feat(sqlite): allow storing the same events in multiple linked chunks

crates/matrix-sdk-sqlite/migrations/event_cache_store/014_event_chunks_event_id_index.sql

@@ -0,0 +1,34 @@
+-- Remove uniqueness constraint on event_id, replace with regular index.
+-- Empties event cache as data migration is not needed.
+
+DELETE FROM linked_chunks;
+DELETE FROM event_chunks;  -- should be done by cascading
+DELETE FROM gap_chunks;    -- should be done by cascading
+
+DROP TABLE event_chunks;
+
+-- Recreate event_chunks with new PRIMARY KEY
+CREATE TABLE "event_chunks" (
+    -- Which linked chunk does this event belong to? (hashed key shared with linked_chunks)
+    "linked_chunk_id" BLOB NOT NULL,
+    -- Which chunk does this event refer to? Corresponds to a `ChunkIdentifier`.
+    "chunk_id" INTEGER NOT NULL,
+
+    -- `OwnedEventId` for events.
+    "event_id" BLOB NOT NULL,
+    -- Position (index) in the chunk.
+    "position" INTEGER NOT NULL,
+
+    -- We need a uniqueness constraint over the `linked_chunk_id`, `chunk_id` and
+    -- `position` tuple because (i) they must be unique, (ii) it dramatically
+    -- improves the performance. Also, we don't have a ROWID, so we must use a PRIMARY KEY, hence
+    -- we use this composite key as the primary key.
+    PRIMARY KEY (linked_chunk_id, chunk_id, position),
+
+    -- If the owning chunk gets deleted, delete the entry too.
+    FOREIGN KEY (linked_chunk_id, chunk_id) REFERENCES linked_chunks(linked_chunk_id, id) ON DELETE CASCADE
+)
+WITHOUT ROWID;
+
+-- Create a non-unique index on `event_id` for query performance.
+CREATE INDEX "event_chunks_event_id_idx" ON "event_chunks" ("event_id");

crates/matrix-sdk-sqlite/src/event_cache_store.rs

@@ -67,7 +67,7 @@ const DATABASE_NAME: &str = "matrix-sdk-event-cache.sqlite3";
 /// This is used to figure whether the SQLite database requires a migration.
 /// Every new SQL migration should imply a bump of this number, and changes in
 /// the [`run_migrations`] function.
-const DATABASE_VERSION: u8 = 13;
+const DATABASE_VERSION: u8 = 14;
 
 /// The string used to identify a chunk of type events, in the `type` field in
 /// the database.
@@ -496,6 +496,16 @@ async fn run_migrations(conn: &SqliteAsyncConn, version: u8) -> Result<()> {
         .await?;
     }
 
+    if version < 14 {
+        conn.with_transaction(|txn| {
+            txn.execute_batch(include_str!(
+                "../migrations/event_cache_store/014_event_chunks_event_id_index.sql"
+            ))?;
+            txn.set_db_version(14)
+        })
+        .await?;
+    }
+
     Ok(())
 }