migrations: schema changes enabling multiple licenses

Add a licenses table to capture license information, along with a join
table that associates packages and licenses.  This caused package
queries in internal/postgres to become pretty complicated, so add a
helper view to offload some of that complexity.

Fixes b/129370884

Change-Id: I7d97b3458b35c7c1b2400a28f5025ba3f83a5af1
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/448258
Reviewed-by: Julie Qiu <julieqiu@google.com>

migrations/000022_add_licenses_table.down.sql

@@ -0,0 +1,7 @@
+-- Copyright 2019 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.
+
+DROP VIEW vw_licensed_packages;
+DROP TABLE package_licenses;
+DROP TABLE licenses;

migrations/000022_add_licenses_table.up.sql

@@ -0,0 +1,51 @@
+-- Copyright 2019 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.
+
+CREATE TABLE licenses (
+	module_path TEXT NOT NULL,
+	version TEXT NOT NULL,
+        file_path TEXT NOT NULL, -- path to license file
+	contents TEXT NOT NULL,
+        type TEXT NOT NULL,
+
+	PRIMARY KEY (module_path, version, file_path),
+	FOREIGN KEY (module_path, version) REFERENCES versions(module_path, version)
+);
+
+-- package_licenses is join table that associates each package with its
+-- applicable licenses.
+CREATE TABLE package_licenses (
+  module_path TEXT NOT NULL,
+  version TEXT NOT NULL,
+  file_path TEXT NOT NULL,
+  package_path TEXT NOT NULL,
+
+  FOREIGN KEY (module_path, version, package_path) REFERENCES packages(module_path, version, path),
+  FOREIGN KEY (module_path, version, file_path) REFERENCES licenses(module_path, version, file_path)
+);
+
+-- vw_licensed_packages is a helper view that captures package licenses when
+-- querying.
+CREATE VIEW vw_licensed_packages AS
+SELECT
+  p.*,
+  -- Aggregate license information into arrays which can later be zipped
+  -- together. The FILTER clause here is necessary due to the left-join.
+  array_agg(l.type) FILTER (WHERE l.version IS NOT NULL) license_types,
+  array_agg(l.file_path) FILTER (WHERE l.version IS NOT NULL) license_paths
+FROM
+  packages p
+LEFT JOIN
+  package_licenses pl
+ON
+  p.module_path = pl.module_path
+  AND p.version = pl.version
+  AND p.path = pl.package_path
+LEFT JOIN
+  licenses l
+ON
+  pl.module_path = l.module_path
+  AND pl.version = l.version
+  AND pl.file_path = l.file_path
+GROUP BY (p.module_path, p.version, p.path);