Document easy room purge benefit of using (room_id, event_id)

[MadLittleMods]

Document easy room purge benefit of using (room_id, event_id)

Discussed in the backend chapter sync as mentioned by @erikjohnston,
https://docs.google.com/document/d/1kmGRzPFfg_gRY6l0sxjYkSLW6UpMFn9ELQX5CtTLWlA/edit#bookmark=id.ciuq6xs2t47

Follow-up to #13701

Dev notes

Database tables which don't have a (room_id, event_id) index,

synapse/synapse/storage/databases/main/purge_events.py

Lines 388 to 412 in fa2f3d8

	# Now we delete tables which lack an index on room_id but have one on event_id
	for table in (
	"event_auth",
	"event_edges",
	"event_json",
	"event_push_actions_staging",
	"event_relations",
	"event_to_state_groups",
	"event_auth_chains",
	"event_auth_chain_to_calculate",
	"redactions",
	"rejections",
	"state_events",
	):
	logger.info("[purge] removing %s from %s", room_id, table)
	txn.execute(
	"""
	DELETE FROM %s WHERE event_id IN (
	SELECT event_id FROM events WHERE room_id=?
	)
	"""
	% (table,),
	(room_id,),
	)

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[DMRobertson]

I'm not sure I follow this reasoning. Is the point that we can do this with a single DELETE FROM ... WHERE ... rather than having to use a subselect or similar which joins to the events table?

[MadLittleMods]

@DMRobertson Seems to be the case so we don't have to do this:

synapse/synapse/storage/databases/main/purge_events.py

Lines 388 to 412 in fa2f3d8

	# Now we delete tables which lack an index on room_id but have one on event_id
	for table in (
	"event_auth",
	"event_edges",
	"event_json",
	"event_push_actions_staging",
	"event_relations",
	"event_to_state_groups",
	"event_auth_chains",
	"event_auth_chain_to_calculate",
	"redactions",
	"rejections",
	"state_events",
	):
	logger.info("[purge] removing %s from %s", room_id, table)
	txn.execute(
	"""
	DELETE FROM %s WHERE event_id IN (
	SELECT event_id FROM events WHERE room_id=?
	)
	"""
	% (table,),
	(room_id,),
	)

From the chapter sync, it was one of the useful benefits people liked from pairing up (room_id, event_id).

Added a note on this part of why it's easier ⏩

[MadLittleMods]

Closing in favor of #13915