Add partial index on identities table to speed up LDAP lookups

For customers with thousands of entries in the table, lookups for an
LDAP identity dominated the PostgreSQL statistics because of sequential
scans. We can speed this up by adding a partial index with the
extern_uid and provider.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59630

changelogs/unreleased/sh-add-index-extern-uid.yml

+---
+title: Add partial index on identities table to speed up LDAP lookups
+merge_request: 26710
+author:
+type: performance

db/post_migrate/20190723105753_add_index_on_identities_lower_extern_uid_and_provider.rb

+# frozen_string_literal: true
+
+class AddIndexOnIdentitiesLowerExternUidAndProvider < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = "index_on_identities_lower_extern_uid_and_provider"
+
+  def up
+    add_concurrent_index(:identities, 'lower(extern_uid), provider', name: INDEX_NAME)
+  end
+
+  def down
+    remove_concurrent_index_by_name(:identities, INDEX_NAME)
+  end
+end

db/schema.rb

@@ -1580,6 +1580,7 @@ ActiveRecord::Schema.define(version: 2019_07_29_090456) do
     t.datetime "updated_at"
     t.integer "saml_provider_id"
     t.string "secondary_extern_uid"
+    t.index "lower((extern_uid)::text), provider", name: "index_on_identities_lower_extern_uid_and_provider"
     t.index ["saml_provider_id"], name: "index_identities_on_saml_provider_id", where: "(saml_provider_id IS NOT NULL)"
     t.index ["user_id"], name: "index_identities_on_user_id"
   end