Add temporary partial index to speed up the migration

Closes #32469

db/post_migrate/20170503004427_upate_retried_for_ci_build.rb

@@ -2,28 +2,42 @@ class UpateRetriedForCiBuild < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false
-  
+
   disable_ddl_transaction!
   def up
     disable_statement_timeout
-    latest_id = <<-SQL.strip_heredoc
-      SELECT MAX(ci_builds2.id)
-        FROM ci_builds ci_builds2
-        WHERE ci_builds.commit_id=ci_builds2.commit_id
-          AND ci_builds.name=ci_builds2.name
-    SQL
-    
-    # This is slow update as it does single-row query
-    # This is designed to be run as idle, or a post deployment migration
-    is_retried = Arel.sql("((#{latest_id}) != ci_builds.id)")
-
-    update_column_in_batches(:ci_builds, :retried, is_retried) do |table, query|
-      query.where(table[:retried].eq(nil))
+    with_temporary_partial_index do
+      latest_id = <<-SQL.strip_heredoc
+        SELECT MAX(ci_builds2.id)
+          FROM ci_builds ci_builds2
+          WHERE ci_builds.commit_id=ci_builds2.commit_id
+            AND ci_builds.name=ci_builds2.name
+      SQL
+
+      # This is slow update as it does single-row query
+      # This is designed to be run as idle, or a post deployment migration
+      is_retried = Arel.sql("((#{latest_id}) != ci_builds.id)")
+
+      update_column_in_batches(:ci_builds, :retried, is_retried) do |table, query|
+        query.where(table[:retried].eq(nil))
+      end
     end
   end
   def down
   end
+
+  def with_temporary_partial_index
+    if Gitlab::Database.postgresql?
+      execute 'CREATE INDEX CONCURRENTLY IF NOT EXISTS index_for_ci_builds_retried_migration ON ci_builds (id) WHERE retried IS NULL;'
+    end
+
+    yield
+
+    if Gitlab::Database.postgresql?
+      execute 'DROP INDEX CONCURRENTLY IF EXISTS index_for_ci_builds_retried_migration'
+    end
+  end
 end