Materialize the number of finished builds

The NrBuilds table tracks the value of ‘select count(*) from Builds
where finished = 0’, keeping it up to date via a trigger.  This is
necessary to make the /all page fast, since otherwise it needs to do a
sequential scan on the Builds table.

src/sql/upgrade-17.sql

@@ -0,0 +1,23 @@
+create table NrBuilds (
+    what  text primary key not null,
+    count integer not null
+);
+
+create function modifyNrBuildsFinished() returns trigger as $$
+  begin
+    if ((tg_op = 'INSERT' and new.finished = 1) or
+        (tg_op = 'UPDATE' and old.finished = 0 and new.finished = 1)) then
+      update NrBuilds set count = count + 1 where what = 'finished';
+    elsif ((tg_op = 'DELETE' and old.finished = 1) or
+           (tg_op = 'UPDATE' and old.finished = 1 and new.finished = 0)) then
+      update NrBuilds set count = count - 1 where what = 'finished';
+    end if;
+    return null;
+  end;
+$$ language plpgsql;
+
+create trigger NrBuildsFinished after insert or update or delete on Builds
+  for each row
+  execute procedure modifyNrBuildsFinished();
+
+insert into NrBuilds(what, count) select 'finished', count(*) from Builds where finished = 1;