explain plan for civiCRM query

+----+--------------------+-----------------------------------------+-----------------+------------------------------------------------------------------------------------+------------------+---------+---------------------------+--------+------------------------------+
| id | select_type        | table                                   | type            | possible_keys                                                                      | key              | key_len | ref                       | rows   | Extra                        |
+----+--------------------+-----------------------------------------+-----------------+------------------------------------------------------------------------------------+------------------+---------+---------------------------+--------+------------------------------+
|  1 | PRIMARY            | contact_a                               | index           | NULL                                                                               | index_is_deleted | 1       | NULL                      | 149732 | Using index; Using temporary |
|  1 | PRIMARY            | civicrm_group_contact-605               | ref             | UI_contact_group                                                                   | UI_contact_group | 4       | step2_crm_db.contact_a.id |      1 | Distinct                     |
|  1 | PRIMARY            | civicrm_group_contact-608               | ref             | UI_contact_group                                                                   | UI_contact_group | 4       | step2_crm_db.contact_a.id |      1 | Distinct                     |
|  1 | PRIMARY            | civicrm_group_contact-612               | ref             | UI_contact_group                                                                   | UI_contact_group | 4       | step2_crm_db.contact_a.id |      1 | Using where; Distinct        |
|  7 | DEPENDENT SUBQUERY | civicrm_group_contact                   | unique_subquery | UI_contact_group,FK_civicrm_group_contact_group_id                                 | UI_contact_group | 8       | func,const                |      1 | Using where                  |
|  6 | DEPENDENT SUBQUERY | civicrm_value_1_riding__circonscription | eq_ref          | unique_entity_id,FK_civicrm_value_1_riding__circonscription_entity_id,INDEX_riding | unique_entity_id | 4       | func                      |      1 | Using where; Using temporary |
|  6 | DEPENDENT SUBQUERY | contact_a                               | eq_ref          | PRIMARY                                                                            | PRIMARY          | 4       | func                      |      1 | Using where; Using index     |
|  5 | DEPENDENT SUBQUERY | civicrm_group_contact                   | unique_subquery | UI_contact_group,FK_civicrm_group_contact_group_id                                 | UI_contact_group | 8       | func,const                |      1 | Using where                  |
|  4 | DEPENDENT SUBQUERY | civicrm_value_1_riding__circonscription | eq_ref          | unique_entity_id,FK_civicrm_value_1_riding__circonscription_entity_id,INDEX_riding | unique_entity_id | 4       | func                      |      1 | Using where; Using temporary |
|  4 | DEPENDENT SUBQUERY | contact_a                               | eq_ref          | PRIMARY                                                                            | PRIMARY          | 4       | func                      |      1 | Using where; Using index     |
|  3 | DEPENDENT SUBQUERY | civicrm_group_contact                   | unique_subquery | UI_contact_group,FK_civicrm_group_contact_group_id                                 | UI_contact_group | 8       | func,const                |      1 | Using where                  |
|  2 | DEPENDENT SUBQUERY | civicrm_value_1_riding__circonscription | eq_ref          | unique_entity_id,FK_civicrm_value_1_riding__circonscription_entity_id,INDEX_riding | unique_entity_id | 4       | func                      |      1 | Using where; Using temporary |
|  2 | DEPENDENT SUBQUERY | contact_a                               | eq_ref          | PRIMARY                                                                            | PRIMARY          | 4       | func                      |      1 | Using where; Using index     |
+----+--------------------+-----------------------------------------+-----------------+------------------------------------------------------------------------------------+------------------+---------+---------------------------+--------+------------------------------+