The query in the previous commit reads the newest line of every room the user is
in, once per room, and Room re-runs the whole thing every time a message lands
anywhere. Without an index on `ChatMessage.chatRoomId` each of those lookups is a
scan of every message on the device: work that grows with the entire history
rather than with the room, on the hot path of every arriving message. A device
with ten rooms and a few thousand messages does tens of thousands of row reads to
redraw a list whose visible change is one line of text.
Room has wanted this index since the foreign key was declared and has said so on
every build -- `chatRoomId column references a foreign key but it is not part of
an index. This may trigger full table scans whenever parent table is modified` --
which is the same warning it still emits for a dozen other `chatRoomId` columns.
Those stay as they are; this one now has a reader that makes it matter.
**Schema v15, and Room writes the migration itself.** Adding an index changes no
columns and moves no rows, which is one of the shapes `AutoMigration` handles
without a spec, so this is an entry in the list rather than another manual
migration alongside MIGRATION_13_14.
The generated 15.json differs from 14.json in exactly one place, checked rather
than assumed: `index_ChatMessage_chatRoomId` appears on ChatMessage, and no
table's fields, createSql or other indices move at all.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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