pgsql/migrations: add index on Vulnerability_Notification.deleted_at

`searchNotificationAvailable` never effectively use any indexes because:
- `notified_at < $1`, where $1 is a recent timestamp, returns the
  majority of the table and therefore it is cheaper for PostgreSQL
  to use a sequential scan on the table.
- there is no index for `deleted_at IS NULL`.
However, when Clair has been running for long enough, the grand majority
of rows (99%+) are expected to have a non-NULL `deleted_at` field. This
commit adds a new index on this very field in order to fetch the
remaining 1% in the blink of an eye.

In other words, instead of realizing a full table scan for each
`searchNotificationAvailable` query, we'll use the small branch of a new
index, reducing the total cost from over 30k to a mere 150 on a Clair
database that has already managed more than 1 000 000 notifications.

database/pgsql/migrations/00004_add_index_notification_deleted_at.go

@@ -0,0 +1,29 @@
+// Copyright 2016 clair authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package migrations
+
+import "github.com/remind101/migrate"
+
+func init() {
+	RegisterMigration(migrate.Migration{
+		ID: 4,
+		Up: migrate.Queries([]string{
+			`CREATE INDEX vulnerability_notification_deleted_at_idx ON Vulnerability_Notification (deleted_at);`,
+		}),
+		Down: migrate.Queries([]string{
+			`DROP INDEX vulnerability_notification_deleted_at_idx;`,
+		}),
+	})
+}