ci_runners.contacted_at is not indexed and used for checking runner activity

For example, we run queries such as:

SELECT COUNT(*) FROM ci_runners WHERE active=true AND contacted_at >= NOW() - '60 minutes'::INTERVAL;

The table ci_runners.contacted_at is not indexed, resulting in the following query plan:

                                                     QUERY PLAN                                                                   
---------------------------------------------------------------------------------------------------------------------             
 Aggregate  (cost=15131.14..15131.15 rows=1 width=8) (actual time=24.513..24.514 rows=1 loops=1)                                  
   ->  Seq Scan on ci_runners  (cost=0.00..15118.05 rows=5237 width=0) (actual time=0.093..23.155 rows=5501 loops=1)              
         Filter: (active AND (contacted_at >= (now() - '01:00:00'::interval)))                                                    
         Rows Removed by Filter: 12959                                                                                            
 Planning time: 0.122 ms                                                                                                          
 Execution time: 24.540 ms                                                                                                        

assigned to @blackst0ne

mentioned in merge request !10876 (merged)

closed via merge request !10876 (merged)

mentioned in commit fc9f6c6d