Refactors median code to work with both single and multiple projects

5 files changed, 135 insertions, 72 deletions

diff --git a/lib/gitlab/cycle_analytics/base_query.rb b/lib/gitlab/cycle_analytics/base_query.rb
index b9075047487..def9885789e 100644
--- a/lib/gitlab/cycle_analytics/base_query.rb
+++ b/lib/gitlab/cycle_analytics/base_query.rb
@@ -8,14 +8,14 @@ module Gitlab
       private
 
       def base_query
-        @base_query ||= stage_query([@project.id]) # rubocop:disable Gitlab/ModuleWithInstanceVariables
+        @base_query ||= stage_query(@project.id) # rubocop:disable Gitlab/ModuleWithInstanceVariables
       end
 
       def stage_query(project_ids)
         query = mr_closing_issues_table.join(issue_table).on(issue_table[:id].eq(mr_closing_issues_table[:issue_id]))
           .join(issue_metrics_table).on(issue_table[:id].eq(issue_metrics_table[:issue_id]))
           .project(issue_table[:project_id].as("project_id"))
-          .where(issue_table[:project_id].in(project_ids))
+          .where(issue_table[:project_id].in(Array(project_ids)))
           .where(issue_table[:created_at].gteq(@options[:from])) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
         # Load merge_requests
diff --git a/lib/gitlab/cycle_analytics/base_stage.rb b/lib/gitlab/cycle_analytics/base_stage.rb
index 45386fee622..5c3fd901f79 100644
--- a/lib/gitlab/cycle_analytics/base_stage.rb
+++ b/lib/gitlab/cycle_analytics/base_stage.rb
@@ -31,8 +31,12 @@ module Gitlab
           interval_query = Arel::Nodes::As.new(cte_table,
             subtract_datetimes(stage_query(project_ids), start_time_attrs, end_time_attrs, name.to_s))
 
-          median_datetimes(cte_table, interval_query, name, :project_id)&.each do |project_id, median|
-            loader.call(project_id, median)
+          if project_ids.size == 1
+            loader.call(@project.id, median_datetime(cte_table, interval_query, name))
+          else
+            median_datetimes(cte_table, interval_query, name, :project_id)&.each do |project_id, median|
+              loader.call(project_id, median)
+            end
           end
         end
       end
diff --git a/lib/gitlab/cycle_analytics/usage_data.rb b/lib/gitlab/cycle_analytics/usage_data.rb
index 0aefd434e14..5122e3417ca 100644
--- a/lib/gitlab/cycle_analytics/usage_data.rb
+++ b/lib/gitlab/cycle_analytics/usage_data.rb
@@ -5,27 +5,21 @@ module Gitlab
 
       attr_reader :projects, :options
 
-      def initialize(projects, options)
-        @projects = projects
-        @options = options
+      def initialize
+        @projects = Project.sorted_by_activity.limit(PROJECTS_LIMIT)
+        @options = { from: 7.days.ago }
       end
 
       def to_json
         total = 0
-        values = {}
 
-        medians_per_stage.each do |stage_name, medians|
-          medians = medians.map(&:presence).compact
+        values =
+          medians_per_stage.each_with_object({}) do |(stage_name, medians), hsh|
+            calculations = stage_values(medians)
 
-          stage_values = {
-            average: calc_average(medians),
-            sd: standard_deviation(medians),
-            missing: projects.length - medians.length
-          }
-
-          total += stage_values.values.compact.sum
-          values[stage_name] = stage_values
-        end
+            total += calculations.values.compact.sum
+            hsh[stage_name] = calculations
+          end
 
         values[:total] = total
 
@@ -43,26 +37,36 @@ module Gitlab
         end
       end
 
+      def stage_values(medians)
+        medians = medians.map(&:presence).compact
+        average = calc_average(medians)
+
+        {
+          average: average,
+          sd: standard_deviation(medians, average),
+          missing: projects.length - medians.length
+        }
+      end
+
       def calc_average(values)
         return if values.empty?
 
