Improve DISTINCT query for finding queue names in PostgreSQL

app/models/solid_queue/queue.rb

@@ -6,14 +6,63 @@ class Queue
 
     class << self
       def all
-        Job.select(:queue_name).distinct.collect do |job|
-          new(job.queue_name)
+        queue_names.collect do |queue_name|
+          new(queue_name)
         end
       end
 
       def find_by_name(name)
         new(name)
       end
+
+      private
+
+      def queue_names
+        # PostgreSQL doesn't perform well with SELECT DISTINCT
+        # => Use recursive common table expressions if possible for better performance (https://wiki.postgresql.org/wiki/Loose_indexscan)
+        if SolidQueue::Record.connection.adapter_name.downcase == "postgresql" && SolidQueue::Record.connection.supports_common_table_expressions?
+          Job.connection.execute(queue_names_recursive_cte_sql).to_a.map { |row| row["queue_name"] }
+        else
+          Job.select(:queue_name).distinct.map(&:queue_name)
+        end
+      end
+
+      def queue_names_recursive_cte_sql
+        # This relies on the fact that queue_name in solid_queue_jobs is NOT NULL
+        # The sql looks something like below:
+        # WITH RECURSIVE t AS (
+        #   (SELECT queue_name FROM solid_queue_jobs ORDER BY queue_name LIMIT 1)  -- parentheses required
+        #   UNION ALL
+        #   SELECT (SELECT queue_name FROM solid_queue_jobs WHERE queue_name > t.queue_name ORDER BY queue_name LIMIT 1)
+        #   FROM t
+        #   WHERE t.queue_name IS NOT NULL
+        # )
+        # SELECT queue_name FROM t WHERE queue_name IS NOT NULL;
+
+        cte_table = Arel::Table.new(:t)
+        jobs_table = Job.arel_table
+
+        cte_base_case = jobs_table.project(jobs_table[:queue_name]).order(jobs_table[:queue_name]).take(1)
+
+        subquery = jobs_table
+          .project(jobs_table[:queue_name])
+          .where(jobs_table[:queue_name].gt(cte_table[:queue_name]))
+          .order(jobs_table[:queue_name])
+          .take(1)
+        cte_recursive_case = cte_table.project(subquery)
+          .where(cte_table[:queue_name].not_eq(nil))
+
+        cte_definition = Arel::Nodes::Cte.new(
+          Arel.sql("t"),
+          Arel::Nodes::UnionAll.new(cte_base_case, cte_recursive_case),
+        )
+
+        cte_table
+          .project(cte_table[:queue_name])
+          .where(cte_table[:queue_name].not_eq(nil))
+          .with(:recursive, cte_definition)
+          .to_sql
+      end
     end
 
     def initialize(name)

test/models/solid_queue/queue_test.rb

@@ -0,0 +1,14 @@
+require "test_helper"
+
+class SolidQueue::QueueTest < ActiveSupport::TestCase
+  test "list all queues" do
+    queue_names = [ "test", "test2", "the_queue", "backend" ]
+    queue_names.each do |queue_name|
+      (SecureRandom.random_number(5) + 1).times do |i|
+        AddToBufferJob.set(queue: queue_name).perform_later(i)
+      end
+    end
+
+    assert_equal queue_names.sort, SolidQueue::Queue.all.map(&:name).sort
+  end
+end