Fix statement timeouts in RemoveRestrictedTodos migration

On GitLab.com, the RemoveRestrictedTodos background migration
encountered about 700+ failures a day due to statement timeouts.

PostgreSQL might perform badly with a LIMIT 1 because the planner is
guessing that scanning the index in ID order will come across the
desired row in less time it will take the planner than using another
index. The order_hint does not affect the search results. For example,
`ORDER BY id ASC, updated_at ASC` means the same thing as `ORDER BY id
ASC`.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52649

4 files changed, 49 insertions, 26 deletions

diff --git a/app/models/concerns/each_batch.rb b/app/models/concerns/each_batch.rb
index 8cf0b8b154d..6314b46a7e3 100644
--- a/app/models/concerns/each_batch.rb
+++ b/app/models/concerns/each_batch.rb
@@ -39,7 +39,15 @@ module EachBatch
     #
     # of - The number of rows to retrieve per batch.
     # column - The column to use for ordering the batches.
-    def each_batch(of: 1000, column: primary_key)
+    # order_hint - An optional column to append to the `ORDER BY id`
+    #   clause to help the query planner. PostgreSQL might perform badly
+    #   with a LIMIT 1 because the planner is guessing that scanning the
+    #   index in ID order will come across the desired row in less time
+    #   it will take the planner than using another index. The
+    #   order_hint does not affect the search results. For example,
+    #   `ORDER BY id ASC, updated_at ASC` means the same thing as `ORDER
+    #   BY id ASC`.
+    def each_batch(of: 1000, column: primary_key, order_hint: nil)
       unless column
         raise ArgumentError,
           'the column: argument must be set to a column name to use for ordering rows'
@@ -48,7 +56,9 @@ module EachBatch
       start = except(:select)
         .select(column)
         .reorder(column => :asc)
-        .take
+
+      start = start.order(order_hint) if order_hint
+      start = start.take
 
       return unless start
 
@@ -60,6 +70,9 @@ module EachBatch
           .select(column)
           .where(arel_table[column].gteq(start_id))
           .reorder(column => :asc)
+
+        stop = stop.order(order_hint) if order_hint
+        stop = stop
           .offset(of)
           .limit(1)
           .take
diff --git a/changelogs/unreleased/sh-fix-issue-52649.yml b/changelogs/unreleased/sh-fix-issue-52649.yml
new file mode 100644
index 00000000000..34b7f74a345
--- /dev/null
+++ b/changelogs/unreleased/sh-fix-issue-52649.yml
@@ -0,0 +1,5 @@
+---
+title: Fix statement timeouts in RemoveRestrictedTodos migration
+merge_request: 22795
+author:
+type: other
diff --git a/lib/gitlab/background_migration/remove_restricted_todos.rb b/lib/gitlab/background_migration/remove_restricted_todos.rb
index 9941c2fe6d9..47579d46c1b 100644
--- a/lib/gitlab/background_migration/remove_restricted_todos.rb
+++ b/lib/gitlab/background_migration/remove_restricted_todos.rb
@@ -67,7 +67,7 @@ module Gitlab
           .where('access_level >= ?', 20)
 
         confidential_issues = Issue.select(:id, :author_id).where(confidential: true, project_id: project_id)
-        confidential_issues.each_batch(of: 100) do |batch|
+        confidential_issues.each_batch(of: 100, order_hint: :confidential) do |batch|
           batch.each do |issue|
             assigned_users = IssueAssignee.select(:user_id).where(issue_id: issue.id)
 
diff --git a/spec/models/concerns/each_batch_spec.rb b/spec/models/concerns/each_batch_spec.rb
index 951690a217b..17224c09693 100644
--- a/spec/models/concerns/each_batch_spec.rb
+++ b/spec/models/concerns/each_batch_spec.rb
@@ -14,40 +14,45 @@ describe EachBatch do
       5.times { create(:user, updated_at: 1.day.ago) }
     end
 
-    it 'yields an ActiveRecord::Relation when a block is given' do
-      model.each_batch do |relation|
-        expect(relation).to be_a_kind_of(ActiveRecord::Relation)
+    shared_examples 'each_batch handling' do |kwargs|
+      it 'yields an ActiveRecord::Relation when a block is given' do
+        model.each_batch(kwargs) do |relation|
+          expect(relation).to be_a_kind_of(ActiveRecord::Relation)
+        end
       end
-    end
 
-    it 'yields a batch index as the second argument' do
-      model.each_batch do |_, index|
-        expect(index).to eq(1)
+      it 'yields a batch index as the second argument' do
+        model.each_batch(kwargs) do |_, index|
+          expect(index).to eq(1)
+        end
       end
-    end
 
-    it 'accepts a custom batch size' do
-      amount = 0
+      it 'accepts a custom batch size' do
+        amount = 0
 
-      model.each_batch(of: 1) { amount += 1 }
+        model.each_batch(kwargs.merge({ of: 1 })) { amount += 1 }
 
-      expect(amount).to eq(5)
-    end
+        expect(amount).to eq(5)
+      end
 
-    it 'does not include ORDER BYs in the yielded relations' do
-      model.each_batch do |relation|
-        expect(relation.to_sql).not_to include('ORDER BY')
+      it 'does not include ORDER BYs in the yielded relations' do
+        model.each_batch do |relation|
+          expect(relation.to_sql).not_to include('ORDER BY')
+        end
       end
-    end
 
-    it 'allows updating of the yielded relations' do
-      time = Time.now
+      it 'allows updating of the yielded relations' do
+        time = Time.now
 
-      model.each_batch do |relation|
-        relation.update_all(updated_at: time)
-      end
+        model.each_batch do |relation|
+          relation.update_all(updated_at: time)
+        end
 
-      expect(model.where(updated_at: time).count).to eq(5)
+        expect(model.where(updated_at: time).count).to eq(5)
+      end
     end
+
+    it_behaves_like 'each_batch handling', {}
+    it_behaves_like 'each_batch handling', { order_hint: :updated_at }
   end
 end