Some queries from Web interface aren't using indexes requiring too much time on v3.1.3

先重現 Web 介面查詢 state=*\u0026archive=true，並針對 MySQL 使用 EXPLAIN 執行提供的 SQL。追蹤產生此陳述式的查詢路徑，然後確認查詢使用適當的索引，且根據 slow log 不再需要超過 90 秒。

This query: state=*&archive=true
Is producing a complex sql statement that it is not using indexes and are taking a long time to complete:

explain SELECT (coalesce((SELECT count FROM patchwork_patchtag WHERE patchwork_patchtag.patch_id=patchwork_patch.id AND patchwork_patchtag.tag_id=1), 0)) AS `tag_1_count`, (coalesce((SELECT count FROM patchwork_patchtag WHERE patchwork_patchtag.patch_id=patchwork_patch.id AND patchwork_patchtag.tag_id=2), 0)) AS `tag_2_count`, (coalesce((SELECT count FROM patchwork_patchtag WHERE patchwork_patchtag.patch_id=patchwork_patch.id AND patchwork_patchtag.tag_id=3), 0)) AS `tag_3_count`, (coalesce((SELECT count FROM patchwork_patchtag WHERE patchwork_patchtag.patch_id=patchwork_patch.id AND patchwork_patchtag.tag_id=4), 0)) AS `tag_4_count`, (coalesce((SELECT count FROM patchwork_patchtag WHERE patchwork_patchtag.patch_id=patchwork_patch.id AND patchwork_patchtag.tag_id=5), 0)) AS `tag_5_count`, `patchwork_patch`.`id`, `patchwork_patch`.`msgid`, `patchwork_patch`.`date`, `patchwork_patch`.`submitter_id`, `patchwork_patch`.`project_id`, `patchwork_patch`.`name`, `patchwork_patch`.`delegate_id`, `patchwork_patch`.`state_id`, `patchwork_patch`.`series_id`, `patchwork_person`.`id`, `patchwork_person`.`email`, `patchwork_person`.`name`, `patchwork_person`.`user_id`, `auth_user`.`id`, `auth_user`.`password`, `auth_user`.`last_login`, `auth_user`.`is_superuser`, `auth_user`.`username`, `auth_user`.`first_name`, `auth_user`.`last_name`, `auth_user`.`email`, `auth_user`.`is_staff`, `auth_user`.`is_active`, `auth_user`.`date_joined`, `patchwork_state`.`id`, `patchwork_state`.`name`, `patchwork_state`.`slug`, `patchwork_state`.`ordering`, `patchwork_state`.`action_required`, `patchwork_series`.`id`, `patchwork_series`.`name` FROM `patchwork_patch` INNER JOIN `patchwork_person` ON (`patchwork_patch`.`submitter_id` = `patchwork_person`.`id`) LEFT OUTER JOIN `auth_user` ON (`patchwork_patch`.`delegate_id` = `auth_user`.`id`) LEFT OUTER JOIN `patchwork_state` ON (`patchwork_patch`.`state_id` = `patchwork_state`.`id`) LEFT OUTER JOIN `patchwork_series` ON (`patchwork_patch`.`series_id` = `patchwork_series`.`id`) WHERE (`patchwork_patch`.`project_id` = 1 AND `patchwork_patch`.`archived`) ORDER BY `patchwork_patch`.`date` DESC LIMIT 1000;
+------+--------------------+--------------------+--------+----------------------------------------------------------------------+--------------------------+---------+----------------------------------------+-------+-----------------------------+
| id   | select_type        | table              | type   | possible_keys                                                        | key                      | key_len | ref                                    | rows  | Extra                       |
+------+--------------------+--------------------+--------+----------------------------------------------------------------------+--------------------------+---------+----------------------------------------+-------+-----------------------------+
|    1 | PRIMARY            | patchwork_patch    | ref    | patchwork_patch_499df97c,patchwork_patch_1a37f020,patch_covering_idx | patchwork_patch_499df97c | 4       | const                                  | 30607 | Using where; Using filesort |
|    1 | PRIMARY            | patchwork_person   | eq_ref | PRIMARY                                                              | PRIMARY                  | 4       | patchwork.patchwork_patch.submitter_id | 1     |                             |
|    1 | PRIMARY            | auth_user          | eq_ref | PRIMARY                                                              | PRIMARY                  | 4       | patchwork.patchwork_patch.delegate_id  | 1     | Using where                 |
|    1 | PRIMARY            | patchwork_state    | eq_ref | PRIMARY                                                              | PRIMARY                  | 4       | patchwork.patchwork_patch.state_id     | 1     | Using where                 |
|    1 | PRIMARY            | patchwork_series   | eq_ref | PRIMARY                                                              | PRIMARY                  | 4       | patchwork.patchwork_patch.series_id    | 1     | Using where                 |
|    6 | DEPENDENT SUBQUERY | patchwork_patchtag | eq_ref | patch_id                                                             | patch_id                 | 8       | patchwork.patchwork_patch.id,const     | 1     |                             |
|    5 | DEPENDENT SUBQUERY | patchwork_patchtag | eq_ref | patch_id                                                             | patch_id                 | 8       | patchwork.patchwork_patch.id,const     | 1     |                             |
|    4 | DEPENDENT SUBQUERY | patchwork_patchtag | eq_ref | patch_id                                                             | patch_id                 | 8       | patchwork.patchwork_patch.id,const     | 1     |                             |
|    3 | DEPENDENT SUBQUERY | patchwork_patchtag | eq_ref | patch_id                                                             | patch_id                 | 8       | patchwork.patchwork_patch.id,const     | 1     |                             |
|    2 | DEPENDENT SUBQUERY | patchwork_patchtag | eq_ref | patch_id                                                             | patch_id                 | 8       | patchwork.patchwork_patch.id,const     | 1     |                             |
+------+--------------------+--------------------+--------+----------------------------------------------------------------------+--------------------------+---------+----------------------------------------+-------+-----------------------------+

As reported by mysql slow log, it takes more than 1:30 mins to complete:

# Thread_id: 5213  Schema: patchwork  QC_hit: No
# Query_time: 92.553408  Lock_time: 0.000355  Rows_sent: 1000  Rows_examined: 101124
# Rows_affected: 0  Bytes_sent: 367857

Issue noticed at https://patchwork.linuxtv.org.