fix(db): Delete recipients without sub query

[ChristophWurst]

MySQL does not use indexes for the "dependent" subquery. It's either a bug or a missing optimization. EXPLAIN for the two queries is fine in isolation but when you DELETE with subquery neither the inner nor the outher query use an index. This causes a full table scan on very large tables.

Before

EXPLAIN DELETE FROM   `oc_mail_recipients` WHERE   `message_id` IN (     SELECT       `id`     FROM       `oc_mail_messages`     WHERE       (`mailbox_id` = 26)       AND (`uid` IN (36923, 10343))   );
+------+--------------------+--------------------+-----------------+-----------------------------------------------------------------------------------------------------------+---------+---------+------+-------+-------------+
| id   | select_type        | table              | type            | possible_keys                                                                                             | key     | key_len | ref  | rows  | Extra       |
+------+--------------------+--------------------+-----------------+-----------------------------------------------------------------------------------------------------------+---------+---------+------+-------+-------------+
|    1 | PRIMARY            | oc_mail_recipients | ALL             | NULL                                                                                                      | NULL    | NULL    | NULL | 55178 | Using where |
|    2 | DEPENDENT SUBQUERY | oc_mail_messages   | unique_subquery | PRIMARY,mail_messages_id_flags,mail_messages_id_flags2,mail_msg_thrd_root_snt_idx,mail_messages_mb_id_uid   | PRIMARY | 8       | func |     1 | Using where |
+------+--------------------+--------------------+-----------------+-----------------------------------------------------------------------------------------------------------+---------+---------+------+-------+-------------+

After

EXPLAIN SELECT `id` FROM `oc_mail_messages` WHERE (`mailbox_id` = 26) AND (`uid` IN (36926, 36924, 36923));
+------+-------------+------------------+-------+---------------------------------------------------------------------------------------------------+-------------------------+---------+------+------+--------------------------+
| id   | select_type | table            | type  | possible_keys                                                                                     | key                     | key_len | ref  | rows | Extra                    |
+------+-------------+------------------+-------+---------------------------------------------------------------------------------------------------+-------------------------+---------+------+------+--------------------------+
|    1 | SIMPLE      | oc_mail_messages | range | mail_messages_id_flags,mail_messages_id_flags2,mail_msg_thrd_root_snt_idx,mail_messages_mb_id_uid | mail_messages_mb_id_uid | 8       | NULL |    3 | Using where; Using index |
+------+-------------+------------------+-------+---------------------------------------------------------------------------------------------------+-------------------------+---------+------+------+--------------------------+
EXPLAIN DELETE FROM `oc_mail_recipients` WHERE `message_id` IN (69049, 69050, 69052);
+------+-------------+--------------------+-------+---------------------------+---------------------------+---------+------+------+-------------+
| id   | select_type | table              | type  | possible_keys             | key                       | key_len | ref  | rows | Extra       |
+------+-------------+--------------------+-------+---------------------------+---------------------------+---------+------+------+-------------+
|    1 | SIMPLE      | oc_mail_recipients | range | mail_recipient_msg_id_idx | mail_recipient_msg_id_idx | 5       | NULL |    6 | Using where |
+------+-------------+--------------------+-------+---------------------------+---------------------------+---------+------+------+-------------+

Fixes #9760

[ChristophWurst]

Does not work because Doctrine silently drops the join: doctrine/orm#10524 refactored to use SELECT+DELETE. It's not ideal but faster than the FTS.

[Altahrim]

Out of curiosity, have you tried something like this?

EXPLAIN
DELETE R.*
FROM oc_mail_recipients R, oc_mail_messages M
WHERE 
  R.message_id = M.id 
  AND M.mailbox_id = 26 
  AND uid IN (36923, 10343)

+------+-------------+-------+-------+-----------------------------------------------------------------------------------------------------------+---------------------------+---------+---------+------+--------------------------+
| id   | select_type | table | type  | possible_keys                                                                                             | key                       | key_len | ref     | rows | Extra                    |
+------+-------------+-------+-------+-----------------------------------------------------------------------------------------------------------+---------------------------+---------+---------+------+--------------------------+
|    1 | SIMPLE      | M     | range | PRIMARY,mail_messages_id_flags,mail_messages_id_flags2,mail_msg_thrd_root_snt_idx,mail_messages_mb_id_uid | mail_messages_mb_id_uid   | 8       | NULL    | 2    | Using where; Using index |
|    1 | SIMPLE      | R     | ref   | mail_recipient_msg_id_idx                                                                                 | mail_recipient_msg_id_idx | 5       | oc.M.id | 1    | Using where              |
+------+-------------+-------+-------+-----------------------------------------------------------------------------------------------------------+---------------------------+---------+---------+------+--------------------------+

However, I am not sure we can generate such query with DBAL…

[ChristophWurst]

/backport to stable3.7