Optimize DB indexes for ES indexing of notes

To index notes, we exclude system notes and use `find_in_batches` to
load them in batches for submission to the ES bulk import API.
These queries often result in DB timeouts because the usage of
`ORDER BY id` results in the `notes_pkey` index being used.

This adds an optimized partial index, and removes the unused index
`index_notes_on_noteable_type` which is already covered for our
usage by the existing `index_notes_on_noteable_id_and_noteable_type`.

Newer versions of PostgreSQL (at least 11) are smarter about this and
use `index_notes_on_project_id_and_noteable_type` instead, so we might
be able to remove the partial index again in the future.

1 files changed, 29 insertions, 0 deletions

diff --git a/db/migrate/20190815093949_remove_index_notes_on_noteable_type.rb b/db/migrate/20190815093949_remove_index_notes_on_noteable_type.rb
new file mode 100644
index 00000000000..158c88e6258
--- /dev/null
+++ b/db/migrate/20190815093949_remove_index_notes_on_noteable_type.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class RemoveIndexNotesOnNoteableType < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index(*index_arguments)
+  end
+
+  def down
+    add_concurrent_index(*index_arguments)
+  end
+
+  private
+
+  def index_arguments
+    [
+      :notes,
+      [:noteable_type],
+      {
+        name: 'index_notes_on_noteable_type'
+      }
+    ]
+  end
+end