SELECT ts, host, json_to_string(j) AS j
FROM legacy_jsonb
ORDER BY ts, host;

+---------------------+------+----------------------------+
| ts                  | host | j                          |
+---------------------+------+----------------------------+
| 2024-01-01T00:00:01 | r1   | {"a":10,"msg":"old-jsonb"} |
| 2024-01-01T00:00:02 | r2   | {"a":20,"msg":"old-jsonb"} |
| 2024-01-01T00:00:03 | r3   | {"msg":"no-a"}             |
+---------------------+------+----------------------------+

SELECT host, json_to_string(j) AS j
FROM legacy_jsonb
WHERE json_get_int(j, 'a') = 10
ORDER BY host;

+------+----------------------------+
| host | j                          |
+------+----------------------------+
| r1   | {"a":10,"msg":"old-jsonb"} |
+------+----------------------------+

SHOW CREATE TABLE legacy_jsonb;

+--------------+---------------------------------------------+
| Table        | Create Table                                |
+--------------+---------------------------------------------+
| legacy_jsonb | CREATE TABLE IF NOT EXISTS "legacy_jsonb" ( |
|              |   "ts" TIMESTAMP(3) NOT NULL,               |
|              |   "host" STRING NULL,                       |
|              |   "j" JSON NULL,                            |
|              |   TIME INDEX ("ts"),                        |
|              |   PRIMARY KEY ("host")                      |
|              | )                                           |
|              |                                             |
|              | ENGINE=mito                                 |
|              |                                             |
+--------------+---------------------------------------------+