Inconsistent schema - indexes

- Fix approval_rules_code_owners_rule_type index

Changelog: fixed

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/349549

db/post_migrate/20220128155814_fix_approval_rules_code_owners_rule_type_index.rb

+# frozen_string_literal: true
+
+class FixApprovalRulesCodeOwnersRuleTypeIndex < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'index_approval_rules_code_owners_rule_type'
+  OLD_INDEX_NAME = 'index_approval_rules_code_owners_rule_type_old'
+  TABLE = :approval_merge_request_rules
+  COLUMN = :merge_request_id
+  WHERE_CONDITION = 'rule_type = 2'
+
+  disable_ddl_transaction!
+
+  def up
+    rename_index TABLE, INDEX_NAME, OLD_INDEX_NAME if index_exists_by_name?(TABLE, INDEX_NAME) && !index_exists_by_name?(TABLE, OLD_INDEX_NAME)
+
+    add_concurrent_index TABLE, COLUMN, where: WHERE_CONDITION, name: INDEX_NAME
+
+    remove_concurrent_index_by_name TABLE, OLD_INDEX_NAME
+  end
+
+  def down
+    # No-op
+  end
+end

db/schema_migrations/20220128155814

+77cc8fc86f2c6a5ed017dde40dd4db796821a35e6ce4d8dcbe24b2cdaccbb5d9
\ No newline at end of file

spec/migrations/20220128155814_fix_approval_rules_code_owners_rule_type_index_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!('fix_approval_rules_code_owners_rule_type_index')
+
+RSpec.describe FixApprovalRulesCodeOwnersRuleTypeIndex, :migration do
+  let(:table_name) { :approval_merge_request_rules }
+  let(:index_name) { 'index_approval_rules_code_owners_rule_type' }
+
+  it 'correctly migrates up and down' do
+    reversible_migration do |migration|
+      migration.before -> {
+        expect(subject.index_exists_by_name?(table_name, index_name)).to be_truthy
+      }
+
+      migration.after -> {
+        expect(subject.index_exists_by_name?(table_name, index_name)).to be_truthy
+      }
+    end
+  end
+
+  context 'when the index already exists' do
+    before do
+      subject.add_concurrent_index table_name, :merge_request_id, where: 'rule_type = 2', name: index_name
+    end
+
+    it 'keeps the index' do
+      migrate!
+
+      expect(subject.index_exists_by_name?(table_name, index_name)).to be_truthy
+    end
+  end
+end