migrations: add documents.tsv_search_tokens

Rather than storing the tsvector data for name, path, synopsis and readme
separately, they will now to be stored together in a tsv_search_tokens,
which has been shown to improve performance for the search query.

Change-Id: I80460b3a9e22efbe642419e37b95e10ed2c73d9f
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/455288
Reviewed-by: Robert Findley <rfindley@google.com>

migrations/000033_create_search_column.down.sql

@@ -0,0 +1,9 @@
+-- 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.
+
+BEGIN;
+
+ALTER TABLE documents DROP COLUMN tsv_search_tokens;
+
+END;

migrations/000033_create_search_column.up.sql

@@ -0,0 +1,16 @@
+-- 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.
+
+BEGIN;
+
+ALTER TABLE documents ADD COLUMN tsv_search_tokens tsvector;
+CREATE INDEX tsv_search_tokens_idx ON documents USING gin(tsv_search_tokens);
+
+UPDATE documents SET tsv_search_tokens =
+	name_tokens ||
+	path_tokens ||
+	synopsis_tokens ||
+	readme_tokens;
+
+END;