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diff --git a/docs/development/database_schema.md b/docs/development/database_schema.md
index d996a7caa2..29945c264e 100644
--- a/docs/development/database_schema.md
+++ b/docs/development/database_schema.md
@@ -191,3 +191,28 @@ There are three separate aspects to this:
    flavour will be accepted by SQLite 3.22, but will give a column whose
    default value is the **string** `"FALSE"` - which, when cast back to a boolean
    in Python, evaluates to `True`.
+
+
+## `event_id` global uniqueness
+
+`event_id`'s can be considered globally unique although there has been a lot of
+debate on this topic in places like
+[MSC2779](https://github.com/matrix-org/matrix-spec-proposals/issues/2779) and
+[MSC2848](https://github.com/matrix-org/matrix-spec-proposals/pull/2848) which
+has no resolution yet (as of 2022-09-01). There are several places in Synapse
+and even in the Matrix APIs like [`GET
+/_matrix/federation/v1/event/{eventId}`](https://spec.matrix.org/v1.1/server-server-api/#get_matrixfederationv1eventeventid)
+where we assume that event IDs are globally unique.
+
+When scoping `event_id` in a database schema, it is often nice to accompany it
+with `room_id` (`PRIMARY KEY (room_id, event_id)` and a `FOREIGN KEY(room_id)
+REFERENCES rooms(room_id)`) which makes flexible lookups easy. For example it
+makes it very easy to find and clean up everything in a room when it needs to be
+purged (no need to use sub-`select` query or join from the `events` table).
+
+A note on collisions: In room versions `1` and `2` it's possible to end up with
+two events with the same `event_id` (in the same or different rooms). After room
+version `3`, that can only happen with a hash collision, which we basically hope
+will never happen (SHA256 has a massive big key space).
+
+