Improve order by json field performance on mysql - mysql

Question

I have this table:
CREATE TABLE `mytable` (
`session_id` mediumint(8) UNSIGNED NOT NULL,
`data` json NOT NULL,
`jobname` varchar(100) COLLATE utf8_unicode_ci GENERATED ALWAYS AS
(json_unquote(json_extract(`data`,'$.jobname'))) VIRTUAL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
PARTITION BY HASH (session_id)
PARTITIONS 10;
ALTER TABLE `mytable`
ADD KEY `session` (`session_id`),
ADD KEY `jobname` (`jobname`);
It has 2 million rows.
When execute this query, it takes around 23 sec to get the result.
SELECT JSON_EXTRACT(f.data, '$.jobdesc') AS jobdesc
FROM mytable f
WHERE f.session_id = 1
ORDER BY jobdesc DESC
I understand that it is slow because there is no index for jobdesc field.
On data's column, I have 12 fields. I want to let user to be able to sort all fields. If I add index for each field, is it good approach?
Is there any way to improve it?
I am using MYSQL 5.7.13.

Answer 1

You would have to create a virtual column with an index for each of your 12 fields, if you want the user to have the option of sorting by.
CREATE TABLE `mytable` (
`session_id` mediumint(8) UNSIGNED NOT NULL,
`data` json NOT NULL,
`jobname` varchar(100) AS (json_unquote(json_extract(`data`,'$.jobname'))),
`jobdesc` varchar(100) AS (json_unquote(json_extract(`data`,'$.jobdesc'))),
...other extracted virtual fields...
KEY (`jobname`),
KEY (`jobdesc`),
...other indexes on virtual columns...
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
PARTITION BY HASH (session_id)
PARTITIONS 10;
This makes me wonder: why bother using JSON? Why not just declare 12 conventional, non-virtual columns with indexes?
CREATE TABLE `mytable` (
`session_id` mediumint(8) UNSIGNED NOT NULL,
...no `data` column needed...
`jobname` varchar(100),
`jobdesc` varchar(100),
...
KEY (`jobname`),
KEY (`jobdesc`),
...
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
PARTITION BY HASH (session_id)
PARTITIONS 10;
JSON is best when you treat it as a single atomic document, and don't try to use SQL operations on fields within it. If you regularly need to access fields within your JSON, make them into conventional columns.