MariaDB [civicrm]> explain SELECT email FROM civicrm_email JOIN civicrm_contact ON civicrm_email.contact_id = civicrm_contact.id JOIN civicrm_contribution ON civicrm_contact.id = civicrm_contribution.contact_id INNER JOIN ccogdill.GCOrderID0315  ON civicrm_contribution.trxn_id LIKE CONCAT('%', ccogdill.GCOrderID0315.id, '%') WHERE civicrm_email.email IS NOT NULL;

+------+-------------+----------------------+--------+--------------------------------------+------------------------------------+---------+----------------------------------+----------+-------------------------------------------------+
| id   | select_type | table                | type   | possible_keys                        | key                                | key_len | ref                              | rows     | Extra                                           |
+------+-------------+----------------------+--------+--------------------------------------+------------------------------------+---------+----------------------------------+----------+-------------------------------------------------+
|    1 | SIMPLE      | GCOrderID0315        | ALL    | NULL                                 | NULL                               | NULL    | NULL                             |     2249 |                                                 |
|    1 | SIMPLE      | civicrm_email        | ALL    | UI_email,FK_civicrm_email_contact_id | NULL                               | NULL    | NULL                             | 10827277 | Using where; Using join buffer (flat, BNL join) |
|    1 | SIMPLE      | civicrm_contact      | eq_ref | PRIMARY                              | PRIMARY                            | 4       | civicrm.civicrm_email.contact_id |        1 | Using index                                     |
|    1 | SIMPLE      | civicrm_contribution | ref    | FK_civicrm_contribution_contact_id   | FK_civicrm_contribution_contact_id | 4       | civicrm.civicrm_email.contact_id |        1 | Using where                                     |
+------+-------------+----------------------+--------+--------------------------------------+------------------------------------+---------+----------------------------------+----------+-------------------------------------------------+