MySQL: index json arrays of variable length?

Question

I want to make a tags column of type json:

e.g.,

id  |  tags
=========================================
1   |  '["tag1", "tag2", "tag3"]'
2   |  '["tag1", "tag3", "tag5", "tag7"]'
3   |  '["tag2", "tag5"]'

I want to index each tag in the arrays, without knowing the length of the arrays (variable length).

So then if I query for rows that contain tag2, it should return rows 1, 3.

https://dev.mysql.com/doc/refman/5.7/en/json.html

JSON columns cannot be indexed. You can work around this restriction by creating an index on a generated column that extracts a scalar value from the JSON column

By "extracts a scalar value", does this mean I must extract & index each item in the arrays individually (meaning I must know the maximum length of the array to index them all)? How do I index a variable length array?

Answer 1

It's now possible with MySQL 8.0.17+

Something like this (not tested)

CREATE TABLE posts (
    id BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    tags JSON,
    INDEX tags( (CAST(tags AS VARCHAR(32) ARRAY)) )
    );

Use it this way:

SELECT * FROM posts 
         WHERE JSON_CONTAINS(tags, CAST('[tag1, tag2]' AS JSON));

More details and samples here: https://dev.mysql.com/doc/refman/8.0/en/json.html