Removed use of subqueries in email analytics queries

[kevinansfield]

closes https://linear.app/tryghost/issue/ENG-790/remove-use-of-sub-queries-in-email-analytics

Avoiding sub queries means we don't have a process tied up for longer than necessary and we can more easily see if one of the queries is non-performant.

• extracted the count queries into separate queries and used the retrieved values in the final update query
• removed a query by moving the email open rate calculation into JS as we've already fetched the necessary data before that point
• optimized calculation of delivered_count by switching from IS NOT NULL to IS NULL to match the typical data stored in that column so it needs to read far fewer rows from the index when counting (see below for before/after)

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delivered_at IS NOT NULL vs delivered_at IS NULL

mysql> explain analyze SELECT COUNT(id) FROM email_recipients WHERE email_id = '00000000846f9cc01e7bd3cc' AND delivered_at IS NOT NULL \G;
*************************** 1. row ***************************
EXPLAIN: -> Aggregate: count(email_recipients.id)  (cost=47067 rows=1) (actual time=71.9..71.9 rows=1 loops=1)
    -> Filter: ((email_recipients.email_id = '00000000846f9cc01e7bd3cc') and (email_recipients.delivered_at is not null))  (cost=32464 rows=146030) (actual time=0.0552..69 rows=75682 loops=1)
        -> Covering index range scan on email_recipients using email_recipients_email_id_delivered_at_index over (email_id = '00000000846f9cc01e7bd3cc' AND NULL < delivered_at)  (cost=32464 rows=146030) (actual time=0.0511..43.2 rows=75682 loops=1)

mysql> explain analyze SELECT COUNT(id) FROM email_recipients WHERE email_id = '00000000846f9cc01e7bd3cc' AND delivered_at IS NULL \G;
*************************** 1. row ***************************
EXPLAIN: -> Aggregate: count(email_recipients.id)  (cost=1477 rows=1) (actual time=4.44..4.44 rows=1 loops=1)
    -> Filter: (email_recipients.delivered_at is null)  (cost=813 rows=6638) (actual time=0.23..4.23 rows=3593 loops=1)
        -> Covering index lookup on email_recipients using email_recipients_email_id_delivered_at_index (email_id='00000000846f9cc01e7bd3cc', delivered_at=NULL)  (cost=813 rows=6638) (actual time=0.229..3.9 rows=3593 loops=1)

[kevinansfield]

There is some potential improvement here by optimising our use of IS NULL vs IS NOT NULL based on typical usage patterns. See #19918 (comment)