Fixed email_recipients indexes to match query usage (#19918)
closes https://linear.app/tryghost/issue/ENG-791/migration-to-fix-email-recipients-indexes
Our indexes over single columns (`delivered_at`, `opened_at`, `failed_at`) were ineffective because the only time we query those is alongside `email_id` meaning we were frequently performing full table scans on very large tables during our email analytics jobs.
- added migration to add new indexes covering `email_id` and the respective columns
- added migration to drop the old indexes that weren't being used in any query plans
Local runtime with ~2M email_recipient rows:
- before: 1.7s
- after: 99ms
Explain output...
before:
```
+----+-------------+------------------+------------+-------+----------------------------------------------------------------------------------+----------------------------------------------+---------+-------+--------+----------+------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+-------+----------------------------------------------------------------------------------+----------------------------------------------+---------+-------+--------+----------+------------------------------------+
| 1 | UPDATE | emails | NULL | index | NULL | PRIMARY | 98 | NULL | 1 | 100.00 | Using where |
| 4 | SUBQUERY | email_recipients | NULL | range | email_recipients_email_id_member_email_index,email_recipients_failed_at_index | email_recipients_failed_at_index | 6 | NULL | 2343 | 7.76 | Using index condition; Using where |
| 3 | SUBQUERY | email_recipients | NULL | ref | email_recipients_email_id_member_email_index,email_recipients_opened_at_index | email_recipients_email_id_member_email_index | 98 | const | 159126 | 50.00 | Using where |
| 2 | SUBQUERY | email_recipients | NULL | ref | email_recipients_email_id_member_email_index,email_recipients_delivered_at_index | email_recipients_email_id_member_email_index | 98 | const | 159126 | 50.00 | Using where |
+----+-------------+------------------+------------+-------+----------------------------------------------------------------------------------+----------------------------------------------+---------+-------+--------+----------+------------------------------------+
```
after:
```
+----+-------------+------------------+------------+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------+---------+------+--------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------+---------+------+--------+----------+--------------------------+
| 1 | UPDATE | emails | NULL | index | NULL | PRIMARY | 98 | NULL | 1 | 100.00 | Using where; |
| 4 | SUBQUERY | email_recipients | NULL | range | email_recipients_email_id_member_email_index,email_recipients_email_id_delivered_at_index,email_recipients_email_id_opened_at_index,email_recipients_email_id_failed_at_index | email_recipients_email_id_failed_at_index | 104 | NULL | 60 | 100.00 | Using where; Using index |
| 3 | SUBQUERY | email_recipients | NULL | range | email_recipients_email_id_member_email_index,email_recipients_email_id_delivered_at_index,email_recipients_email_id_opened_at_index,email_recipients_email_id_failed_at_index | email_recipients_email_id_opened_at_index | 104 | NULL | 119496 | 100.00 | Using where; Using index |
| 2 | SUBQUERY | email_recipients | NULL | range | email_recipients_email_id_member_email_index,email_recipients_email_id_delivered_at_index,email_recipients_email_id_opened_at_index,email_recipients_email_id_failed_at_index | email_recipients_email_id_delivered_at_index | 104 | NULL | 146030 | 100.00 | Using where; Using index |
+----+-------------+------------------+------------+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------+---------+------+--------+----------+--------------------------+
```
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