migrations: add index on units.path_id

An index is added to units.path_id to improve performance when joining
with the paths table.

Change-Id: I03410af9deef8fcac6ec5090876a07c8ba1d46d7
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/267685
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000048_add_idx_units_path_id.down.sql

@@ -0,0 +1,9 @@
+-- Copyright 2020 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+BEGIN;
+
+DROP INDEX idx_units_path_id;
+
+END;

migrations/000048_add_idx_units_path_id.up.sql

@@ -0,0 +1,9 @@
+-- Copyright 2020 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+BEGIN;
+
+CREATE INDEX idx_units_path_id ON units(path_id);
+
+END;