         (values.sum / values.length).to_i
       end
 
-      def sample_variance(values)
+      def standard_deviation(values, average)
+        Math.sqrt(sample_variance(values, average)).to_i
+      end
+
+      def sample_variance(values, average)
         return 0 if values.length <= 1
 
-        avg = calc_average(values)
         sum = values.inject(0) do |acc, val|
-          acc + (val - avg)**2
+          acc + (val - average)**2
         end
 
         sum / (values.length - 1)
       end
-
-      def standard_deviation(values)
-        Math.sqrt(sample_variance(values)).to_i
-      end
     end
   end
 end
diff --git a/lib/gitlab/database/median.rb b/lib/gitlab/database/median.rb
index de2cb040ad6..2f9dd914674 100644
--- a/lib/gitlab/database/median.rb
+++ b/lib/gitlab/database/median.rb
@@ -2,32 +2,34 @@
 module Gitlab
   module Database
     module Median
+      def median_datetime(arel_table, query_so_far, column_sym)
+        extract_median(execute_queries(arel_table, query_so_far, column_sym)).presence
+      end
+
       def median_datetimes(arel_table, query_so_far, column_sym, partition_column)
-        median_queries =
-          if Gitlab::Database.postgresql?
-            pg_median_datetime_sql(arel_table, query_so_far, column_sym, partition_column)
-          elsif Gitlab::Database.mysql?
-            mysql_median_datetime_sql(arel_table, query_so_far, column_sym)
-          end
-
-        results = Array.wrap(median_queries).map do |query|
-          ActiveRecord::Base.connection.execute(query)
-        end
-        extract_medians(results).presence
+        extract_medians(execute_queries(arel_table, query_so_far, column_sym, partition_column)).presence
       end
 
-      def extract_medians(results)
+      def extract_median(results)
         result = results.compact.first
 
         if Gitlab::Database.postgresql?
-          result.values.map do |id, median|
-            [id.to_i, median&.to_f]
-          end.to_h
+          result = result.first.presence
+
+          result['median']&.to_f if result
         elsif Gitlab::Database.mysql?
           result.to_a.flatten.first
         end
       end
 
+      def extract_medians(results)
+        return {} if Gitlab::Database.mysql?
+
+        results.compact.first.values.map do |id, median|
+          [id.to_i, median&.to_f]
+        end.to_h
+      end
+
       def mysql_median_datetime_sql(arel_table, query_so_far, column_sym)
         query = arel_table
                 .from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name))
@@ -53,7 +55,7 @@ module Gitlab
         ]
       end
 
-      def pg_median_datetime_sql(arel_table, query_so_far, column_sym, partition_column)
+      def pg_median_datetime_sql(arel_table, query_so_far, column_sym, partition_column = nil)
         # Create a CTE with the column we're operating on, row number (after sorting by the column
         # we're operating on), and count of the table we're operating on (duplicated across) all rows
         # of the CTE. For example, if we're looking to find the median of the `projects.star_count`
@@ -64,49 +66,107 @@ module Gitlab
         #           5 |      1 |  3
         #           9 |      2 |  3
         #          15 |      3 |  3
+        #
+        #  If a partition column is used we will do the same operation but for separate partitions,
+        #  when that happens the CTE might look like this:
+        #
+        #  project_id | star_count | row_id | ct
+        # ------------+------------+--------+----
+        #           1 |          5 |     1 |  2
+        #           1 |          9 |     2 |  2
+        #           2 |         10 |     1 |  3
+        #           2 |         15 |     2 |  3
+        #           2 |         20 |     3 |  3
         cte_table = Arel::Table.new("ordered_records")
+
         cte = Arel::Nodes::As.new(
           cte_table,
-          arel_table
-            .project(
-              arel_table[partition_column],
-              arel_table[column_sym].as(column_sym.to_s),
-              Arel::Nodes::Over.new(Arel::Nodes::NamedFunction.new("rank", []),
-                                    Arel::Nodes::Window.new.partition(arel_table[partition_column])
-                                      .order(arel_table[column_sym])).as('row_id'),
-              arel_table.from(arel_table.alias)
-                .project("COUNT(*)")
-                .where(arel_table[partition_column].eq(arel_table.alias[partition_column])).as('ct')).
+          arel_table.project(*rank_rows(arel_table, column_sym, partition_column)).
             # Disallow negative values
             where(arel_table[column_sym].gteq(zero_interval)))
 
