Fix performance on instances list in admin UI (#15282)

- Reduce duplicate queries
- Remove n+1 queries
- Add accounts count to detailed view
- Add separate action log entry for updating existing domain blocks

db/migrate/20201206004238_create_instances.rb

@@ -0,0 +1,9 @@
+class CreateInstances < ActiveRecord::Migration[5.2]
+  def change
+    create_view :instances, materialized: true
+
+    # To be able to refresh the view concurrently,
+    # at least one unique index is required
+    safety_assured { add_index :instances, :domain, unique: true }
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_10_17_234926) do
+ActiveRecord::Schema.define(version: 2020_12_06_004238) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1045,4 +1045,29 @@ ActiveRecord::Schema.define(version: 2020_10_17_234926) do
   add_foreign_key "web_push_subscriptions", "users", on_delete: :cascade
   add_foreign_key "web_settings", "users", name: "fk_11910667b2", on_delete: :cascade
   add_foreign_key "webauthn_credentials", "users"
+
+  create_view "instances", materialized: true, sql_definition: <<-SQL
+      WITH domain_counts(domain, accounts_count) AS (
+           SELECT accounts.domain,
+              count(*) AS accounts_count
+             FROM accounts
+            WHERE (accounts.domain IS NOT NULL)
+            GROUP BY accounts.domain
+          )
+   SELECT domain_counts.domain,
+      domain_counts.accounts_count
+     FROM domain_counts
+  UNION
+   SELECT domain_blocks.domain,
+      COALESCE(domain_counts.accounts_count, (0)::bigint) AS accounts_count
+     FROM (domain_blocks
+       LEFT JOIN domain_counts ON (((domain_counts.domain)::text = (domain_blocks.domain)::text)))
+  UNION
+   SELECT domain_allows.domain,
+      COALESCE(domain_counts.accounts_count, (0)::bigint) AS accounts_count
+     FROM (domain_allows
+       LEFT JOIN domain_counts ON (((domain_counts.domain)::text = (domain_allows.domain)::text)));
+  SQL
+  add_index "instances", ["domain"], name: "index_instances_on_domain", unique: true
+
 end

db/views/instances_v01.sql

@@ -0,0 +1,17 @@
+WITH domain_counts(domain, accounts_count)
+AS (
+  SELECT domain, COUNT(*) as accounts_count
+  FROM accounts
+  WHERE domain IS NOT NULL
+  GROUP BY domain
+)
+SELECT domain, accounts_count
+FROM domain_counts
+UNION
+SELECT domain_blocks.domain, COALESCE(domain_counts.accounts_count, 0)
+FROM domain_blocks
+LEFT OUTER JOIN domain_counts ON domain_counts.domain = domain_blocks.domain
+UNION
+SELECT domain_allows.domain, COALESCE(domain_counts.accounts_count, 0)
+FROM domain_allows
+LEFT OUTER JOIN domain_counts ON domain_counts.domain = domain_allows.domain