def _message_index(message_path: Path) -> dict:
    revision = _database_revision(message_path)
    def build():
        connection = connect_sqlite(str(message_path))
        try:
            columns = _columns(connection, "message_table")
            result = {"columns": columns, "total_messages": 0, "conversations": {},
                      "ordered": [], "previews": {}, "revision": revision}
            if not {"conversation_id", "send_time"}.issubset(columns):
                return result
            result["total_messages"] = _message_counts(message_path)[0]
            # Sort/group inside SQLite instead of allocating one Python tuple
            # per message. Only compact row IDs and one timestamp per chat are
            # retained; message bodies are fetched for the visible page only.
            cursor = connection.execute(
                "SELECT conversation_id,GROUP_CONCAT(message_rowid),MAX(message_time) "
                "FROM (SELECT rowid AS message_rowid,conversation_id,CAST(send_time AS REAL) AS message_time "
                "FROM message_table WHERE conversation_id IS NOT NULL AND conversation_id<>'' "
                "AND conversation_id NOT LIKE 'Y:%' "
                "ORDER BY conversation_id,CAST(send_time AS REAL),rowid) GROUP BY conversation_id"
            )
            for conversation_id, encoded_rows, timestamp in cursor:
                row_ids = array("q", map(int, str(encoded_rows).split(",")))
                timestamp = float(timestamp or 0)
                if not math.isfinite(timestamp):
                    timestamp = 0.0
                result["conversations"][str(conversation_id)] = {
                    "rows": row_ids, "lastTimestamp": timestamp, "lastRowId": row_ids[-1],
                }
            result["ordered"] = sorted(result["conversations"], key=lambda key: (
                result["conversations"][key]["lastTimestamp"],
                result["conversations"][key]["lastRowId"], key), reverse=True)
            return result
        finally:
            connection.close()
    return _cached("index", revision[0], revision, build)