How do I properly optimize this query?

I’m trying to optimize the following query. I’ve used EXPLAIN to troubleshoot but it keeps showing up in our logs as a non-indexed query. What am I missing? I’ve created indexes on all columns referenced, but it’s still showing up.

select `users`.`nickname`, `users`.`userId`, `userdata`.`avatarUpdatedAt` 
from `users` 
inner join `userdata` on `userdata`.`userId` = `users`.`userId` 
order by `lastActivity` desc 
limit 28

+---+---+---+---+---+---+---+---+---+---+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+---+---+---+---+---+---+---+---+---+---+
| 1 | SIMPLE | users | index | PRIMARY,userId | lastActivity | 8 | NULL | 28 |  |
| 1 | SIMPLE | userdata | eq_ref | userdata_userid_unique | userdata_userid_unique | 8 | prod.users.userId | 1 | Using index condition |
+---+---+---+---+---+---+---+---+---+---+

# Time: 200703  6:14:18
# User@Host: prod[prod] @ localhost [127.0.0.1]
# Thread_id: 17988  Schema: prod  QC_hit: No
# Query_time: 0.000201  Lock_time: 0.000011  Rows_sent: 28  Rows_examined: 56
# Rows_affected: 0  Bytes_sent: 1039
# Full_scan: Yes  Full_join: No  Tmp_table: No  Tmp_table_on_disk: No
# Filesort: No  Filesort_on_disk: No  Merge_passes: 0  Priority_queue: No

Answer

If you have a big table which contains millions of row, it would be the best to have where condition to filter out the data to make query run faster.

Indexing will help in retrieving data so just having index does not make query to run faster.

select 
    u.nickname, 
    u.userId, 
    ud.avatarUpdatedAt 
from users u
inner join userdata ud
on ud.userId = u.userId
where ud.avatarUpdatedAt >= '2020-01-01'
and ud.avatarUpdatedAt <= '2020-05-01'
order by lastActivity desc