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Hi there,

During upgrades, I've noticed that when running online_data_migrations
with "infinite-until-done" mode, it loops over all of the migrations
one by one.

However, one of the online data migrations
(instance_obj.populate_missing_availability_zones) makes a query that
takes a really long time as it seems inefficient (which eventually
results in 0, cause it already ran), which means as it loops in
"blocks" of 50, there's almost a 2-3 to 8 minute wait in really large
environments.

The question ends up in specific:

SELECT count(*) AS count_1
FROM (SELECT instance_extra.created_at AS instance_extra_created_at,
instance_extra.updated_at AS instance_extra_updated_at,
instance_extra.deleted_at AS instance_extra_deleted_at,
instance_extra.deleted AS instance_extra_deleted, instance_extra.id AS
instance_extra_id, instance_extra.instance_uuid AS
instance_extra_instance_uuid
FROM instance_extra
WHERE instance_extra.keypairs IS NULL AND instance_extra.deleted = 0) AS anon_1

The explain for the DB query in this example:

+------+-------------+----------------+------+---------------+------+---------+------+--------+-------------+
| id   | select_type | table          | type | possible_keys | key  |
key_len | ref  | rows   | Extra       |
+------+-------------+----------------+------+---------------+------+---------+------+--------+-------------+
|    1 | SIMPLE      | instance_extra | ALL  | NULL          | NULL |
NULL    | NULL | 382473 | Using where |
+------+-------------+----------------+------+---------------+------+---------+------+--------+-------------+

It's possible that it can be ever worse, as this number is from
another very-long running environments.

+------+-------------+----------------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table          | type | possible_keys | key  |
key_len | ref  | rows    | Extra       |
+------+-------------+----------------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | instance_extra | ALL  | NULL          | NULL |
NULL    | NULL | 3008741 | Using where |
+------+-------------+----------------+------+---------------+------+---------+------+---------+-------------+

I'm not the SQL expert, could we not optimize this?  Alternatively,
could we update the online data migrations code to "pop out" any of
the migrations that return 0 for the next iteration, that way it only
works on those online_data_migrations that *have* to be done, and
ignore those it knows are done?

Thanks,
Mohammed

-- 
Mohammed Naser — vexxhost
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