Fix stage index migration failing in PostgreSQL 10

As discussed in
https://www.postgresql.org/message-id/9922.1353433645%40sss.pgh.pa.us,
the PostgreSQL window function last_value may not consider the
right rows:

Note that first_value, last_value, and nth_value consider only the rows
within the "window frame", which by default contains the rows from the
start of the partition through the last peer of the current row. This is
likely to give unhelpful results for last_value and sometimes also
nth_value. You can redefine the frame by adding a suitable frame
specification (RANGE or ROWS) to the OVER clause. See Section 4.2.8 for
more information about frame specifications.

This query could be fixed by adding `RANGE BETWEEN UNBOUNDED PRECEDING
AND UNBOUNDED FOLLOWING`, but that's quite verbose. It's simpler just to
use the first_value function.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59985

lib/gitlab/background_migration/migrate_stage_index.rb

@@ -21,8 +21,8 @@ module Gitlab
                   AND stage_idx IS NOT NULL
                 GROUP BY stage_id, stage_idx
             ), indexes AS (
-              SELECT DISTINCT stage_id, last_value(stage_idx)
-                OVER (PARTITION BY stage_id ORDER BY freq ASC) AS index
+              SELECT DISTINCT stage_id, first_value(stage_idx)
+                OVER (PARTITION BY stage_id ORDER BY freq DESC) AS index
                 FROM freqs
             )
 

spec/lib/gitlab/background_migration/migrate_stage_index_spec.rb

@@ -30,6 +30,6 @@ describe Gitlab::BackgroundMigration::MigrateStageIndex, :migration, schema: 201
 
     described_class.new.perform(100, 101)
 
-    expect(stages.all.pluck(:position)).to eq [2, 3]
+    expect(stages.all.order(:id).pluck(:position)).to eq [2, 3]
   end
 end