Add SQL index to states.event_id

[amelchio]

Description:

The States.event_id field references Event.event_id but until now there was no index on the former. This potentially makes Event purging slow (as reported in #12797) because each removed event does a States table scan to check that it is not referenced.

MySQL users will have this index already, automatically added due to the foreign key. However, because we did not actually write States.event_id values until #12580, this index can be useless (ignored due to low cardinality statistics). I tested that the migration code properly removes this automatic index.

This index should also make it possible to return and even improve the optimization in #12608.

Related issue (if applicable): fixes #12797

Checklist:

• The code change is tested and works locally.

If the code does not interact with devices:

• Local tests with tox run successfully.
• Tests have been added to verify that the new code works.

[corneyl]

How can I temporarily add this index using a query? Purging now takes endlessly to complete.

Edit: I think this did it for me (PostgreSQL): CREATE INDEX ix_states_event_id ON states (event_id);

[amelchio]

@corneyl That is correct. Did it improve purge performance for you?

[corneyl]

Without index the query did not finish within 9 hours, with index it took I think less than a minute to purge around 200k records. So yes, definitely!