This index is added on `(target_project_id, iid)` and has a `WHERE state
= 'opened'` condition. Using this index we can drastically improve the
performance of the query used to count the total number of merge
requests in a group. Without this index the query would eventually
perform the following:
-> Index Scan using index_merge_requests_on_target_project_id_and_iid on merge_requests (cost=0.43..4.89 rows=7 width=4) (actual time=0.058..0.353 rows=6 loops=228)
Index Cond: (target_project_id = projects.id)
Filter: ((state)::text = 'opened'::text)
Rows Removed by Filter: 141
Buffers: shared hit=34351 dirtied=1
Out of the ~180 milliseconds the entire query would take, around 170
milliseconds was spent in just this segment. With the index in place,
the above segment is turned into the following:
-> Index Only Scan using yorick_test on merge_requests (cost=0.42..0.55 rows=7 width=4) (actual time=0.004..0.010 rows=6 loops=228)
Index Cond: (target_project_id = projects.id)
Heap Fetches: 419
Buffers: shared hit=1381
The index also reduces the total query time to roughly 10 milliseconds.
Yorick Peterse