I'm not sure when this became a problem, but thanks to T120880 we've discovered that many civicrm_contribution rows are lacking an associated wmf_contribution_extra row. This is supposed to be a 1:1 association.
Out of 16483354 contributions, 1100514 are missing the extra table. The most recent is 2013-01-15.
We need to backfill these rows, to avoid adding complexity to scripts that assume this data is present.
I just checked
select c.id,contact_id, total_amount,receive_date FROM civicrm.civicrm_contribution c LEFT JOIN civicrm.wmf_contribution_extra x ON c.id = x.entity_id WHERE x.id IS NULL
-> ;
+----------+------------+--------------+---------------------+
| id | contact_id | total_amount | receive_date |
+----------+------------+--------------+---------------------+
| 52049669 | 8794787 | 1.35 | 2020-10-29 23:23:55 |
+----------+------------+--------------+---------------------+
Just the one - and of course it is 2020 - guess we did this