Fix PG::GroupingError with label filters

This change fixes the PG::GroupingError when two label filters are given
in the merge request analytics filters.

app/models/concerns/issuable.rb

@@ -352,12 +352,15 @@ module Issuable
     #
     # Returns an array of arel columns
     def grouping_columns(sort)
+      sort = sort.to_s
       grouping_columns = [arel_table[:id]]
 
       if %w(milestone_due_desc milestone_due_asc milestone).include?(sort)
         milestone_table = Milestone.arel_table
         grouping_columns << milestone_table[:id]
         grouping_columns << milestone_table[:due_date]
+      elsif %w(merged_at_desc merged_at_asc).include?(sort)
+        grouping_columns << MergeRequest::Metrics.arel_table[:merged_at]
       end
 
       grouping_columns

changelogs/unreleased/280535-fix-mr-analytics-query-error.yml

+---
+title: Fix merge request analytics page filtering when multiple labels are given
+merge_request: 51060
+author:
+type: fixed

spec/graphql/resolvers/merge_requests_resolver_spec.rb

@@ -231,6 +231,16 @@ RSpec.describe Resolvers::MergeRequestsResolver do
         it 'sorts merge requests descending' do
           expect(resolve_mr(project, sort: :merged_at_desc)).to eq [merge_request_3, merge_request_1, merge_request_with_milestone, merge_request_6, merge_request_5, merge_request_4, merge_request_2]
         end
+
+        context 'when label filter is given and the optimized_issuable_label_filter feature flag is off' do
+          before do
+            stub_feature_flags(optimized_issuable_label_filter: false)
+          end
+
+          it 'does not raise PG::GroupingError' do
+            expect { resolve_mr(project, sort: :merged_at_desc, labels: %w[a b]) }.not_to raise_error
+          end
+        end
       end
     end
   end