Add unique index for note_id to system note metadata table

In issue 1, the sequential scan was taking 250+ ms.

1 files changed, 17 insertions, 0 deletions

diff --git a/db/migrate/20170419001229_add_index_to_system_note_metadata.rb b/db/migrate/20170419001229_add_index_to_system_note_metadata.rb
new file mode 100644
index 00000000000..c68fd920fff
--- /dev/null
+++ b/db/migrate/20170419001229_add_index_to_system_note_metadata.rb
@@ -0,0 +1,17 @@
+class AddIndexToSystemNoteMetadata < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    # MySQL automatically creates an index on a foreign-key constraint; PostgreSQL does not
+    add_concurrent_index :system_note_metadata, :note_id, unique: true if Gitlab::Database.postgresql?
+  end
+
+  def down
+    remove_concurrent_index :system_note_metadata, :note_id, unique: true if Gitlab::Database.postgresql?
+  end
+end