Add lifecycle index to usage event tables

Add a composite [state, <guid>, id] index on app_usage_events and
service_usage_events to support the keep-running cleanup's correlated lookups
of related lifecycle events and the backfill's existence checks. Created
concurrently on Postgres.

The task lifecycle's correlated lookups (keyed by task_guid) are served by
the existing app_usage_events_task_guid_index -- a task has only a handful of
events, so probing by task_guid alone stays cheap and no [state, task_guid,
id] index is needed.

Author: joyvuu-dave

db/migrations/20260601120000_add_lifecycle_index_to_usage_events.rb

@@ -0,0 +1,68 @@
+Sequel.migration do
+  no_transaction # to use the 'concurrently' option
+
+  up do
+    if database_type == :postgres
+      VCAP::Migration.with_concurrent_timeout(self) do
+        add_index :app_usage_events, %i[state app_guid id],
+                  name: :app_usage_events_lifecycle_index,
+                  if_not_exists: true,
+                  concurrently: true
+
+        add_index :service_usage_events, %i[state service_instance_guid id],
+                  name: :service_usage_events_lifecycle_index,
+                  if_not_exists: true,
+                  concurrently: true
+      end
+
+    elsif database_type == :mysql
+      alter_table :app_usage_events do
+        # rubocop:disable Sequel/ConcurrentIndex
+        add_index %i[state app_guid id], name: :app_usage_events_lifecycle_index unless @db.indexes(:app_usage_events).include?(:app_usage_events_lifecycle_index)
+        # rubocop:enable Sequel/ConcurrentIndex
+      end
+
+      alter_table :service_usage_events do
+        # rubocop:disable Sequel/ConcurrentIndex
+        unless @db.indexes(:service_usage_events).include?(:service_usage_events_lifecycle_index)
+          add_index %i[state service_instance_guid id],
+                    name: :service_usage_events_lifecycle_index
+        end
+        # rubocop:enable Sequel/ConcurrentIndex
+      end
+    end
+  end
+
+  down do
+    if database_type == :postgres
+      VCAP::Migration.with_concurrent_timeout(self) do
+        drop_index :app_usage_events, %i[state app_guid id],
+                   name: :app_usage_events_lifecycle_index,
+                   if_exists: true,
+                   concurrently: true
+
+        drop_index :service_usage_events, %i[state service_instance_guid id],
+                   name: :service_usage_events_lifecycle_index,
+                   if_exists: true,
+                   concurrently: true
+      end
+    end
+
+    if database_type == :mysql
+      alter_table :app_usage_events do
+        # rubocop:disable Sequel/ConcurrentIndex
+        drop_index %i[state app_guid id], name: :app_usage_events_lifecycle_index if @db.indexes(:app_usage_events).include?(:app_usage_events_lifecycle_index)
+        # rubocop:enable Sequel/ConcurrentIndex
+      end
+
+      alter_table :service_usage_events do
+        # rubocop:disable Sequel/ConcurrentIndex
+        if @db.indexes(:service_usage_events).include?(:service_usage_events_lifecycle_index)
+          drop_index %i[state service_instance_guid id],
+                     name: :service_usage_events_lifecycle_index
+        end
+        # rubocop:enable Sequel/ConcurrentIndex
+      end
+    end
+  end
+end

spec/migrations/20260601120000_add_lifecycle_index_to_usage_events_spec.rb

@@ -0,0 +1,44 @@
+require 'spec_helper'
+require 'migrations/helpers/migration_shared_context'
+
+RSpec.describe 'migration to add the lifecycle index to the usage event tables', isolation: :truncation, type: :migration do
+  include_context 'migration' do
+    let(:migration_filename) { '20260601120000_add_lifecycle_index_to_usage_events.rb' }
+  end
+
+  let(:run_migration) do
+    Sequel::Migrator.run(db, migrations_path, target: current_migration_index, allow_missing_migration_files: true)
+  end
+
+  let(:revert_migration) do
+    Sequel::Migrator.run(db, migrations_path, target: current_migration_index - 1, allow_missing_migration_files: true)
+  end
+
+  it 'adds the lifecycle index to both usage event tables (idempotently) and removes it on revert' do
+    # Before migration: the lifecycle indexes should not exist.
+    expect(db.indexes(:app_usage_events)).not_to include(:app_usage_events_lifecycle_index)
+    expect(db.indexes(:service_usage_events)).not_to include(:service_usage_events_lifecycle_index)
+
+    # Up migration adds both indexes with the expected column order.
+    expect { run_migration }.not_to raise_error
+    expect(db.indexes(:app_usage_events)).to include(:app_usage_events_lifecycle_index)
+    expect(db.indexes(:service_usage_events)).to include(:service_usage_events_lifecycle_index)
+    expect(db.indexes(:app_usage_events)[:app_usage_events_lifecycle_index][:columns]).to eq(%i[state app_guid id])
+    expect(db.indexes(:service_usage_events)[:service_usage_events_lifecycle_index][:columns]).to eq(%i[state service_instance_guid id])
+
+    # Up migration is idempotent: running again does not fail.
+    expect { run_migration }.not_to raise_error
+    expect(db.indexes(:app_usage_events)).to include(:app_usage_events_lifecycle_index)
+    expect(db.indexes(:service_usage_events)).to include(:service_usage_events_lifecycle_index)
+
+    # Down migration removes both indexes.
+    expect { revert_migration }.not_to raise_error
+    expect(db.indexes(:app_usage_events)).not_to include(:app_usage_events_lifecycle_index)
+    expect(db.indexes(:service_usage_events)).not_to include(:service_usage_events_lifecycle_index)
+
+    # Down migration is idempotent: running again does not fail.
+    expect { revert_migration }.not_to raise_error
+    expect(db.indexes(:app_usage_events)).not_to include(:app_usage_events_lifecycle_index)
+    expect(db.indexes(:service_usage_events)).not_to include(:service_usage_events_lifecycle_index)
+  end
+end