         # From the CTE, select either the middle row or the middle two rows (this is accomplished
         # by 'where cte.row_id between cte.ct / 2.0 AND cte.ct / 2.0 + 1'). Find the average of the
         # selected rows, and this is the median value.
-        cte_table
-          .project(cte_table[partition_column])
-          .project(average([extract_epoch(cte_table[column_sym])], "median"))
-          .where(
-            Arel::Nodes::Between.new(
-              cte_table[:row_id],
-              Arel::Nodes::And.new(
-                [(cte_table[:ct] / Arel.sql('2.0')),
-                 (cte_table[:ct] / Arel.sql('2.0') + 1)]
+        result =
+          cte_table
+            .project(*median_projections(cte_table, column_sym, partition_column))
+            .where(
+              Arel::Nodes::Between.new(
+                cte_table[:row_id],
+                Arel::Nodes::And.new(
+                  [(cte_table[:ct] / Arel.sql('2.0')),
+                   (cte_table[:ct] / Arel.sql('2.0') + 1)]
+                )
               )
             )
-          )
-          .with(query_so_far, cte)
-          .group(cte_table[partition_column])
-          .order(cte_table[partition_column])
-          .to_sql
+            .with(query_so_far, cte)
+
+        result.group(cte_table[partition_column]).order(cte_table[partition_column]) if partition_column
+
+        result.to_sql
       end
 
       private
 
+      def median_queries(arel_table, query_so_far, column_sym, partition_column = nil)
+        if Gitlab::Database.postgresql?
+          pg_median_datetime_sql(arel_table, query_so_far, column_sym, partition_column)
+        elsif Gitlab::Database.mysql?
+          mysql_median_datetime_sql(arel_table, query_so_far, column_sym)
+        end
+      end
+
+      def execute_queries(arel_table, query_so_far, column_sym, partition_column = nil)
+        queries = median_queries(arel_table, query_so_far, column_sym, partition_column)
+
+        Array.wrap(queries).map { |query| ActiveRecord::Base.connection.execute(query) }
+      end
+
       def average(args, as)
         Arel::Nodes::NamedFunction.new("AVG", args, as)
       end
 
+      def rank_rows(arel_table, column_sym, partition_column)
+        column_row = arel_table[column_sym].as(column_sym.to_s)
+
+        if partition_column
+          partition_row = arel_table[partition_column]
+          row_id =
+            Arel::Nodes::Over.new(
+              Arel::Nodes::NamedFunction.new('rank', []),
+              Arel::Nodes::Window.new.partition(arel_table[partition_column])
+                .order(arel_table[column_sym])
+            ).as('row_id')
+
+          count = arel_table.from(arel_table.alias)
+                    .project('COUNT(*)')
+                    .where(arel_table[partition_column].eq(arel_table.alias[partition_column]))
+                    .as('ct')
+
+          [partition_row, column_row, row_id, count]
+        else
+          row_id =
+            Arel::Nodes::Over.new(
+              Arel::Nodes::NamedFunction.new('row_number', []),
+              Arel::Nodes::Window.new.order(arel_table[column_sym])
+            ).as('row_id')
+
+          count = arel_table.project("COUNT(1)").as('ct')
+
+          [column_row, row_id, count]
+        end
+      end
+
+      def median_projections(table, column_sym, partition_column)
+        if partition_column
+          [table[partition_column],
+           average([extract_epoch(table[column_sym])], "median")]
+        else
+          [average([extract_epoch(table[column_sym])], "median")]
+        end
+      end
+
       def extract_epoch(arel_attribute)
         Arel.sql(%Q{EXTRACT(EPOCH FROM "#{arel_attribute.relation.name}"."#{arel_attribute.name}")})
       end
diff --git a/lib/gitlab/usage_data.rb b/lib/gitlab/usage_data.rb
index dae0e20e156..37d3512990e 100644
--- a/lib/gitlab/usage_data.rb
+++ b/lib/gitlab/usage_data.rb
@@ -73,12 +73,7 @@ module Gitlab
       end
 
       def cycle_analytics_usage_data
-        # We only want to generate this data for instances that use PostgreSQL
-        return {} if Gitlab::Database.mysql?
-
-        projects = Project.sorted_by_activity.limit(Gitlab::CycleAnalytics::UsageData::PROJECTS_LIMIT)
-
-        Gitlab::CycleAnalytics::UsageData.new(projects, { from: 7.days.ago }).to_json
+        Gitlab::CycleAnalytics::UsageData.new.to_json
       end
 
       def features_usage_data