Output:

==  AddForeignKeyFromNotificationSettingsToUsers: migrating ===================
-- 8040 batches
-- 17 rows deleted
==  AddForeignKeyFromNotificationSettingsToUsers: migrated (63.5747s) =========

Initial query:

gitlabhq_production=> EXPLAIN (ANALYZE, BUFFERS) SELECT  "notification_settings"."id" FROM "notification_settings"  ORDER BY "notification_settings"."id" ASC LIMIT 1;
                                                                               QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.43..0.45 rows=1 width=4) (actual time=9.176..9.177 rows=1 loops=1)
   Buffers: shared hit=2 read=2
   I/O Timings: read=9.008
   ->  Index Only Scan using notification_settings_pkey on notification_settings  (cost=0.43..167145.07 rows=8037744 width=4) (actual time=9.175..9.175 rows=1 loops=1)
         Heap Fetches: 0
         Buffers: shared hit=2 read=2
         I/O Timings: read=9.008
 Planning time: 0.866 ms
 Execution time: 9.218 ms
(9 rows)

Arbitrary batch:

gitlabhq_production=> EXPLAIN (ANALYZE, BUFFERS) SELECT  "notification_settings"."id" FROM "notification_settings" WHERE ("notification_settings"."id" >= 469549)  ORDER BY "notification_settings"."id" ASC LIMIT 1 OFFSET 1000;
                                                                                 QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=23.73..23.75 rows=1 width=4) (actual time=16.060..16.060 rows=1 loops=1)
   Buffers: shared hit=8 read=5
   I/O Timings: read=15.621
   ->  Index Only Scan using notification_settings_pkey on notification_settings  (cost=0.43..177271.18 rows=7608457 width=4) (actual time=11.687..16.006 rows=1001 loops=1)
         Index Cond: (id >= 469549)
         Heap Fetches: 0
         Buffers: shared hit=8 read=5
         I/O Timings: read=15.621
 Planning time: 0.106 ms
 Execution time: 16.082 ms
(10 rows)