraints_from_partitioned_table.to_a).to match_array(constraints_from_source_table.to_a)
+  end
+
   it 'inserts the same record as the one in the source table', :aggregate_failures do
     expect { create(:audit_event) }.to change { partitioned_table.count }.by(1)
 
@@ -22,4 +29,13 @@ RSpec.describe AuditEventPartitioned do
 
     expect(event_from_partitioned_table).to eq(event_from_source_table)
   end
+
+  def null_constraints(table)
+    table.connection.select_all(<<~SQL)
+      SELECT c.column_name, c.is_nullable
+      FROM information_schema.columns c
+      WHERE c.table_name = '#{table.table_name}'
+      AND c.column_name != 'created_at'
+    SQL
+  end
 end