This commit adds an index for the query which the chat plugin executes
multiple times when preloading user data in `Chat::ChatChannelFetcher.unread_counts`.
Sample query plan from a query I grabbed from one of our production
instance.
Before:
```
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=10.77..696.67 rows=7 width=16) (actual time=7.735..7.736 rows=0 loops=1)
Group Key: cc.id
-> Nested Loop (cost=10.77..696.54 rows=12 width=8) (actual time=7.734..7.735 rows=0 loops=1)
Join Filter: (cc.id = cm.chat_channel_id)
-> Nested Loop (cost=0.56..76.44 rows=1 width=16) (actual time=0.011..0.037 rows=7 loops=1)
-> Index Only Scan using chat_channels_pkey on chat_channels cc (cost=0.28..22.08 rows=7 width=8) (actual time=0.004..0.014 rows=7 loops=1)
Index Cond: (id = ANY ('{192,300,228,727,8,612,1633}'::bigint[]))
Heap Fetches: 0
-> Index Scan using user_chat_channel_unique_memberships on user_chat_channel_memberships uccm (cost=0.28..7.73 rows=1 width=8) (actual time=0.003..0.003 rows=1 loops=7)
Index Cond: ((user_id = 1338) AND (chat_channel_id = cc.id))
-> Bitmap Heap Scan on chat_messages cm (cost=10.21..618.98 rows=89 width=12) (actual time=1.096..1.097 rows=0 loops=7)
Recheck Cond: (chat_channel_id = uccm.chat_channel_id)
Filter: ((deleted_at IS NULL) AND (user_id <> 1338) AND (id > COALESCE(uccm.last_read_message_id, 0)))
Rows Removed by Filter: 2085
Heap Blocks: exact=7106
-> Bitmap Index Scan on index_chat_messages_on_chat_channel_id_and_created_at (cost=0.00..10.19 rows=270 width=0) (actual time=0.114..0.114 rows=2085 loops=7)
Index Cond: (chat_channel_id = uccm.chat_channel_id)
Planning Time: 0.408 ms
Execution Time: 7.762 ms
(19 rows)
```
After:
```
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=5.84..367.39 rows=7 width=16) (actual time=0.130..0.131 rows=0 loops=1)
Group Key: cc.id
-> Nested Loop (cost=5.84..367.26 rows=12 width=8) (actual time=0.129..0.130 rows=0 loops=1)
Join Filter: (cc.id = cm.chat_channel_id)
-> Nested Loop (cost=0.56..76.44 rows=1 width=16) (actual time=0.038..0.069 rows=7 loops=1)
-> Index Only Scan using chat_channels_pkey on chat_channels cc (cost=0.28..22.08 rows=7 width=8) (actual time=0.011..0.022 rows=7 loops=1)
Index Cond: (id = ANY ('{192,300,228,727,8,612,1633}'::bigint[]))
Heap Fetches: 0
-> Index Scan using user_chat_channel_unique_memberships on user_chat_channel_memberships uccm (cost=0.28..7.73 rows=1 width=8) (actual time=0.006..0.006 rows=1 loops=7)
Index Cond: ((user_id = 1338) AND (chat_channel_id = cc.id))
-> Bitmap Heap Scan on chat_messages cm (cost=5.28..289.71 rows=89 width=12) (actual time=0.008..0.008 rows=0 loops=7)
Recheck Cond: ((chat_channel_id = uccm.chat_channel_id) AND (id > COALESCE(uccm.last_read_message_id, 0)) AND (deleted_at IS NULL))
Filter: (user_id <> 1338)
-> Bitmap Index Scan on index_chat_messages_on_chat_channel_id_and_id (cost=0.00..5.26 rows=90 width=0) (actual time=0.008..0.008 rows=0 loops=7)
Index Cond: ((chat_channel_id = uccm.chat_channel_id) AND (id > COALESCE(uccm.last_read_message_id, 0)))
Planning Time: 1.217 ms
Execution Time: 0.188 ms
(17 rows)
```
80d289eb8b