Add a latest_build_status table

This will avoid many queries trying to figure out what the latest build status
is, which will hopefuly simplify queries as well as improving performance.

sqitch/deploy/create_latest_build_status.sql

@@ -0,0 +1,27 @@
+-- Deploy guix-data-service:create_latest_build_status to pg
+
+BEGIN;
+
+CREATE TABLE latest_build_status (
+    build_id integer PRIMARY KEY NOT NULL REFERENCES builds(id),
+    "timestamp" timestamp without time zone DEFAULT clock_timestamp(),
+    status guix_data_service.buildstatus NOT NULL
+);
+
+INSERT INTO latest_build_status
+SELECT DISTINCT build_id,
+                first_value(timestamp) OVER rows_for_build AS timestamp,
+                first_value(status) OVER rows_for_build AS status
+FROM build_status
+WINDOW rows_for_build AS (
+  PARTITION BY build_id
+  ORDER BY
+    timestamp DESC,
+    CASE WHEN status = 'scheduled' THEN -2
+         WHEN status = 'started' THEN -1
+         ELSE 0
+    END DESC
+    RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
+);
+
+COMMIT;

sqitch/revert/create_latest_build_status.sql

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

sqitch/sqitch.plan

@@ -73,3 +73,4 @@ change_autovacuum_config 2020-10-01T21:24:46Z Christopher Baines <mail@cbaines.n
 change_derivation_source_file_nars_constraint 2020-10-02T17:12:58Z Christopher Baines <mail@cbaines.net> # Change derivation source file nars constraint
 add_derivation_sources_derivation_source_file_id_index 2020-10-02T19:11:59Z Christopher Baines <mail@cbaines.net> # Add derivation_sources.derivation_source_file_id index
 git_repositories_add_fetch_with_authentication_field 2020-10-07T17:31:20Z Christopher Baines <mail@cbaines.net> # Add git_repositories.fetch_with_authentication
+create_latest_build_status 2020-10-13T17:22:39Z Christopher Baines <mail@cbaines.net> # Create the latest_build_status table

sqitch/verify/create_latest_build_status.sql

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