Fix issuable search optimization in PG12

Uses a materialized CTE so that our optimization fence actually works.

Recursive CTEs are materialized by default in PG12 but we're not using
this recursively here so we use Gitlab::SQL::CTE which handles adding of
the MATERIALIZED keyword when needed.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61880

app/finders/issuable_finder.rb

@@ -334,8 +334,7 @@ class IssuableFinder
     return items if items.is_a?(ActiveRecord::NullRelation)
 
     if use_cte_for_search?
-      cte = Gitlab::SQL::RecursiveCTE.new(klass.table_name)
-      cte << items
+      cte = Gitlab::SQL::CTE.new(klass.table_name, items)
 
       items = klass.with(cte.to_arel).from(klass.table_name)
     end

changelogs/unreleased/331089-issue-search-materialized-cte.yml

+---
+title: Fix issuable search optimization in PG12
+merge_request: 61880
+author:
+type: fixed

spec/finders/issues_finder_spec.rb

@@ -1178,6 +1178,7 @@ RSpec.describe IssuesFinder do
 
         it 'returns true' do
           expect(finder.use_cte_for_search?).to be_truthy
+          expect(finder.execute.to_sql).to match(/^WITH "issues" AS #{Gitlab::Database::AsWithMaterialized.materialized_if_supported}/)
         end
       end
 
@@ -1186,6 +1187,7 @@ RSpec.describe IssuesFinder do
 
         it 'returns true' do
           expect(finder.use_cte_for_search?).to be_truthy
+          expect(finder.execute.to_sql).to match(/^WITH "issues" AS #{Gitlab::Database::AsWithMaterialized.materialized_if_supported}/)
         end
       end
     end