fix: Increase the parallelism config to fix flaky tests, revert bad commits (#13410)

The specs break only in Circle CI, we have to figure out the root cause
for the same. At the moment, I have increased the parallelism to fix
this.

app/builders/v2/reports/first_response_time_distribution_builder.rb

@@ -16,28 +16,27 @@ class V2::Reports::FirstResponseTimeDistributionBuilder
 
   def build_distribution
     results = fetch_aggregated_counts
-    format_results(results)
+    map_to_channel_types(results)
   end
 
   def fetch_aggregated_counts
     ReportingEvent
-      .joins('INNER JOIN inboxes ON reporting_events.inbox_id = inboxes.id')
       .where(account_id: account.id, name: 'first_response')
       .where(range_condition)
-      .group('inboxes.channel_type')
+      .group(:inbox_id)
       .select(
-        'inboxes.channel_type',
+        :inbox_id,
         bucket_case_statements
       )
   end
 
   def bucket_case_statements
     <<~SQL.squish
-      COUNT(CASE WHEN reporting_events.value < 3600 THEN 1 END) AS bucket_0_1h,
-      COUNT(CASE WHEN reporting_events.value >= 3600 AND reporting_events.value < 14400 THEN 1 END) AS bucket_1_4h,
-      COUNT(CASE WHEN reporting_events.value >= 14400 AND reporting_events.value < 28800 THEN 1 END) AS bucket_4_8h,
-      COUNT(CASE WHEN reporting_events.value >= 28800 AND reporting_events.value < 86400 THEN 1 END) AS bucket_8_24h,
-      COUNT(CASE WHEN reporting_events.value >= 86400 THEN 1 END) AS bucket_24h_plus
+      COUNT(CASE WHEN value < 3600 THEN 1 END) AS bucket_0_1h,
+      COUNT(CASE WHEN value >= 3600 AND value < 14400 THEN 1 END) AS bucket_1_4h,
+      COUNT(CASE WHEN value >= 14400 AND value < 28800 THEN 1 END) AS bucket_4_8h,
+      COUNT(CASE WHEN value >= 28800 AND value < 86400 THEN 1 END) AS bucket_8_24h,
+      COUNT(CASE WHEN value >= 86400 THEN 1 END) AS bucket_24h_plus
     SQL
   end
 
@@ -45,15 +44,25 @@ class V2::Reports::FirstResponseTimeDistributionBuilder
     range.present? ? { created_at: range } : {}
   end
 
-  def format_results(results)
+  def inbox_channel_types
+    @inbox_channel_types ||= account.inboxes.pluck(:id, :channel_type).to_h
+  end
+
+  def map_to_channel_types(results)
     results.each_with_object({}) do |row, hash|
-      hash[row.channel_type] = {
-        '0-1h' => row.bucket_0_1h,
-        '1-4h' => row.bucket_1_4h,
-        '4-8h' => row.bucket_4_8h,
-        '8-24h' => row.bucket_8_24h,
-        '24h+' => row.bucket_24h_plus
-      }
+      channel_type = inbox_channel_types[row.inbox_id]
+      next unless channel_type
+
+      hash[channel_type] ||= empty_buckets
+      hash[channel_type]['0-1h'] += row.bucket_0_1h
+      hash[channel_type]['1-4h'] += row.bucket_1_4h
+      hash[channel_type]['4-8h'] += row.bucket_4_8h
+      hash[channel_type]['8-24h'] += row.bucket_8_24h
+      hash[channel_type]['24h+'] += row.bucket_24h_plus
     end
   end
+
+  def empty_buckets
+    { '0-1h' => 0, '1-4h' => 0, '4-8h' => 0, '8-24h' => 0, '24h+' => 0 }
+  end
 end