more notes on relations

doc/impl-thoughts/RELATIONS.md

@@ -1,7 +1,9 @@
 Relations and redactions
 
 events that refer to another event will need support in the SyncWriter, Timeline and SendQueue I think.
-SyncWriter will need to resolve the related remote id to a [fragmentId, eventIndex] and persist that on the event that relates to some other. Same for SendQueue? If unknown remote id, not much to do. However, once the remote id comes in, how do we handle it correctly? We might need a index on m.relates_to/event_id? I'd rather avoid that if possible, as that becomes useless once we have the target event of the relationship (we store the relations on the target event (see "One fetch" below) and have the target event id on the relation so can go both ways). We should look into what part of the relationships will be present on the event once it is received from the server (e.g. m.replace might be evident, but not all the reaction events?). If not, we could add a object store with missing relation targets.
+SyncWriter will need to resolve the related remote id to a [fragmentId, eventIndex] and persist that on the event that relates to some other. Same for SendQueue? If unknown remote id, not much to do. However, once the remote id comes in, how do we handle it correctly? We might need a index on m.relates_to/event_id? I'd rather avoid that if possible, as that becomes useless once we have the target event of the relationship (we store the relations on the target event (see "One fetch" below) and have the target event id on the relation so can go both ways). I'm not sure this index will be completely useless actually. For edits, we'll want to be able to list all edits. For reactions, we'll want to fetch the authors and timestamps. For replies, we want to render the origin event and not use the fallback text? It is true though that only a minority of the events will have a related_to event id, so I wonder if it is faster to put it in a different store? Perhaps a prototype can clarify ...
+
+We should look into what part of the relationships will be present on the event once it is received from the server (e.g. m.replace might be evident, but not all the reaction events?). If not, we could add a object store with missing relation targets.
 
 The timeline can take incoming events from both the SendQueue and SyncWriter, and see if their related to fragmentId/eventIndex is in view, and then update it?