migrations: create documentation_symbols

For golang/go#37102

Change-Id: I75fbb6393664ade2ab14f70d8df0e4503c5adea9
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/295953
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000073_create_documentation_symbols.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 TABLE documentation_symbols;
+
+END;

migrations/000073_create_documentation_symbols.up.sql

@@ -0,0 +1,20 @@
+-- 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 TABLE documentation_symbols (
+    id INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
+    documentation_id INTEGER NOT NULL,
+    package_symbol_id INTEGER NOT NULL,
+
+    UNIQUE(documentation_id, package_symbol_id),
+    -- TODO: add FK to documentation.id in a future migration, once the UNIQUE
+    -- constraint has been added.
+    FOREIGN KEY (package_symbol_id) REFERENCES package_symbols(id) ON DELETE CASCADE
+);
+COMMENT ON TABLE documentation_symbols IS
+'TABLE documentation_symbols contains symbols for a given row in the documentation table.';
+
+END;