Merge ~pappacena/launchpad:gitrepo-status-column into launchpad:db-devel

[Thiago F. Pappacena]

Database patch to create GitRepository.status column.

This MP is just adding the column, a quick operation that should be done on a fast downtime deployment. The NOT NULL and default value constraint, together with an index, is being created in a hot db patch here: https://code.launchpad.net/~pappacena/launchpad/+git/launchpad/+merge/385198.

[Thiago F. Pappacena (pappacena)]

Pushed the requested changes with comments.

[William Grant (wgrant)]

review: Approve (db)

diff --git a/database/schema/patch-2210-17-0.sql b/database/schema/patch-2210-17-0.sql
new file mode 100644
index 0000000..3f5946a
--- /dev/null
+++ b/database/schema/patch-2210-17-0.sql
@@ -0,0 +1,17 @@
+-- Copyright 2020 Canonical Ltd.  This software is licensed under the
+-- GNU Affero General Public License version 3 (see the file LICENSE).
+
+SET client_min_messages=ERROR;
+
+ALTER TABLE GitRepository ADD COLUMN status INTEGER;
+
+-- ALTER COLUMN ... SET DEFAULT doesn't trigger a table rewrite,
+-- while ADD COLUMN ... DEFAULT xx does. In pg <11 this operation is slow.
+-- That's why we first create, and then we set the default value.
+-- Data backfilling will be done in a hot patch instead of a fast downtime.
+ALTER TABLE GitRepository ALTER COLUMN status SET DEFAULT 2;
+
+COMMENT ON COLUMN GitRepository.status
+    IS 'The current status of this git repository.';
+
+INSERT INTO LaunchpadDatabaseRevision VALUES (2210, 17, 0);