Make a constraint deferrable

To allow for deferring it in a transaction when deleting derivations.

sqitch/deploy/make_some_constraints_deferrable.sql

@@ -0,0 +1,8 @@
+-- Deploy guix-data-service:make_some_constraints_deferrable to pg
+
+BEGIN;
+
+ALTER TABLE derivations_by_output_details_set
+    ALTER CONSTRAINT derivations_by_output_details_set_derivation_id_fkey DEFERRABLE INITIALLY IMMEDIATE;
+
+COMMIT;

sqitch/revert/make_some_constraints_deferrable.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:make_some_constraints_deferrable from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;

sqitch/sqitch.plan

@@ -51,3 +51,4 @@ change_package_derivations_by_guix_revision_range_target 2020-02-08T10:13:07Z Ch
 allow_including_and_excluding_branches_for_repositories 2020-02-08T11:30:02Z Christopher Baines <mail@cbaines.net> # Allow including and excluding branches for repositories
 channel_instance_derivations 2020-02-10T07:59:03Z Christopher Baines <mail@cbaines.net> # Add tables to store derivations for channel instances
 update_build_servers_build_config 2020-02-13T20:07:19Z Christopher Baines <mail@cbaines.net> # Update build_servers_build_config values
+make_some_constraints_deferrable 2020-02-16T10:54:22Z Christopher Baines <mail@cbaines.net> # Make some constraints deferrable

sqitch/verify/make_some_constraints_deferrable.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:make_some_constraints_deferrable on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;