* MM-40799: Improve ReplyCount query
This PR optimizes the reply count query to use only
the indexed column to count the rows rather than using
the ID column. This results in an index-only scan
rather than an index scan. As a result, the query
can be satisfied directly by scanning the index
and there is no need to touch the heap.
Count(*) also gives the same result, but is not recommended
as it also verifies non-null columns and can take slightly
longer.
Before:
```
explain analyze SELECT p.*, (SELECT COUNT(Posts.Id) FROM Posts WHERE Posts.RootId = (CASE WHEN p.RootId = '' THEN p.Id ELSE p.RootId END) AND Posts.DeleteAt = 0) AS ReplyCount FROM Posts p WHERE (CreateAt < (SELECT CreateAt FROM Posts WHERE Id = 'bguxt6zzcjytpffgza3dc7xody') AND p.ChannelId = '5xmynf36cinrzksuzhq7my1nbc' AND DeleteAt = 0) ORDER BY p.ChannelId, DeleteAt, CreateAt DESC LIMIT 30 OFFSET 0;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=9.14..130616.04 rows=30 width=922) (actual time=19.208..19.454 rows=30 loops=1)
InitPlan 2 (returns $2)
-> Index Scan using posts_pkey on posts posts_1 (cost=0.56..8.58 rows=1 width=8) (actual time=0.028..0.030 rows=1 loops=1)
Index Cond: ((id)::text = 'bguxt6zzcjytpffgza3dc7xody'::text)
-> Index Scan Backward using idx_posts_channel_id_delete_at_create_at on posts p (cost=0.56..2720977.61 rows=625 width=922) (actual time=0.075..0.317 rows=30 loops=1)
Index Cond: (((channelid)::text = '5xmynf36cinrzksuzhq7my1nbc'::text) AND (deleteat = 0) AND (createat < $2))
SubPlan 1
-> Aggregate (cost=4349.49..4349.50 rows=1 width=8) (actual time=0.007..0.007 rows=1 loops=30)
-> Index Scan using idx_posts_root_id_delete_at on posts (cost=0.56..4346.36 rows=1253 width=27) (actual time=0.007..0.007 rows=0 loops=30)
Index Cond: (((rootid)::text = (CASE WHEN ((p.rootid)::text = ''::text) THEN p.id ELSE p.rootid END)::text) AND (deleteat = 0))
```
After:
```
explain analyze SELECT p.*, (SELECT COUNT(Posts.RootId) FROM Posts WHERE Posts.RootId = (CASE WHEN p.RootId = '' THEN p.Id ELSE p.RootId END) AND Posts.DeleteAt = 0) AS ReplyCount FROM Posts p WHERE (CreateAt < (SELECT CreateAt FROM Posts WHERE Id = 'bguxt6zzcjytpffgza3dc7xody') AND p.ChannelId = '5xmynf36cinrzksuzhq7my1nbc' AND DeleteAt = 0) ORDER BY p.ChannelId, DeleteAt, CreateAt DESC LIMIT 30 OFFSET 0;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=9.14..130616.04 rows=30 width=922) (actual time=12.936..13.188 rows=30 loops=1)
InitPlan 2 (returns $2)
-> Index Scan using posts_pkey on posts posts_1 (cost=0.56..8.58 rows=1 width=8) (actual time=0.029..0.030 rows=1 loops=1)
Index Cond: ((id)::text = 'bguxt6zzcjytpffgza3dc7xody'::text)
-> Index Scan Backward using idx_posts_channel_id_delete_at_create_at on posts p (cost=0.56..2720977.61 rows=625 width=922) (actual time=0.076..0.324 rows=30 loops=1)
Index Cond: (((channelid)::text = '5xmynf36cinrzksuzhq7my1nbc'::text) AND (deleteat = 0) AND (createat < $2))
SubPlan 1
-> Aggregate (cost=4349.49..4349.50 rows=1 width=8) (actual time=0.008..0.008 rows=1 loops=30)
-> Index Only Scan using idx_posts_root_id_delete_at on posts (cost=0.56..4346.36 rows=1253 width=8) (actual time=0.007..0.007 rows=0 loops=30)
Index Cond: ((rootid = (CASE WHEN ((p.rootid)::text = ''::text) THEN p.id ELSE p.rootid END)::text) AND (deleteat = 0))
Heap Fetches: 14
```
https://mattermost.atlassian.net/browse/MM-40799
```release-note
NONE
```
* Use COUNT(*) for faster
```release-note
NONE
```
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