1 files changed, 79 insertions, 0 deletions
diff --git a/lib/gitlab/database/count/reltuples_count_strategy.rb b/lib/gitlab/database/count/reltuples_count_strategy.rb
new file mode 100644
index 00000000000..c3a674aeb7e
--- /dev/null
+++ b/lib/gitlab/database/count/reltuples_count_strategy.rb
@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module Count
+      class PgClass < ActiveRecord::Base
+        self.table_name = 'pg_class'
+      end
+
+      # This strategy counts based on PostgreSQL's statistics in pg_stat_user_tables.
+      #
+      # Specifically, it relies on the column reltuples in said table. An additional
+      # check is performed to make sure statistics were updated within the last hour.
+      #
+      # Otherwise, this strategy skips tables with outdated statistics.
+      #
+      # There are no guarantees with respect to the accuracy of this strategy. Runtime
+      # however is guaranteed to be "fast", because it only looks up statistics.
+      class ReltuplesCountStrategy
+        attr_reader :models
+        def initialize(models)
+          @models = models
+        end
+
+        # Returns a hash of the table names that have recently updated tuples.
+        #
+        # @returns [Hash] Table name to count mapping (e.g. { 'projects' => 5, 'users' => 100 })
+        def count
+          size_estimates
+        rescue *CONNECTION_ERRORS
+          {}
+        end
+
+        def self.enabled?
+          Gitlab::Database.postgresql?
+        end
+
+        private
+
+        def table_names
+          models.map(&:table_name)
+        end
+
+        def size_estimates(check_statistics: true)
+          table_to_model = models.each_with_object({}) { |model, h| h[model.table_name] = model }
+
+          # Querying tuple stats only works on the primary. Due to load balancing, the
+          # easiest way to do this is to start a transaction.
+          ActiveRecord::Base.transaction do
+            get_statistics(table_names, check_statistics: check_statistics).each_with_object({}) do |row, data|
+              model = table_to_model[row.table_name]
+              data[model] = row.estimate
+            end
+          end
+        end
+
+        # Generates the PostgreSQL query to return the tuples for tables
+        # that have been vacuumed or analyzed in the last hour.
+        #
+        # @param [Array] table names
+        # @returns [Hash] Table name to count mapping (e.g. { 'projects' => 5, 'users' => 100 })
+        def get_statistics(table_names, check_statistics: true)
+          time = 1.hour.ago
+
+          query = PgClass.joins("LEFT JOIN pg_stat_user_tables USING (relname)")
+            .where(relname: table_names)
+            .select('pg_class.relname AS table_name, reltuples::bigint AS estimate')
+
+          if check_statistics
+            query = query.where('last_vacuum > ? OR last_autovacuum > ? OR last_analyze > ? OR last_autoanalyze > ?',
+                                time, time, time, time)
+          end
+
+          query
+        end
+      end
+    end
+  end
+end