migrations: add idx_search_documents_tsv_path_tokens
A GIN index is added on search_documents.tsv_path_tokens to improve
performance for multi-word searches.
This should reduce the query below from 1.6s to 250ms.
```
SELECT *
FROM search_documents
WHERE tsv_path_tokens @@ to_tsquery('symbols', 'blake2b');
```
For golang/go#44142
Change-Id: I130df35b36ba9420e12610323334c329f69f211e
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/343290
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>
diff --git a/migrations/000141_add_idx_search_documents_tsv_path_tokens.down.sql b/migrations/000141_add_idx_search_documents_tsv_path_tokens.down.sql
new file mode 100644
index 0000000..a42dd91
--- /dev/null
+++ b/migrations/000141_add_idx_search_documents_tsv_path_tokens.down.sql
@@ -0,0 +1,9 @@
+-- Copyright 2021 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_path_documents_tsv_path_tokens;
+
+END;
diff --git a/migrations/000141_add_idx_search_documents_tsv_path_tokens.up.sql b/migrations/000141_add_idx_search_documents_tsv_path_tokens.up.sql
new file mode 100644
index 0000000..9a1847f
--- /dev/null
+++ b/migrations/000141_add_idx_search_documents_tsv_path_tokens.up.sql
@@ -0,0 +1,9 @@
+-- Copyright 2021 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_path_documents_tsv_path_tokens ON search_documents USING gin (tsv_path_tokens);
+
+END;
