migrations: drop imports table

The imports table is no longer used and is dropped.

This frees up 136 GB of table space and 478 GB of indexes.

For golang/go#39629

Change-Id: I5a381832d9d4d69408fb62052782097436c40371
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/265242
Trust: Julie Qiu <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Jamal Carvalho <[email protected]>

Author: julieqiu

migrations/000038_drop_imports_table.down.sql

@@ -0,0 +1,23 @@
+-- Copyright 2020 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 imports (
+    from_path text NOT NULL,
+    from_module_path text NOT NULL,
+    from_version text NOT NULL,
+    to_path text NOT NULL,
+    PRIMARY KEY (to_path, from_path, from_version, from_module_path),
+    FOREIGN KEY (from_path, from_module_path, from_version)
+        REFERENCES packages(path, module_path, version) ON DELETE CASCADE
+);
+COMMENT ON TABLE imports IS
+'TABLE imports contains the imports for a package in the packages table. Package (from_path), in module (from_module_path) at version (from_version), imports package (to_path). We do not store the version and module at which to_path is imported because it is hard to compute.';
+
+CREATE INDEX idx_imports_from_path_from_version ON imports (from_path, from_version);
+COMMENT ON INDEX idx_imports_from_path_from_version IS
+'INDEX idx_imports_from_path_from_version is used to improve performance of the imports tab.';
+
+END;

migrations/000038_drop_imports_table.up.sql

@@ -0,0 +1,9 @@
+-- Copyright 2020 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 imports;
+
+END;