Enable PgHero Query Stats

This does the work to enable historical query stats in PgHero, but the
Postgres instance backing the service must also be configured, see:

https://github.com/ankane/pghero/blob/master/guides/Rails.md#query-stats

We are:

- adding the tables to store stats
- setting up a GoodJob cron job to collect the stats and clean old ones
(the demo app generates a lot of queries)

For reference, the recommended settings for `postgres.conf`:

```
shared_preload_libraries = 'pg_stat_statements'
pg_stat_statements.track = all
pg_stat_statements.max = 10000
track_activity_query_size = 2048
```

See:

https://github.com/ankane/pghero/blob/master/guides/Query-Stats.md

demo/app/jobs/pg_hero_maintenance_job.rb

@@ -0,0 +1,15 @@
+class PgHeroMaintenanceJob < ApplicationJob
+  include GoodJob::ActiveJobExtensions::Concurrency
+
+  good_job_control_concurrency_with(
+    key: "pg_hero_maintenance",
+    total_limit: 1
+  )
+
+  discard_on StandardError
+
+  def perform
+    PgHero.capture_query_stats
+    PgHero.clean_query_stats
+  end
+end

demo/config/initializers/good_job.rb

@@ -82,6 +82,11 @@
       complex_schedule: {
         cron: -> (last_ran) { last_ran ? last_ran + 17.hours : Time.now},
         class: "OtherJob",
+      },
+      pg_hero_maintenance: {
+        cron: "*/10 * * * *", # Every 10 minutes
+        class: "PgHeroMaintenanceJob",
+        description: "Runs PG Hero maintenance",
       }
     }
   end

demo/db/migrate/20240321162554_create_pghero_query_stats.rb

@@ -0,0 +1,15 @@
+class CreatePgheroQueryStats < ActiveRecord::Migration[7.1]
+  def change
+    create_table :pghero_query_stats do |t|
+      t.text :database
+      t.text :user
+      t.text :query
+      t.integer :query_hash, limit: 8
+      t.float :total_time
+      t.integer :calls, limit: 8
+      t.timestamp :captured_at
+    end
+
+    add_index :pghero_query_stats, [:database, :captured_at]
+  end
+end

demo/db/schema.rb

@@ -10,8 +10,9 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2024_01_14_221613) do
+ActiveRecord::Schema.define(version: 2024_03_21_162554) do
   # These are extensions that must be enabled in order to support this database
+  enable_extension "pg_stat_statements"
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
 
@@ -94,4 +95,15 @@
     t.index ["scheduled_at"], name: "index_good_jobs_on_scheduled_at", where: "(finished_at IS NULL)"
   end
 
+  create_table "pghero_query_stats", force: :cascade do |t|
+    t.text "database"
+    t.text "user"
+    t.text "query"
+    t.bigint "query_hash"
+    t.float "total_time"
+    t.bigint "calls"
+    t.datetime "captured_at", precision: nil
+    t.index ["database", "captured_at"], name: "index_pghero_query_stats_on_database_and_captured_at"
+  end
+
 end