Add index on lower path to redirect routes

This will speed up the LIKE query in
RedirectRoute.matching_path_and_descendants

db/post_migrate/20180306164012_add_path_index_to_redirect_routes.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddPathIndexToRedirectRoutes < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_redirect_routes_on_path_unique_text_pattern_ops'
+
+  # Indexing on LOWER(path) varchar_pattern_ops speeds up the LIKE query in
+  # RedirectRoute.matching_path_and_descendants
+  #
+  # This same index is also added in the `ReworkRedirectRoutesIndexes` so this
+  # is a no-op in most cases. But this migration is also called from the
+  # `setup_postgresql.rake` task when setting up a new database, in which case
+  # we want to create the index.
+  def up
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+
+    if_not_exists = Gitlab::Database.version.to_f >= 9.5 ? "IF NOT EXISTS" : ""
+
+    # Unique index on lower(path) across both types of redirect_routes:
+    execute("CREATE UNIQUE INDEX CONCURRENTLY #{if_not_exists} #{INDEX_NAME} ON redirect_routes (lower(path) varchar_pattern_ops);")
+  end
+
+  def down
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+
+    execute("DROP INDEX IF EXISTS #{INDEX_NAME};")
+  end
+end