Added organization_id index to notifications

[lightalloy]

What type of PR is this? (check all applicable)

• Optimization

Description

Added organization_id index to the notifications table.

Explaining query without an index:

Notification.where("organization_id = 3 and read = false").select("count(*)").explain
------------------------------------------------------------------------
 Aggregate  (cost=780.32..780.33 rows=1 width=8)
   ->  Seq Scan on notifications  (cost=0.00..775.94 rows=1753 width=0)
         Filter: ((NOT read) AND (organization_id = 3))
(3 rows)

With the provided index:

Notification.where("organization_id = 3 and read = false").select("count(*)").explain
-----------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=338.36..338.37 rows=1 width=8)
   ->  Index Scan using index_notifications_on_organization_id on notifications  (cost=0.28..333.98 rows=1753 width=0)
         Index Cond: (organization_id = 3)
         Filter: (NOT read)
(4 rows)