From ea41405209a2b6aa4c89ee80c12b642f224d516a Mon Sep 17 00:00:00 2001
From: Pau Perez <saulopefa@gmail.com>
Date: Mon, 16 Sep 2019 13:03:17 +0200
Subject: [PATCH] Index spree_orders on various columns

The following query

```sql
SELECT spree_orders . *
FROM spree_orders
WHERE spree_orders . user_id = ?
AND spree_orders . completed_at IS ?
AND spree_orders . created_by_id = ?
ORDER BY created_at DESC LIMIT ?
```

performs quite badly even though LIMIT is always 1 because:

* ORDER BY requires sorting by a column which is not indexed therefore
a sequential scan is performed.
* Although `completed_at` is indexed, `user_id` and `created_by_id` are
not causing a sequential scan.

To make it worse this query is executed very often in the following
controllers among others also related to checkout:

* CartController#populate
* EnterprisesController#Shop
* LineItemsController#bought
* ShopController#products
* ShopController#order_cycle

In some cases this query alone accounts for 66.8% of the total time
of the endpoint.

Results

See by yourself. We move from 56.643ms to 0.077ms. Pretty neat.

```
openfoodnetwork=> explain analyze SELECT "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."user_id" = 1 AND "spree_orders"."completed_at" IS NULL AND "spree_orders"."created_by_id" = 1 ORDER BY created_at DESC LIMIT 1;
                                                        QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=11753.03..11753.04 rows=1 width=195) (actual time=56.580..56.580 rows=0 loops=1)
   ->  Sort  (cost=11753.03..11753.04 rows=1 width=195) (actual time=56.578..56.578 rows=0 loops=1)
         Sort Key: created_at DESC
         Sort Method: quicksort  Memory: 25kB
         ->  Seq Scan on spree_orders  (cost=0.00..11753.02 rows=1 width=195) (actual time=56.571..56.571 rows=0 loops=1)
               Filter: ((completed_at IS NULL) AND (user_id = 1) AND (created_by_id = 1))
               Rows Removed by Filter: 256135
 Planning time: 0.252 ms
 Execution time: 56.643 ms
(9 rows)

openfoodnetwork=> CREATE INDEX ON spree_orders (completed_at, user_id, created_by_id, created_at);
CREATE INDEX
openfoodnetwork=> explain analyze SELECT "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."user_id" = 1 AND "spree_orders"."completed_at" IS NULL AND "spree_orders"."created_by_id" = 1 ORDER BY created_at DESC LIMIT 1;
mit  (cost=8.45..8.46 rows=1 width=195) (actual time=0.030..0.030 rows=0 loops=1)
   ->  Sort  (cost=8.45..8.46 rows=1 width=195) (actual time=0.029..0.029 rows=0 loops=1)
         Sort Key: created_at DESC
         Sort Method: quicksort  Memory: 25kB
         ->  Index Scan using spree_orders_completed_at_user_id_created_by_id_created_at_idx on spree_orders  (cost=0.42..8.44 rows=1 width=195) (actual time=0.021..0.021 rows=0 loops=1)
               Index Cond: ((completed_at IS NULL) AND (user_id = 1) AND (created_by_id = 1))
 Planning time: 0.199 ms
 Execution time: 0.077 ms
```
---
 ...0916105416_add_long_compound_index_on_spree_orders.rb | 9 +++++++++
 ...0916110029_drop_completed_at_index_on_spree_orders.rb | 5 +++++
 db/schema.rb                                             | 4 ++--
 3 files changed, 16 insertions(+), 2 deletions(-)
 create mode 100644 db/migrate/20190916105416_add_long_compound_index_on_spree_orders.rb
 create mode 100644 db/migrate/20190916110029_drop_completed_at_index_on_spree_orders.rb

diff --git a/db/migrate/20190916105416_add_long_compound_index_on_spree_orders.rb b/db/migrate/20190916105416_add_long_compound_index_on_spree_orders.rb
new file mode 100644
index 0000000000..5ff04483ec
--- /dev/null
+++ b/db/migrate/20190916105416_add_long_compound_index_on_spree_orders.rb
@@ -0,0 +1,9 @@
+class AddLongCompoundIndexOnSpreeOrders < ActiveRecord::Migration
+  def change
+    add_index(
+      :spree_orders,
+      [:completed_at, :user_id, :created_by_id, :created_at],
+      name: 'spree_orders_completed_at_user_id_created_by_id_created_at_idx'
+    )
+  end
+end
diff --git a/db/migrate/20190916110029_drop_completed_at_index_on_spree_orders.rb b/db/migrate/20190916110029_drop_completed_at_index_on_spree_orders.rb
new file mode 100644
index 0000000000..47b5517295
--- /dev/null
+++ b/db/migrate/20190916110029_drop_completed_at_index_on_spree_orders.rb
@@ -0,0 +1,5 @@
+class DropCompletedAtIndexOnSpreeOrders < ActiveRecord::Migration
+  def change
+    remove_index :spree_orders, :completed_at
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 81684ade4c..99f98e7c59 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20190913023137) do
+ActiveRecord::Schema.define(:version => 20190916110029) do
 
   create_table "adjustment_metadata", :force => true do |t|
     t.integer "adjustment_id"
@@ -576,7 +576,7 @@ ActiveRecord::Schema.define(:version => 20190913023137) do
     t.integer  "created_by_id"
   end
 
-  add_index "spree_orders", ["completed_at"], :name => "index_spree_orders_on_completed_at"
+  add_index "spree_orders", ["completed_at", "user_id", "created_by_id", "created_at"], :name => "spree_orders_completed_at_user_id_created_by_id_created_at_idx"
   add_index "spree_orders", ["customer_id"], :name => "index_spree_orders_on_customer_id"
   add_index "spree_orders", ["number"], :name => "index_orders_on_number"