Fix stage index migration failing in PostgreSQL 10sh-fix-issue-59985

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

2 files changed, 3 insertions, 3 deletions

diff --git a/lib/gitlab/background_migration/migrate_stage_index.rb b/lib/gitlab/background_migration/migrate_stage_index.rb
index f90f35a913d..f921233460d 100644
--- a/lib/gitlab/background_migration/migrate_stage_index.rb
+++ b/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
             )
 
diff --git a/spec/lib/gitlab/background_migration/migrate_stage_index_spec.rb b/spec/lib/gitlab/background_migration/migrate_stage_index_spec.rb
index f8107dd40b9..4db829b1e7b 100644
--- a/spec/lib/gitlab/background_migration/migrate_stage_index_spec.rb
+++ b/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