Speed up searching environments by ref deployed

Current query looks over all project deployments
and aggregates by environment_id

New query loops over project's environments and
tries to find existing deployment for it

We also add a dedicated index on
[environment_id, ref] to speed up this new query.

Changelog: added

app/finders/environments/environments_by_deployments_finder.rb

@@ -23,12 +23,18 @@ module Environments
           deployments.none
         end
 
-      environment_ids = deployments
-        .group(:environment_id)
-        .select(:environment_id)
+      environments =
+        if Feature.enabled?(:environments_by_deployments_finder_exists_optimization, default_enabled: :yaml)
+          project.environments.available
+            .where('EXISTS (?)', deployments.where('environment_id = environments.id'))
+        else
+          environment_ids = deployments
+            .group(:environment_id)
+            .select(:environment_id)
 
-      environments = project.environments.available
-        .where(id: environment_ids)
+          project.environments.available
+            .where(id: environment_ids)
+        end
 
       if params[:find_latest]
         find_one(environments.order_by_last_deployed_at_desc)

config/feature_flags/development/environments_by_deployments_finder_exists_optimization.yml

+---
+name: environments_by_deployments_finder_exists_optimization
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72781/
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/343544
+milestone: '14.5'
+type: development
+group: group::release
+default_enabled: false

db/post_migrate/20211021131217_add_deployments_environment_id_and_ref_index.rb

+# frozen_string_literal: true
+
+class AddDeploymentsEnvironmentIdAndRefIndex < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'index_deployments_on_environment_id_and_ref'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :deployments, [:environment_id, :ref], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :deployments, INDEX_NAME
+  end
+end

db/schema_migrations/20211021131217

+87834e00821eb1ed8489c1d772dc3ac743bcf84669e78c04c7988f6f761970b8
\ No newline at end of file

db/structure.sql

@@ -25120,6 +25120,8 @@ CREATE INDEX index_deployments_on_environment_id_and_id ON deployments USING btr
 
 CREATE INDEX index_deployments_on_environment_id_and_iid_and_project_id ON deployments USING btree (environment_id, iid, project_id);
 
+CREATE INDEX index_deployments_on_environment_id_and_ref ON deployments USING btree (environment_id, ref);
+
 CREATE INDEX index_deployments_on_environment_id_status_and_finished_at ON deployments USING btree (environment_id, status, finished_at);
 
 CREATE INDEX index_deployments_on_environment_id_status_and_id ON deployments USING btree (environment_id, status, id);

spec/finders/environments/environments_by_deployments_finder_spec.rb

@@ -11,7 +11,7 @@ RSpec.describe Environments::EnvironmentsByDeploymentsFinder do
     project.add_maintainer(user)
   end
 
-  describe '#execute' do
+  shared_examples 'execute' do
     context 'tagged deployment' do
       let(:environment_two) { create(:environment, project: project) }
       # Environments need to include commits, so rewind two commits to fit
@@ -124,4 +124,16 @@ RSpec.describe Environments::EnvironmentsByDeploymentsFinder do
       end
     end
   end
+
+  describe "#execute" do
+    include_examples 'execute'
+
+    context 'when environments_by_deployments_finder_exists_optimization is disabled' do
+      before do
+        stub_feature_flags(environments_by_deployments_finder_exists_optimization: false)
+      end
+
+      include_examples 'execute'
+    end
+  end
 end