MySQL: index json arrays of variable length?

Question

I want to make a tags column of type json:

e.g.,

id  |  tags
=========================================
1   |  \'["tag1"         


        

Answer 1

By "extracts a scalar value", does this mean I must extract & index each item in the arrays individually [...]?

You can extract as many items as you want. They will be stored as scalars (e.g. string), rather than as compound values (which JSON is).

CREATE TABLE mytags (
    id INT NOT NULL AUTO_INCREMENT,
    tags JSON,
    PRIMARY KEY (id)
);

INSERT INTO mytags (tags) VALUES
    ('["tag1", "tag2", "tag3"]'),
    ('["tag1", "tag3", "tag5", "tag7"]'),
    ('["tag2", "tag5"]');

SELECT * FROM mytags;

+----+----------------------------------+
| id | tags                             |
+----+----------------------------------+
|  1 | ["tag1", "tag2", "tag3"]         |
|  2 | ["tag1", "tag3", "tag5", "tag7"] |
|  3 | ["tag2", "tag5"]                 |
+----+----------------------------------+

Let's create an index with one item only (first value from the JSON object):

ALTER TABLE mytags
    ADD COLUMN tags_scalar VARCHAR(255) GENERATED ALWAYS AS (json_extract(tags, '$[0]')),
    ADD INDEX tags_index (tags_scalar);

SELECT * FROM mytags;

+----+----------------------------------+-------------+
| id | tags                             | tags_scalar |
+----+----------------------------------+-------------+
|  1 | ["tag1", "tag2", "tag3"]         | "tag1"      |
|  2 | ["tag1", "tag3", "tag5", "tag7"] | "tag1"      |
|  3 | ["tag2", "tag5"]                 | "tag2"      |
+----+----------------------------------+-------------+

Now you have an index on the VARCHAR column tags_scalar. The value contains quotes, which can also be skipped:

ALTER TABLE mytags DROP COLUMN tags_scalar, DROP INDEX tags_index;

ALTER TABLE mytags
    ADD COLUMN tags_scalar VARCHAR(255) GENERATED ALWAYS AS (json_unquote(json_extract(tags, '$[0]'))),
    ADD INDEX tags_index (tags_scalar);

SELECT * FROM mytags;

+----+----------------------------------+-------------+
| id | tags                             | tags_scalar |
+----+----------------------------------+-------------+
|  1 | ["tag1", "tag2", "tag3"]         | tag1        |
|  2 | ["tag1", "tag3", "tag5", "tag7"] | tag1        |
|  3 | ["tag2", "tag5"]                 | tag2        |
+----+----------------------------------+-------------+

As you can already imagine, the generated column can include more items from the JSON:

ALTER TABLE mytags DROP COLUMN tags_scalar, DROP INDEX tags_index;

ALTER TABLE mytags
    ADD COLUMN tags_scalar VARCHAR(255) GENERATED ALWAYS AS (json_extract(tags, '$[0]', '$[1]', '$[2]')),
    ADD INDEX tags_index (tags_scalar);

SELECT * from mytags;

+----+----------------------------------+--------------------------+
| id | tags                             | tags_scalar              |
+----+----------------------------------+--------------------------+
|  1 | ["tag1", "tag2", "tag3"]         | ["tag1", "tag2", "tag3"] |
|  2 | ["tag1", "tag3", "tag5", "tag7"] | ["tag1", "tag3", "tag5"] |
|  3 | ["tag2", "tag5"]                 | ["tag2", "tag5"]         |
+----+----------------------------------+--------------------------+

or use any other valid expression to auto-generate a string out of the JSON structure, in order to obtain something that can be easily indexed and searched like "tag1tag3tag5tag7".

[...](meaning I must know the maximum length of the array to index them all)?

As explained above, you don't need to know - NULL values can be skipped by using any valid expression. But of course it's always better to know.
Now there's the architecture decision: Is JSON data type the most appropriate to achieve the goal? To solve this particular problem? Is JSON the right tool here? Is it going to speed up searching?

How do I index a variable length array?

If you insist, cast string:

ALTER TABLE mytags DROP COLUMN tags_scalar, DROP INDEX tags_index;

ALTER TABLE mytags
    ADD COLUMN tags_scalar VARCHAR(255) GENERATED ALWAYS AS (replace(replace(replace(cast(tags as char), '"', ''), '[', ''), ']', '')),
    ADD INDEX tags_index (tags_scalar);

SELECT * from mytags;

+----+----------------------------------+------------------------+
| id | tags                             | tags_scalar            |
+----+----------------------------------+------------------------+
|  1 | ["tag1", "tag2", "tag3"]         | tag1, tag2, tag3       |
|  2 | ["tag1", "tag3", "tag5", "tag7"] | tag1, tag3, tag5, tag7 |
|  3 | ["tag2", "tag5"]                 | tag2, tag5             |
+----+----------------------------------+------------------------+

This way or another you end up with a VARCHAR or TEXT column, where you apply the most applicable index structure (some options).

Further reading:

• Indexing a Generated Column to Provide a JSON Column Index
• Functions That Search JSON Values