Performance suggestions for stats feature

[polothy]

Hello - found some potential performance problems if this plugin was used on a very active site:

• In auth_plugin_googleoauth2::get_stats function, the query to auth_googleoauth2_logins table could return a lot of records. An improvement would be to use a count SQL query, EG: SELECT subtype, COUNT(id) logins FROM {auth_googleoauth2_logins} WHERE time > :time GROUP BY subtype and the subtype that is null would be for your moodle logins. Think you would have to use get_recordset_sql to run that SQL as get_records_sql likely wouldn't like the null subtype.
• In the auth_googleoauth2_observer, ideally the get_record on user table just pulls down their auth field instead of the whole record, EG: use $DB->get_field(...) instead. You might be able to get away with using $USER global as well and avoid the DB read altogether.
• The auth_googleoauth2_logins table is never cleaned, so could get quite large over time. I would suggest adding a scheduled task to delete old records. Would probably look very similar to \logstore_standard\task\cleanup_task.
• Adding an index to the time column on the auth_googleoauth2_logins table would really improve query performance once the table size gets larger.

Please let me know if anything is unclear, cheers!

[mouneyrac]

All clear, feel free to send a PR, cheers :)